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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.
Debian July 5, 2015 LOCKING(9)