man/man9/locking.9

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.Dd May 25, 2012
.Dt LOCKING 9
.Os
.Sh NAME
.Nm locking
.Nd kernel synchronization primitives
.Sh DESCRIPTION
The
.Em FreeBSD
kernel is written to run across multiple CPUs and as such requires
several different synchronization primitives to allow the developers
to safely access and manipulate the many data types required.
.Ss Mutexes
Mutexes (also erroneously called "sleep mutexes") are the most commonly used
synchronization primitive in the kernel.
A thread acquires (locks) a mutex before accessing data shared with other
threads (including interrupt threads), and releases (unlocks) it afterwards.
If the mutex cannot be acquired, the thread requesting it will wait.
Mutexes are by default adaptive, meaning that
if the owner of a contended mutex is currently running on another CPU,
then a thread attempting to acquire the mutex will briefly spin
in the hope that the owner is only briefly holding it,
and might release it shortly.
If the owner does not do so, the waiting thread proceeds to yield the processor,
allowing other threads to run.
If the owner is not currently actually running then the spin step is skipped.
Mutexes fully support priority propagation.
.Pp
See
.Xr mutex 9
for details.
.Ss Spin mutexes
Spin mutexes are variation of basic mutexes; the main difference between
the two is that spin mutexes never yield the processor - instead, they spin,
waiting for the thread holding the lock,
(which must be running on another CPU), to release it.
Spin mutexes disable interrupts while the held so as to not get pre-empted.
Since disabling interrupts is expensive, they are also generally slower.
Spin mutexes should be used only when necessary, e.g. to protect data shared
with interrupt filter code (see
.Xr bus_setup_intr 9
for details).
.Ss Pool mutexes
With most synchronization primitives, such as mutexes, programmer must
provide a piece of allocated memory to hold the primitive.
For example, a mutex may be embedded inside the structure it protects.
Pool mutex is a variant of mutex without this requirement - to lock or unlock
a pool mutex, one uses address of the structure being protected with it,
not the mutex itself.
Pool mutexes are seldom used.
.Pp
See
.Xr mtx_pool 9
for details.
.Ss Reader/writer locks
Reader/writer locks allow shared access to protected data by multiple threads,
or exclusive access by a single thread.
The threads with shared access are known as
.Em readers
since they should only read the protected data.
A thread with exclusive access is known as a
.Em writer
since it may modify protected data.
.Pp
Reader/writer locks can be treated as mutexes (see above and
.Xr mutex 9 )
with shared/exclusive semantics.
More specifically, regular mutexes can be
considered to be equivalent to a write-lock on an
.Em rw_lock.
The
.Em rw_lock
locks have priority propagation like mutexes, but priority
can be propagated only to an exclusive holder.
This limitation comes from the fact that shared owners
are anonymous.
Another important property is that shared holders of
.Em rw_lock
can recurse, but exclusive locks are not allowed to recurse.
This ability should not be used lightly and
.Em may go away.
.Pp
See
.Xr rwlock 9
for details.
.Ss Read-mostly locks
Mostly reader locks are similar to
.Em reader/writer
locks but optimized for very infrequent write locking.
.Em Read-mostly
locks implement full priority propagation by tracking shared owners
using a caller-supplied
.Em tracker
data structure.
.Pp
See
.Xr rmlock 9
for details.
.Ss Shared/exclusive locks
Shared/exclusive locks are similar to reader/writer locks; the main difference
between them is that shared/exclusive locks may be held during unbounded sleep
(and may thus perform an unbounded sleep).
They are inherently less efficient than mutexes, reader/writer locks
and read-mostly locks.
They don't support priority propagation.
They should be considered to be closely related to
.Xr sleep 9 .
They could in some cases be
considered a conditional sleep.
.Pp
See
.Xr sx 9
for details.
.Ss Counting semaphores
Counting semaphores provide a mechanism for synchronizing access
to a pool of resources.
Unlike mutexes, semaphores do not have the concept of an owner,
so they can be useful in situations where one thread needs
to acquire a resource, and another thread needs to release it.
They are largely deprecated.
.Pp
See
.Xr sema 9
for details.
.Ss Condition variables
Condition variables are used in conjunction with mutexes to wait for
conditions to occur.
A thread must hold the mutex before calling the
.Fn cv_wait* ,
functions.
When a thread waits on a condition, the mutex
is atomically released before the thread yields the processor,
then reacquired before the function call returns.
.Pp
See
.Xr condvar 9
for details.
.Ss Giant
Giant is an instance of a mutex, with some special characteristics:
.Bl -enum
.It
It is recursive.
.It
Drivers and filesystems can request that Giant be locked around them
by not marking themselves MPSAFE.
Note that infrastructure to do this is slowly going away as non-MPSAFE
drivers either became properly locked or disappear.
.It
Giant must be locked first before other locks.
.It
It is OK to hold Giant while performing unbounded sleep; in such case,
Giant will be dropped before sleeping and picked up after wakeup.
.It
There are places in the kernel that drop Giant and pick it back up
again.
Sleep locks will do this before sleeping.
Parts of the network or VM code may do this as well, depending on the
setting of a sysctl.
This means that you cannot count on Giant keeping other code from
running if your code sleeps, even if you want it to.
.El
.Ss Sleep/wakeup
The functions
.Fn tsleep ,
.Fn msleep ,
.Fn msleep_spin ,
.Fn pause ,
.Fn wakeup ,
and
.Fn wakeup_one
handle event-based thread blocking.
If a thread must wait for an external event, it is put to sleep by
.Fn tsleep ,
.Fn msleep ,
.Fn msleep_spin ,
or
.Fn pause .
Threads may also wait using one of the locking primitive sleep routines
.Xr mtx_sleep 9 ,
.Xr rw_sleep 9 ,
or
.Xr sx_sleep 9 .
.Pp
The parameter
.Fa chan
is an arbitrary address that uniquely identifies the event on which
the thread is being put to sleep.
All threads sleeping on a single
.Fa chan
are woken up later by
.Fn wakeup ,
often called from inside an interrupt routine, to indicate that the
resource the thread was blocking on is available now.
.Pp
Several of the sleep functions including
.Fn msleep ,
.Fn msleep_spin ,
and the locking primitive sleep routines specify an additional lock
parameter.
The lock will be released before sleeping and reacquired
before the sleep routine returns.
If
.Fa priority
includes the
.Dv PDROP
flag, then the lock will not be reacquired before returning.
The lock is used to ensure that a condition can be checked atomically,
and that the current thread can be suspended without missing a
change to the condition, or an associated wakeup.
In addition, all of the sleep routines will fully drop the
.Va Giant
mutex
(even if recursed)
while the thread is suspended and will reacquire the
.Va Giant
mutex before the function returns.
.Pp
See
.Xr sleep 9
for details.
.Ss Lockmanager locks
Shared/exclusive locks, used mostly in
.Xr VFS 9 ,
in particular as a
.Xr vnode 9
lock.
They have features other lock types don't have, such as sleep timeout,
writer starvation avoidance, draining, and interlock mutex, but this makes them
complicated to implement; for this reason, they are deprecated.
.Pp
See
.Xr lock 9
for details.
.Sh INTERACTIONS
The primitives interact and have a number of rules regarding how
they can and can not be combined.
Many of these rules are checked using the
.Xr witness 4
code.
.Ss Bounded vs. unbounded sleep
The following primitives perform bounded sleep:
 mutexes, pool mutexes, reader/writer locks and read-mostly locks.
.Pp
The following primitives may perform an unbounded sleep:
shared/exclusive locks, counting semaphores, condition variables, sleep/wakeup and lockmanager locks.
.Pp
It is an error to do any operation that could result in yielding the processor
while holding a spin mutex.
.Pp
As a general rule, it is an error to do any operation that could result
in unbounded sleep while holding any primitive from the 'bounded sleep' group.
For example, it is an error to try to acquire shared/exclusive lock while
holding mutex, or to try to allocate memory with M_WAITOK while holding
read-write lock.
.Pp
As a special case, it is possible to call
.Fn sleep
or
.Fn mtx_sleep
while holding a single mutex.
It will atomically drop that mutex and reacquire it as part of waking up.
This is often a bad idea because it generally relies on the programmer having
good knowledge of all of the call graph above the place where
.Fn mtx_sleep
is being called and assumptions the calling code has made.
Because the lock gets dropped during sleep, one must re-test all
the assumptions that were made before, all the way up the call graph to the
place where the lock was acquired.
.Pp
It is an error to do any operation that could result in yielding of
the processor when running inside an interrupt filter.
.Pp
It is an error to do any operation that could result in unbounded sleep when
running inside an interrupt thread.
.Ss Interaction table
The following table shows what you can and can not do while holding
one of the synchronization primitives discussed:
.Bl -column ".Ic xxxxxxxxxxxxxxxx" ".Xr XXXXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXX" -offset indent
.It Em "       You want:" Ta spin-mtx Ta mutex Ta rwlock Ta rmlock Ta sx Ta sleep
.It Em "You have:     " Ta ------ Ta ------ Ta ------ Ta ------ Ta ------ Ta ------
.It spin mtx  Ta \&ok-1 Ta \&no Ta \&no Ta \&no Ta \&no Ta \&no-3
.It mutex     Ta \&ok Ta \&ok-1 Ta \&ok Ta \&ok Ta \&no Ta \&no-3
.It rwlock    Ta \&ok Ta \&ok Ta \&ok-2 Ta \&ok Ta \&no Ta \&no-3
.It rmlock    Ta \&ok Ta \&ok Ta \&ok Ta \&ok-2 Ta \&no-5 Ta \&no-5
.It sx        Ta \&ok Ta \&ok Ta \&ok Ta \&ok Ta \&no-2 Ta \&ok-4
.El
.Pp
.Em *1
Recursion is defined per lock.
Lock order is important.
.Pp
.Em *2
Readers can recurse though writers can not.
Lock order is important.
.Pp
.Em *3
There are calls that atomically release this primitive when going to sleep
and reacquire it on wakeup (e.g.
.Fn mtx_sleep ,
.Fn rw_sleep
and
.Fn msleep_spin ) .
.Pp
.Em *4
Though one can sleep holding an sx lock, one can also use
.Fn sx_sleep
which will atomically release this primitive when going to sleep and
reacquire it on wakeup.
.Pp
.Em *5
.Em Read-mostly
locks can be initialized to support sleeping while holding a write lock.
See
.Xr rmlock 9
for details.
.Ss Context mode table
The next table shows what can be used in different contexts.
At this time this is a rather easy to remember table.
.Bl -column ".Ic Xxxxxxxxxxxxxxxxxxx" ".Xr XXXXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXXX" ".Xr XXXXXX" -offset indent
.It Em "Context:"  Ta spin mtx Ta mutex Ta sx Ta rwlock Ta rmlock Ta sleep
.It interrupt filter:  Ta \&ok Ta \&no Ta \&no Ta \&no Ta \&no Ta \&no
.It interrupt thread:  Ta \&ok Ta \&ok Ta \&no Ta \&ok Ta \&ok Ta \&no
.It callout:    Ta \&ok Ta \&ok Ta \&no Ta \&ok Ta \&no Ta \&no
.It syscall:    Ta \&ok Ta \&ok Ta \&ok Ta \&ok Ta \&ok Ta \&ok
.El
.Sh SEE ALSO
.Xr witness 4 ,
.Xr condvar 9 ,
.Xr lock 9 ,
.Xr mtx_pool 9 ,
.Xr mutex 9 ,
.Xr rmlock 9 ,
.Xr rwlock 9 ,
.Xr sema 9 ,
.Xr sleep 9 ,
.Xr sx 9 ,
.Xr BUS_SETUP_INTR 9 ,
.Xr LOCK_PROFILING 9
.Sh HISTORY
These
functions appeared in
.Bsx 4.1
through
.Fx 7.0 .
.Sh BUGS
There are too many locking primitives to choose from.