LOCKING(9) FreeBSD Kernel Developer's Manual LOCKING(9)
NAME
locking - kernel synchronization primitives
DESCRIPTION
The FreeBSD kernel is written to run across multiple CPUs and as such
provides several different synchronization primitives to allow developers
to safely access and manipulate many data types.
Mutexes
Mutexes (also called "blocking 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
adaptive by default, meaning that if the owner of a contended mutex is
currently running on another CPU, then a thread attempting to acquire the
mutex will spin rather than yielding the processor. Mutexes fully
support priority propagation.
See mutex(9) for details.
Spin Mutexes
Spin mutexes are a variation of basic mutexes; the main difference
between the two is that spin mutexes never block. Instead, they spin
while waiting for the lock to be released. To avoid deadlock, a thread
that holds a spin mutex must never yield its CPU. Unlike ordinary
mutexes, spin mutexes disable interrupts when acquired. Since disabling
interrupts can be expensive, they are generally slower to acquire and
release. Spin mutexes should be used only when absolutely necessary,
e.g. to protect data shared with interrupt filter code (see
bus_setup_intr(9) for details), or for scheduler internals.
Mutex Pools
With most synchronization primitives, such as mutexes, the programmer
must provide memory to hold the primitive. For example, a mutex may be
embedded inside the structure it protects. Mutex pools provide a
preallocated set of mutexes to avoid this requirement. Note that mutexes
from a pool may only be used as leaf locks.
See mtx_pool(9) for details.
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 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. Reader/writer locks support priority
propagation like mutexes, but priority is propagated only to an exclusive
holder. This limitation comes from the fact that shared owners are
anonymous.
See rwlock(9) for details.
Read-Mostly Locks
Read-mostly 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
data structure.
See rmlock(9) for details.
Sleepable Read-Mostly Locks
Sleepable read-mostly locks are a variation on read-mostly locks.
Threads holding an exclusive lock may sleep, but threads holding a shared
lock may not. Priority is propagated to shared owners but not to
exclusive owners.
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. Acquiring a contested shared/exclusive lock can perform
an unbounded sleep. These locks do not support priority propagation.
See sx(9) for details.
Lockmanager locks
Lockmanager locks are sleepable shared/exclusive locks used mostly in
VFS(9) (as a vnode(9) lock) and in the buffer cache (BUF_LOCK(9)). They
have features other lock types do not have such as sleep timeouts,
blocking upgrades, writer starvation avoidance, draining, and an
interlock mutex, but this makes them complicated both to use and to
implement; for this reason, they should be avoided.
See lock(9) for details.
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.
See sema(9) for details.
Condition variables
Condition variables are used in conjunction with locks to wait for a
condition to become true. A thread must hold the associated lock before
calling one of the cv_wait(), functions. When a thread waits on a
condition, the lock is atomically released before the thread yields the
processor and reacquired before the function call returns. Condition
variables may be used with blocking mutexes, reader/writer locks, read-
mostly locks, and shared/exclusive locks.
See condvar(9) for details.
Sleep/Wakeup
The functions tsleep(), msleep(), msleep_spin(), pause(), wakeup(), and
wakeup_one() also handle event-based thread blocking. Unlike condition
variables, arbitrary addresses may be used as wait channels and a
dedicated structure does not need to be allocated. However, care must be
taken to ensure that wait channel addresses are unique to an event. 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
sx_sleep(9).
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 event the thread was blocking on
has occurred.
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 (restoring any recursion) before the function returns.
The pause() function is a special sleep function that waits for a
specified amount of time to pass before the thread resumes execution.
This sleep cannot be terminated early by either an explicit wakeup() or a
signal.
See sleep(9) for details.
Giant
Giant is a special mutex used to protect data structures that do not yet
have their own locks. Since it provides semantics akin to the old spl(9)
interface, Giant has special characteristics:
1. It is recursive.
2. Drivers 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 before other non-sleepable locks.
4. Giant is dropped during unbounded sleeps and reacquired after
wakeup.
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. This means that you cannot
count on Giant keeping other code from running if your code sleeps,
even if you want it to.
INTERACTIONS
The primitives can interact and have a number of rules regarding how they
can and can not be combined. Many of these rules are checked by
witness(4).
Bounded vs. Unbounded Sleep
In a bounded sleep (also referred to as "blocking") the only resource
needed to resume execution of a thread is CPU time for the owner of a
lock that the thread is waiting to acquire. In an unbounded sleep (often
referred to as simply "sleeping") a thread waits for an external event or
for a condition to become true. In particular, a dependency chain of
threads in bounded sleeps should always make forward progress, since
there is always CPU time available. This requires that no thread in a
bounded sleep is waiting for a lock held by a thread in an unbounded
sleep. To avoid priority inversions, a thread in a bounded sleep lends
its priority to the owner of the lock that it is waiting for.
The following primitives perform bounded sleeps: mutexes, reader/writer
locks and read-mostly locks.
The following primitives perform unbounded sleeps: sleepable read-mostly
locks, shared/exclusive locks, lockmanager locks, counting semaphores,
condition variables, and sleep/wakeup.
General Principles
• It is an error to do any operation that could result in yielding the
processor while holding a spin mutex.
• 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 a shared/exclusive lock
while holding a mutex, or to try to allocate memory with M_WAITOK
while holding a reader/writer lock.
Note that the lock passed to one of the sleep() or cv_wait()
functions is dropped before the thread enters the unbounded sleep and
does not violate this rule.
• It is an error to do any operation that could result in yielding of
the processor 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.
Interaction table
The following table shows what you can and can not do while holding one
of the locking primitives discussed. Note that "sleep" includes
sema_wait(), sema_timedwait(), any of the cv_wait() functions, and any of
the sleep() functions.
You want: spin mtx mutex/rw rmlock sleep rm sx/lk sleep
You have: -------- -------- ------ -------- ------ ------
spin mtx ok no no no no no-1
mutex/rw ok ok ok no no no-1
rmlock ok ok ok no no no-1
sleep rm ok ok ok ok-2 ok-2 ok-2/3
sx ok ok ok ok ok ok-3
lockmgr ok ok ok ok ok ok
*1 There are calls that atomically release this primitive when going to
sleep and reacquire it on wakeup (mtx_sleep(), rw_sleep(), msleep_spin(),
etc.).
*2 These cases are only allowed while holding a write lock on a sleepable
read-mostly lock.
*3 Though one can sleep while holding this lock, one can also use a
sleep() function to atomically release this primitive when going to sleep
and reacquire it on wakeup.
Note that non-blocking try operations on locks are always permitted.
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/rw rmlock sleep
rm sx/lk sleep
interrupt filter: ok no no no no no
interrupt thread: ok ok ok no no no
callout: ok ok ok no no no
direct callout: ok no no no no no
system call: ok ok ok ok ok ok
SEE ALSO
witness(4), BUS_SETUP_INTR(9), condvar(9), lock(9), LOCK_PROFILING(9),
mtx_pool(9), mutex(9), rmlock(9), rwlock(9), sema(9), sleep(9), sx(9),
timeout(9)
HISTORY
These functions appeared in BSD/OS 4.1 through FreeBSD 7.0.
BUGS
There are too many locking primitives to choose from.
FreeBSD 13.1-RELEASE-p6 July 5, 2015 FreeBSD 13.1-RELEASE-p6
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