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locking — kernel synchronization primitives
The 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.
Mutexes (also called "sleep mutexes") are the most commonly used
synchronization primitive in the kernel. 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 sleep. Mutexes fully support
See mutex(9) for details.
Spin mutexes are variation of basic mutexes; the main difference between
the two is that spin mutexes never sleep - instead, they spin, waiting
for the thread holding the lock, which runs on another CPU, to release
it. Differently from ordinary mutex, spin mutexes disable interrupts
when acquired. Since disabling interrupts is expensive, they are also
generally slower. Spin mutexes should be used only when neccessary, e.g.
to protect data shared with interrupt filter code (see bus_setup_intr(9)
With most synchronisaton 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.
See mtx_pool(9) for details.
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 readers since they should only read the protected
data. A thread with exclusive access is known as a writer since it may
modify protected data.
Reader/writer locks can be treated as mutexes (see above and mutex(9))
with shared/exclusive semantics. More specifically, regular mutexes can
be considered to be equivalent to a write-lock on an rw_lock. The 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 rw_lock can recurse, but exclusive locks are not
allowed to recurse. This ability should not be used lightly and may go
See rwlock(9) for details.
Mostly reader locks are similar to reader/writer locks but optimized for
very infrequent write locking. Read-mostly locks implement full priority
propagation by tracking shared owners using a caller-supplied tracker
See rmlock(9) for details.
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 sleep(9). In fact it could in some
cases be considered a conditional sleep.
See sx(9) for details.
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
See sema(9) for details.
Condition variables are used in conjunction with mutexes to wait for
conditions to occur. A thread must hold the mutex before calling the
cv_wait*(), functions. When a thread waits on a condition, the mutex is
atomically released before the thread is blocked, then reacquired before
the function call returns.
See condvar(9) for details.
Giant is an instance of a mutex, with some special characteristics:
1. It is recursive.
2. 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.
3. Giant must be locked first before other locks.
4. It is OK to hold Giant while performing unbounded sleep; in such
case, Giant will be dropped before sleeping and picked up after
5. 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.
The functions tsleep(), msleep(), msleep_spin(), pause(), wakeup(), and
wakeup_one() handle event-based thread blocking. If a thread must wait
for an external event, it is put to sleep by tsleep(), msleep(),
msleep_spin(), or pause(). Threads may also wait using one of the
locking primitive sleep routines mtx_sleep(9), rw_sleep(9), or
The parameter 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 chan are woken up later by wakeup(), often called from inside an
interrupt routine, to indicate that the resource the thread was blocking
on is available now.
Several of the sleep functions including msleep(), 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 priority includes the 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 Giant
mutex (even if recursed) while the thread is suspended and will reacquire
the Giant mutex before the function returns.
See sleep(9) for details.
Shared/exclusive locks, used mostly in VFS(9), in particular as a
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.
See lock(9) for details.
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
Bounded vs. unbounded sleep
The following primitives perform bounded sleep: mutexes, pool mutexes,
reader/writer locks and read-mostly locks.
The following primitives block (perform unbounded sleep):
shared/exclusive locks, counting semaphores, condition variables,
sleep/wakeup and lockmanager locks.
It is an error to do any operation that could result in any kind of sleep
while holding spin mutex.
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.
As a special case, it is possible to call sleep() or 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 mtx_sleep() is being called and assumptions the
calling code has made. Because the lock gets dropped during sleep, one
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.
It is an error to do any operation that could result in any kind of sleep
when running inside an interrupt filter.
It is an error to do any operation that could result in unbounded sleep
when running inside an interrupt thread.
The following table shows what you can and can not do while holding one
of the synchronization primitives discussed:
You have: You want: spin mtx mutex sx rwlock rmlock sleep
spin mtx ok-1 no no no no no-3
mutex ok ok-1 no ok ok no-3
sx ok ok ok-2 ok ok ok-4
rwlock ok ok no ok-2 ok no-3
rmlock ok ok no ok ok-2 no
*1 Recursion is defined per lock. Lock order is important.
*2 Readers can recurse though writers can not. Lock order is important.
*3 There are calls that atomically release this primitive when going to
sleep and reacquire it on wakeup (e.g. mtx_sleep(), rw_sleep() and
*4 Though one can sleep holding an sx lock, one can also use sx_sleep()
which will atomically release this primitive when going to sleep and
reacquire it on wakeup.
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.
Context: spin mtx mutex sx rwlock rmlock sleep
interrupt filter: ok no no no no no
ithread: ok ok no ok ok no
callout: ok ok no ok no no
syscall: ok ok ok ok ok ok
witness(4), condvar(9), lock(9), mtx_pool(9), mutex(9), rmlock(9),
rwlock(9), sema(9), sleep(9), sx(9), LOCK_PROFILING(9)
These functions appeared in BSD/OS 4.1 through FreeBSD 7.0
There are too many locking primitives to choose from.