Description of VFS                            Thomas Veerman 18-12-2012

Table of contents
1 ..... General description of responsibilities
2 ..... General architecture
3 ..... Worker threads
4 ..... Locking
4.1 .... Locking requirements
4.2 .... Three-level Lock
4.3 .... Data structures subject to locking
4.4 .... Locking order
4.5 .... Vmnt (file system) locking
4.6 .... Vnode (open file) locking
4.7 .... Filp (file position) locking
4.8 .... Lock characteristics per request type
5 ..... Recovery from driver crashes
5.1 .... Recovery from block drivers crashes
5.2 .... Recovery from character driver crashes
5.3 .... Recovery from File Server crashes

1 General description of responsibilities
VFS implements the file system in cooperation with one or more File Servers
(FS). The File Servers take care of the actual file system on a partition. That
is, they interpret the data structure on disk, write and read data to/from
disk, etc. VFS sits on top of those File Servers and communicates with
them. Looking inside VFS, we can identify several roles. First, a role of VFS
is to handle most POSIX system calls that are supported by Minix. Additionally,
it supports a few calls necessary for libc. The following system calls are
handled by VFS:
access, chdir, chmod, chown, chroot, close, creat, fchdir, fcntl, fstat,
fstatfs, fstatvfs, fsync, ftruncate getdents, ioctl, link, llseek, lseek,
lstat, mkdir, mknod, mount, open, pipe, read, readlink, rename, rmdir, select,
stat, statvfs, symlink, sync, truncate, umask, umount, unlink, utime, write.
Second, it maintains part of the state belonging to a process (process state is
spread out over the kernel, VM, PM, and VFS). For example, it maintains state
for select(2) calls, file descriptors and file positions. Also, it cooperates
with the Process Manager to handle the fork, exec, and exit system calls.
Third, VFS keeps track of endpoints that are supposed to be drivers for
character or block special files. File Servers can be regarded as drivers for
block special files, although they are handled entirely different compared
to other drivers.

The following diagram depicts how a read() on a file in /home is being handled:

      ----------------
      | user process |
      ----------------
             ^      ^
             |      |
           read(2)   \
             |        \
             V         \
      ----------------  |
      |      VFS     |  |
      ----------------  |
                    ^   |
                    |   |
                    V   |
  ------- -------- ---------
  | MFS | |  MFS | |  MFS  |
  |  /  | | /usr | | /home |
  ------- -------- ---------
Diagram 1: handling of read(2) system call

The user process executes the read system call which is delivered to VFS. VFS
verifies the read is done on a valid (open) file and forwards the request
to the FS responsible for the file system on which the file resides. The FS
reads the data, copies it directly to the user process, and replies to VFS
it has executed the request. Subsequently, VFS replies to the user process
the operation is done and the user process continues to run.

2 General architecture
VFS works roughly identical to every other server and driver in Minix; it
fetches a message (internally referred to as a job in some cases), executes
the request embedded in the message, returns a reply, and fetches the next
job. There are several sources for new jobs: from user processes, from PM, from
the kernel, and from suspended jobs inside VFS itself (suspended operations
on pipes, locks, or character special files). File Servers are regarded as
normal user processes in this case, but their abilities are limited. This
is to prevent deadlocks. Once a job is received, a worker thread starts
executing it. During the lifetime of a job, the worker thread might need
to talk to several File Servers. The protocol VFS speaks with File Servers
is fully documented on the Wiki at [0]. The protocol fields are defined in
<minix/vfsif.h>. If the job is an operation on a character or block special
file and the need to talk to a driver arises, VFS uses the Character and
Block Device Protocol. See [1]. This is sadly not official documentation,
but it is an accurate description of how it works. Luckily, driver writers
can use the libchardriver and libblockdriver libraries and don't have to
know the details of the protocol.

3 Worker threads
Upon start up, VFS spawns a configurable amount of worker threads. The
main thread fetches requests and replies, and hands them off to idle or
reply-pending workers, respectively. If no worker threads are available,
the request is queued. There are 3 types of worker threads: normal, a system
worker, and a deadlock resolver. All standard system calls are handled by
normal worker threads. Jobs from PM and notifications from the kernel are taken
care of by the system worker. The deadlock resolver handles jobs from system
processes (i.e., File Servers and drivers) when there are no normal worker
threads available; all normal threads might be blocked on a single worker
thread that caused a system process to send a request on its own. To unblock
all normal threads, we need to reserve one thread to handle that situation.
VFS drives all File Servers and drivers asynchronously. While waiting for
a reply, a worker thread is blocked and other workers can keep processing
requests. Upon reply the worker thread is unblocked.
As mentioned above, the main thread is responsible for retrieving new jobs and
replies to current jobs and start or unblock the proper worker thread. Given
how many sources for new jobs and replies there are, the work for the main
thread is quite complicated. Consider Table 1.

---------------------------------------------------------
| From                 |  normal  | deadlock |  system  |
---------------------------------------------------------
 msg is new job
---------------------------------------------------------
| PM                   |          |          |    X     |
+----------------------+----------+----------+----------+
| Notification from    |          |          |          |
| the kernel           |          |          |    X     |
+----------------------+----------+----------+----------+
| Notification from    |          |          |          |
| DS or system process |    X     |    X     |          |
+----------------------+----------+----------+----------+
| User process         |    X     |          |          |
+----------------------+----------+----------+----------+
| Unsuspended process  |    X     |          |          |
---------------------------------------------------------
 msg is reply
---------------------------------------------------------
| File Server reply    |  resume  |          |          |
+----------------------+----------+----------+----------+
| Sync. driver reply   |  resume  |          |          |
+----------------------+----------+----------+----------+
| Async. driver reply  | resume/X |    X     |          |
---------------------------------------------------------
Table 1: VFS' message fetching main loop. X means 'start thread'.

The reason why asynchronous driver replies get their own thread is for the
following. In some cases, a reply has a thread blocked waiting for it which
can be resumed (e.g., open). In another case there's a lot of work to be
done which involves sending new messages (e.g., select replies). Finally,
DEV_REVIVE replies unblock suspended processes which in turn generate new jobs
to be handled by the main loop (e.g., suspended reads and writes). So depending
on the reply a new thread has to be started. Having all this logic in the main
loop is messy, so we start a thread regardless of the actual reply contents.
When there are no worker threads available and there is no need to invoke
the deadlock resolver (i.e., normal system calls), the request is queued in
the fproc table. This works because a process can send only one system call
at a time. When implementing kernel threads, one has to take this assumption
into account.
The protocol PM speaks with VFS is asynchronous and PM is allowed to
send as many request to VFS as it wants. It is impossible to use the same
queueing mechanism as normal processes use, because that would allow for
just 1 queued message. Instead, the system worker maintains a linked list
of pending requests. Moreover, this queueing mechanism is also the reason
why notifications from the kernel are handled by the system worker; the
kernel has no corresponding fproc table entry (so we can't store it there)
and the linked list has no dependencies on that table.
Communication with drivers is asynchronous even when the driver uses the
synchronous driver protocol. However, to guarantee identical behavior,
access to synchronous drivers is serialized. File Servers are treated
differently. VFS was designed to be able to send requests concurrently to
File Servers, although at the time of writing there are no File Servers that
can actually make use of that functionality. To identify which reply from an
FS belongs to which worker thread, all requests have an embedded transaction
identification number (a magic number + thread id encoded in the mtype field
of a message) which the FS has to echo upon reply. Because the range of valid
transaction IDs is isolated from valid system call numbers, VFS can use that
ID to differentiate between replies from File Servers and actual new system
calls from FSes. Using this mechanism VFS is able to support FUSE and ProcFS.

4 Locking
To ensure correct execution of system calls, worker threads sometimes need
certain objects within VFS to remain unchanged during thread suspension
and resumption (i.e., when they need to communicate with a driver or File
Server). Threads keep most state on the stack, but there are a few global
variables that require protection: the fproc table, vmnt table, vnode table,
and filp table. Other tables such as lock table, select table, and dmap table
don't require protection by means of exclusive access. There it's required
and enough to simply mark an entry in use.

4.1 Locking requirements
VFS implements the locking model described in [2]. For completeness of this
document we'll describe it here, too. The requirements are based on a threading
package that is non-preemptive. VFS must guarantee correct functioning with
several, semi-concurrently executing threads in any arbitrary order. The
latter requirement follows from the fact that threads need service from
other components like File Servers and drivers, and they may take any time
to complete requests.
1) Consistency of replicated values. Several system calls rely on VFS keeping
a replicated representation of data in File Servers (e.g., file sizes,
file modes, etc.).
2) Isolation of system calls. Many system calls involve multiple requests to
FSes. Concurrent requests from other processes must not lead to otherwise
impossible results (e.g., a chmod operation on a file cannot fail halfway
through because it's suddenly unlinked or moved).
3) Integrity of objects. From the point of view of threads, obtaining mutual
exclusion is a potentially blocking operation. The integrity of any objects
used across blocking calls must be guaranteed (e.g., the file mode in a vnode
must remain intact not only when talking to other components, but also when
obtaining a lock on a filp).
4) No deadlock. Not one call may cause another call to never complete. Deadlock
situations are typically the result of two or more threads that each hold
exclusive access to one resource and want exclusive access to the resource
held by the other thread. These resources are a) data (global variables)
and b) worker threads.
4a) Conflicts between locking of different types of objects can be avoided by
keeping a locking order: objects of different type must always be locked in
the same order. If multiple objects of the same type are to be locked, then
first a "common denominator" higher up in the locking order must be locked.
4b) Some threads can only run to completion when another thread does work on
their behalf. Examples of this are drivers and file servers that do system
calls on their own (e.g., ProcFS, PFS/UNIX Domain Sockets, FUSE) or crashing
components (e.g., a driver for a character special file that crashes during
a request; a second thread is required to handle resource clean up or driver
restart before the first thread can abort or retry the request).
5) No starvation. VFS must guarantee that every system call completes in finite
time (e.g., an infinite stream of reads must never completely block writes).
Furthermore, we want to maximize parallelism to improve performance. This
leads to:
6) A request to one File Server must not block access to other FS
processes. This means that most forms of locking cannot take place at a
global level, and must at most take place on the file system level.
7) No read-only operation on a regular file must block an independent read
call to that file. In particular, (read-only) open and close operations may
not block such reads, and multiple independent reads on the same file must
be able to take place concurrently (i.e., reads that do not share a file
position between their file descriptors).

4.2 Three-level Lock
From the requirements it follows that we need at least two locking types: read
and write locks. Concurrent reads are allowed, but writes are exclusive both
from reads and from each other. However, in a lot of cases it possible to use
a third locking type that is in between read and write lock: the serialize
lock. This is implemented in the three-level lock [2]. The three-level
lock provides:
TLL_READ: allows an unlimited number of threads to hold the lock with the
same type (both the thread itself and other threads); N * concurrent.
TLL_READSER: also allows an unlimited number of threads with type TLL_READ,
but only one thread can obtain serial access to the lock; N * concurrent +
1 * serial.
TLL_WRITE: provides full mutual exclusion; 1 * exclusive + 0 * concurrent +
0 * serial.
In absence of TLL_READ locks, a TLL_READSER is identical to TLL_WRITE. However,
TLL_READSER never blocks concurrent TLL_READ access. TLL_READSER can be
upgraded to TLL_WRITE; the thread will block until the last TLL_READ lock
leaves and new TLL_READ locks are blocked. Locks can be downgraded to a
lower type. The three-level lock is implemented using two FIFO queues with
write-bias. This guarantees no starvation.

4.3 Data structures subject to locking
VFS has a number of global data structures. See Table 2.

--------------------------------------------------------------------
| Structure  | Object description                                  |
+------------+-----------------------------------------------------|
| fproc      | Process (includes process's file descriptors)       |
+------------+-----------------------------------------------------|
| vmnt       | Virtual mount; a mounted file system                |
+------------+-----------------------------------------------------|
| vnode      | Virtual node; an open file                          |
+------------+-----------------------------------------------------|
| filp       | File position into an open file                     |
+------------+-----------------------------------------------------|
| lock       | File region locking state for an open file          |
+------------+-----------------------------------------------------|
| select     | State for an in-progress select(2) call             |
+------------+-----------------------------------------------------|
| dmap       | Mapping from major device number to a device driver |
--------------------------------------------------------------------
Table 2: VFS object types.

An fproc object is a process. An fproc object is created by fork(2)
and destroyed by exit(2) (which may, or may not, be instantiated from the
process itself). It is identified by its endpoint number ('fp_endpoint')
and process id ('fp_pid'). Both are unique although in general the endpoint
number is used throughout the system.
A vmnt object is a mounted file system. It is created by mount(2) and destroyed
by umount(2). It is identified by a device number ('m_dev') and FS endpoint
number ('m_fs_e'); both are unique to each vmnt object. There is always a
single process that handles a file system on a device and a device cannot
be mounted twice.
A vnode object is the VFS representation of an open inode on the file
system. A vnode object is created when a first process opens or creates the
corresponding file and is destroyed when the last process, which has that
file open, closes it. It is identified by a combination of FS endpoint number
('v_fs_e') and inode number of that file system ('v_inode_nr'). A vnode
might be mapped to another file system; the actual reading and writing is
handled by a different endpoint. This has no effect on locking.
A filp object contains a file position within a file. It is created when a file
is opened or anonymous pipe created and destroyed when the last user (i.e.,
process) closes it. A file descriptor always points to a single filp. A filp
always point to a single vnode, although not all vnodes are pointed to by a
filp. A filp has a reference count ('filp_count') which is identical to the
number of file descriptors pointing to it. It can be increased by a dup(2)
or fork(2). A filp can therefore be shared by multiple processes.
A lock object keeps information about locking of file regions. This has
nothing to do with the threading type of locking. The lock objects require
no locking protection and won't be discussed further.
A select object keeps information on a select(2) operation that cannot
be fulfilled immediately (waiting for timeout or file descriptors not
ready). They are identified by their owner ('requestor'); a pointer to the
fproc table. A null pointer means not in use. A select object can be used by
only one process and a process can do only one select(2) at a time. Select(2)
operates on filps and is organized in such a way that it is sufficient to
apply locking on individual filps and not on select objects themselves. They
won't be discussed further.
A dmap object is a mapping from a device number to a device driver. A device
driver can have multiple device numbers associated (e.g., TTY). Access to
a driver is exclusive when it uses the synchronous driver protocol.

4.4 Locking order
Based on the description in the previous section, we need protection for
fproc, vmnt, vnode, and filp objects. To prevent deadlocks as a result of
object locking, we need to define a strict locking order. In VFS we use the
following order:

fproc -> [exec] -> vmnt -> vnode -> filp -> [block special file] -> [dmap]

That is, no thread may lock an fproc object while holding a vmnt lock,
and no thread may lock a vmnt object while holding an (associated) vnode, etc.
Fproc needs protection because processes themselves can initiate system
calls, but also PM can cause system calls that have to be executed in their
name. For example, a process might be busy reading from a character device
and another process sends a termination signal. The exit(2) that follows is
sent by PM and is to be executed by the to-be-killed process itself. At this
point there is contention for the fproc object that belongs to the process,
hence the need for protection.
The exec(2) call is protected by a mutex for the following reason. VFS uses a
number of variables on the heap to read ELF headers. They are on the heap due
to their size; putting them on the stack would increase stack size demands for
worker threads. The exec call does blocking read calls and thus needs exclusive
access to these variables. However, only the exec(2) syscall needs this lock.
Access to block special files needs to be exclusive. File Servers are
responsible for handling reads from and writes to block special files; if
a block special file is on a device that is mounted, the FS responsible for
that mount point takes care of it, otherwise the FS that handles the root of
the file system is responsible. Due to mounting and unmounting file systems,
the FS handling a block special file may change. Locking the vnode is not
enough since the inode can be on an entirely different File Server. Therefore,
access to block special files must be mutually exclusive from concurrent
mount(2)/umount(2) operations. However, when we're not accessing a block
special file, we don't need this lock.

4.5 Vmnt (file system) locking
Vmnt locking cannot be seen completely separately from vnode locking. For
example, umount(2) fails if there are still in-use vnodes, which means that
FS requests [0] only involving in-use inodes do not have to acquire a vmnt
lock. On the other hand, all other request do need a vmnt lock. Extrapolating
this to system calls this means that all system calls involving a file
descriptor don't need a vmnt lock and all other system calls (that make FS
requests) do need a vmnt lock.

-------------------------------------------------------------------------------
| Category          | System calls                                            |
+-------------------+---------------------------------------------------------+
| System calls with | access, chdir, chmod, chown, chroot, creat, dumpcore*,  |
| a path name       | exec, link, lstat, mkdir, mknod, mount, open, readlink, |
| argument          | rename, rmdir, stat, statvfs, symlink, truncate, umount,|
|                   | unlink, utime                                           |
+-------------------+---------------------------------------------------------+
| System calls with | close, fchdir, fcntl, fstat, fstatvfs, ftruncate,       |
| a file descriptor | getdents, ioctl, llseek, pipe, read, select, write      |
| argument          |                                                         |
+-------------------+---------------------------------------------------------+
| System calls with | fsync**, sync, umask                                    |
| other or no       |                                                         |
| arguments         |                                                         |
-------------------------------------------------------------------------------
Table 3: System call categories
* path name argument is implicit, the path name is "core.<pid>"
** although fsync actually provides a file descriptor argument, it's only
used to find the vmnt and not to do any actual operations on

Before we describe what kind of vmnt locks VFS applies to system calls with a
path name or other arguments, we need to make some notes on path lookup. Path
lookups take arbitrary paths as input (relative and absolute). They can start
at any vmnt (based on root directory and working directory of the process doing
the lookup) and visit any file system in arbitrary order, possibly visiting
the same file system more than once. As such, VFS can never tell in advance
at which File Server a lookup will end. This has the following consequences:
 - In the lookup procedure, only one vmnt must be locked at a time. When
 moving from one vmnt to another, the first vmnt has to be unlocked before
 acquiring the next lock to prevent deadlocks.
 - The lookup procedure must lock each visited file system with TLL_READSER
 and downgrade or upgrade to the lock type desired by the caller for the
 destination file system (as VFS cannot know which file system is final). This
 is to prevent deadlocks when a thread acquires a TLL_READSER on a vmnt and
 another thread TLL_READ on the same vmnt. If the second thread is blocked
 on the first thread due to it acquiring a lock on a vnode, the first thread
 will be unable to upgrade a TLL_READSER lock to TLL_WRITE.

We use the following mapping for vmnt locks onto three-level lock types:
-------------------------------------------------------------------------------
| Lock type  |  Mapped to  | Used for                                         |
+------------+-------------+--------------------------------------------------+
| VMNT_READ  | TLL_READ    | Read-only operations and fully independent write |
|            |             | operations                                       |
+------------+-------------+--------------------------------------------------+
| VMNT_WRITE | TLL_READSER | Independent create and modify operations         |
+------------+-------------+--------------------------------------------------+
| VMNT_EXCL  | TLL_WRITE   | Delete and dependent write operations            |
-------------------------------------------------------------------------------
Table 4: vmnt to tll lock mapping

The following table shows a sub-categorization of system calls without a
file descriptor argument, together with their locking types and motivation
as used by VFS.
-------------------------------------------------------------------------------
| Group       | System calls | Lock type  | Motivation                        |
+-------------+--------------+------------+-----------------------------------+
| File open   | chdir,       | VMNT_READ  | These operations do not interfere |
| ops.        | chroot, exec,|            | with each other, as vnodes can be |
| (non-create)| open         |            | opened concurrently, and open     |
|             |              |            | operations do not affect          |
|             |              |            | replicated state.                 |
+-------------+--------------+------------+-----------------------------------+
| File create-| creat,       | VMNT_EXCL  | File create ops. require mutual   |
| and-open    | open(O_CREAT)| for create | exclusion from concurrent file    |
| ops         |              | VMNT_WRITE | open ops. If the file already     |
|             |              | for open   | existed, the VMNT_WRITE lock that |
|             |              |            | is necessary for the lookup is    |
|             |              |            | not upgraded                      |
+-------------+--------------+------------+-----------------------------------+
| File create-| pipe         | VMNT_READ  | These create nameless inodes      |
| unique-and- |              |            | which cannot be opened by means   |
| open ops.   |              |            | of a path. Their creation         |
|             |              |            | therefore does not interfere with |
|             |              |            | anything else                     |
+-------------+--------------+------------+-----------------------------------+
| File create-| mkdir, mknod,| VMNT_WRITE | These operations do not affect    |
| only ops.   | slink        |            | any VFS state, and can therefore  |
|             |              |            | take place concurrently with open |
|             |              |            | operations                        |
+-------------+--------------+------------+-----------------------------------+
| File info   | access, lstat| VMNT_READ  | These operations do not interfere |
| retrieval or| readlink,stat|            | with each other and do not modify |
| modification| utime        |            | replicated state                  |
+-------------+--------------+------------+-----------------------------------+
| File        | chmod, chown,| VMNT_READ  | These operations do not interfere |
| modification| truncate     |            | with each other. They do need     |
|             |              |            | exclusive access on the vnode     |
|             |              |            | level                             |
+-------------+--------------+------------+-----------------------------------+
| File link   | link         | VMNT_WRITE | Identical to file create-only     |
| ops.        |              |            | operations                        |
+-------------+--------------+------------+-----------------------------------+
| File unlink | rmdir, unlink| VMNT_EXCL  | These must not interfere with     |
| ops.        |              |            | file create operations, to avoid  |
|             |              |            | the scenario where inodes are     |
|             |              |            | reused immediately. However, due  |
|             |              |            | to necessary path checks, the     |
|             |              |            | vmnt is first locked VMNT_WRITE   |
|             |              |            | and then upgraded                 |
+-------------+--------------+------------+-----------------------------------+
| File rename | rename       | VMNT_EXCL  | Identical to file unlink          |
| ops.        |              |            | operations                        |
+-------------+--------------+------------+-----------------------------------+
| Non-file    | sync, umask  | VMNT_READ  | umask does not involve the file   |
| ops.        |              | or none    | system, so it does not need       |
|             |              |            | locks. sync does not alter state  |
|             |              |            | in VFS and  is atomic at the FS   |
|             |              |            | level                             |
-------------------------------------------------------------------------------
Table 5: System call without file descriptor argument sub-categorization

4.6 Vnode (open file) locking
Compared to vmnt locking, vnode locking is relatively straightforward. All
read-only accesses to vnodes that merely read the vnode object's fields are
allowed to be concurrent. Consequently, all accesses that change fields
of a vnode object must be exclusive. This leaves us with creation and
destruction of vnode objects (and related to that, their reference counts);
it's sufficient to serialize these accesses. This follows from the fact
that a vnode is only created when the first user opens it, and destroyed
when the last user closes it. A open file in process A cannot be be closed
by process B. Note that this also relies on the fact that a process can do
only one system call at a time. Kernel threads would violate this assumption.

We use the following mapping for vnode locks onto three-level lock types:
-------------------------------------------------------------------------------
| Lock type  |  Mapped to  | Used for                                         |
+------------+-------------+--------------------------------------------------+
| VNODE_READ | TLL_READ    | Read access to previously opened vnodes          |
+------------+-------------+--------------------------------------------------+
| VNODE_OPCL | TLL_READSER | Creation, opening, closing, and destruction of   |
|            |             | vnodes                                           |
+------------+-------------+--------------------------------------------------+
| VNODE_WRITE| TLL_WRITE   | Write access to previously opened vnodes         |
-------------------------------------------------------------------------------
Table 6: vnode to tll lock mapping

When vnodes are destroyed, they are initially locked with VNODE_OPCL. After
all, we're going to alter the reference count, so this must be serialized. If
the reference count then reaches zero we obtain exclusive access. This should
always be immediately possible unless there is a consistency problem. See
section 4.8 for an exhaustive listing of locking methods for all operations on
vnodes.

4.7 Filp (file position) locking
The main fields of a filp object that are shared between various processes
(and by extension threads), and that can change after object creation,
are filp_count and filp_pos. Writes to and reads from filp object must be
mutually exclusive, as all system calls have to use the latest version. For
example, a read(2) call changes the file position (i.e., filp_pos), so two
concurrent reads must obtain exclusive access. Consequently, as even read
operations require exclusive access, filp object don't use three-level locks,
but only mutexes.

System calls that involve a file descriptor often access both the filp and
the corresponding vnode. The locking order requires us to first lock the
vnode and then the filp. This is taken care of at the filp level. Whenever
a filp is locked, a lock on the vnode is acquired first. Conversely, when
a filp is unlocked, the corresponding vnode is also unlocked. A convenient
consequence is that whenever a vnode is locked exclusively (VNODE_WRITE),
all corresponding filps are implicitly locked. This is of particular use
when multiple filps must be locked at the same time:
  - When opening a named pipe, VFS must make sure that there is at most one
  filp for the reader end and one filp for the writer end.
  - Pipe readers and writers must be suspended in the absence of (respectively)
  writers and readers.
  - To prevent pipe file sizes to grow too large and wrap, the file size is
  reset to zero when the pipe is empty. This can happen after a read(2).
Because both filps are linked to the same vnode object (they are for the same
pipe), it suffices to exclusively lock that vnode instead of both filp objects.

In some cases it can happen that a function that operates on a locked filp,
calls another function that triggers another lock on a different filp for
the same vnode. For example, close_filp. At some point, close_filp() calls
release() which in turn will loop through the filp table looking for pipes
being select(2)ed on. If there are, the select code will lock the filp and do
operations on it. This works fine when doing a select(2) call, but conflicts
with close(2) or exit(2). Lock_filp() makes an exception for this situation;
if you've already locked a vnode with VNODE_OPCL or VNODE_WRITE when locking
a filp, you obtain a "soft lock" on the vnode for this filp. This means
that lock_filp won't actually try to lock the vnode (which wouldn't work),
but flags the vnode as "skip unlock_vnode upon unlock_filp." Upon unlocking
the filp, the vnode remains locked, the soft lock is removed, and the filp
mutex is released. Note that this scheme does not violate the locking order;
the vnode is (already) locked before the filp.

A similar problem arises with do_pipe. In this case we obtain a new vnode
object, lock it, and obtain two new, locked, filp objects. If everything works
out and the filp objects are linked to the same vnode, we run into trouble
when unlocking both filps. The first filp being unlocked would work; the
second filp doesn't have an associated vnode that's locked anymore. Therefore
we introduced a plural unlock_filps(filp1, filp2) that can unlock two filps
that both point to the same vnode.

4.8 Lock characteristics per request type
For File Servers that support concurrent requests, it's useful to know which
locking guarantees VFS provides for vmnts and vnodes, so it can take that
into account when protecting internal data structures. READ = TLL_READ,
READSER = TLL_READSER, WRITE = TLL_WRITE. The vnode locks applies to the
REQ_INODE_NR field in requests, unless the notes say otherwise.

------------------------------------------------------------------------------
| request      | vmnt    | vnode   | notes                                   |
+--------------+---------+---------+-----------------------------------------+
| REQ_BREAD    |         | READ    | VFS serializes reads from and writes to |
|              |         |         | block special files                     |
+--------------+---------+---------+-----------------------------------------+
| REQ_BWRITE   |         | WRITE   | VFS serializes reads from and writes to |
|              |         |         | block special files                     |
+--------------+---------+---------+-----------------------------------------+
| REQ_CHMOD    | READ    | WRITE   | vmnt is only locked if file is not      |
|              |         |         | already opened                          |
+--------------+---------+---------+-----------------------------------------+
| REQ_CHOWN    | READ    | WRITE   | vmnt is only locked if file is not      |
|              |         |         | already opened                          |
+--------------+---------+---------+-----------------------------------------+
| REQ_CREATE   | WRITE   | WRITE   | The directory in which the file is      |
|              |         |         | created is write locked                 |
+--------------+---------+---------+-----------------------------------------+
| REQ_FLUSH    |         |         | Mutually exclusive to REQ_BREAD and     |
|              |         |         | REQ_BWRITE                              |
+--------------+---------+---------+-----------------------------------------+
| REQ_FSTATFS  |         |         |                                         |
+--------------+---------+---------+-----------------------------------------+
| REQ_FTRUNC   | READ    | WRITE   | vmnt is only locked if file is not      |
|              |         |         | already opened                          |
+--------------+---------+---------+-----------------------------------------+
| REQ_GETDENTS | READ    | READ    | vmnt is only locked if file is not      |
|              |         |         | already opened                          |
+--------------+---------+---------+-----------------------------------------+
| REQ_INHIBREAD|         | READ    |                                         |
+--------------+---------+---------+-----------------------------------------+
| REQ_LINK     | READSER | WRITE   | REQ_INODE_NR is locked READ             |
|              |         |         | REQ_DIR_INO is locked WRITE             |
+--------------+---------+---------+-----------------------------------------+
| REQ_LOOKUP   | READSER |         |                                         |
+--------------+---------+---------+-----------------------------------------+
| REQ_MKDIR    | READSER | WRITE   |                                         |
+--------------+---------+---------+-----------------------------------------+
| REQ_MKNOD    | READSER | WRITE   |                                         |
+--------------+---------+---------+-----------------------------------------+
|REQ_MOUNTPOINT| WRITE   | WRITE   |                                         |
+--------------+---------+---------+-----------------------------------------+
|REQ_NEW_DRIVER|         |         |                                         |
+--------------+---------+---------+-----------------------------------------+
| REQ_NEWNODE  |         |         | Only sent to PFS                        |
+--------------+---------+---------+-----------------------------------------+
| REQ_PUTNODE  |         | READSER | READSER when dropping all but one       |
|              |         | or WRITE| references. WRITE when final reference  |
|              |         |         | is dropped (i.e., no longer in use)     |
+--------------+---------+---------+-----------------------------------------+
| REQ_RDLINK   | READ    | READ    | In some circumstances stricter locking  |
|              |         |         | might be applied, but not guaranteed    |
+--------------+---------+---------+-----------------------------------------+
| REQ_READ     |         | READ    |                                         |
+--------------+---------+---------+-----------------------------------------+
|REQ_READSUPER | WRITE   |         |                                         |
+--------------+---------+---------+-----------------------------------------+
| REQ_RENAME   | WRITE   | WRITE   |                                         |
+--------------+---------+---------+-----------------------------------------+
| REQ_RMDIR    | WRITE   | WRITE   |                                         |
+--------------+---------+---------+-----------------------------------------+
| REQ_SLINK    | READSER | READ    |                                         |
+--------------+---------+---------+-----------------------------------------+
| REQ_STAT     | READ    | READ    | vmnt is only locked if file is not      |
|              |         |         | already opened                          |
+--------------+---------+---------+-----------------------------------------+
| REQ_STATVFS  | READ    | READ    | vmnt is only locked if file is not      |
|              |         |         | already opened                          |
+--------------+---------+---------+-----------------------------------------+
| REQ_SYNC     | READ    |         |                                         |
+--------------+---------+---------+-----------------------------------------+
| REQ_UNLINK   | WRITE   | WRITE   |                                         |
+--------------+---------+---------+-----------------------------------------+
| REQ_UNMOUNT  | WRITE   |         |                                         |
+--------------+---------+---------+-----------------------------------------+
| REQ_UTIME    | READ    | READ    |                                         |
+--------------+---------+---------+-----------------------------------------+
| REQ_WRITE    |         | WRITE   |                                         |
-----------------------------------------------------------------------------+
Table 7: VFS-FS requests locking guarantees

5 Recovery from driver crashes
VFS can recover from block special file and character special file driver
crashes. It can recover to some degree from a crashed File Server (which we
can regard as a driver).

5.1 Recovery from block drivers crashes
When reading or writing, VFS doesn't communicate with block drivers directly,
but always through a File Server (the root File Server being default). If the
block driver crashes, the File Server does most of the work of the recovery
procedure. VFS loops through all open files for block special files that
were handled by this driver and reopens them. After that it sends the new
endpoint to the File Server so it can finish the recover procedure. Finally,
the File Server will retry pending requests if possible. However, reopening
files can cause the block driver to crash again. When that happens, VFS will
stop the recovery. A driver can return ERESTART to VFS to tell it to retry
a request. VFS does this with an arbitrary maximum of 5 attempts.

5.2 Recovery from character driver crashes
Character special files are treated differently. Once VFS has found out a
driver has been restarted, it will stop the current request (if there is
any). It makes no sense to retry requests due to the nature of character
special files. If a character special driver can restart without changing
endpoints, this merely results in the current request (e.g., read, write, or
ioctl) failing and allows the user process to reissue the same request. On
the other hand, if a driver restart causes the driver to change endpoint
number, all associated file descriptors are marked invalid and subsequent
operations on them will always fail with a bad file descriptor error.

5.3 Recovery from File Server crashes
At the time of writing we cannot recover from crashed File Servers. When
VFS detects it has to clean up the remnants of a File Server process (i.e.,
through an exit(2)), it marks all associated file descriptors as invalid
and cancels ongoing and pending requests to that File Server. Resources that
were in use by the File Server are cleaned up.

[0] http://wiki.minix3.org/en/DevelopersGuide/VfsFsProtocol
[1] http://www.cs.vu.nl/~dcvmoole/minix/blockchar.txt
[2] http://www.minix3.org/theses/moolenbroek-multimedia-support.pdf
