mutex(9)

NAME

mutex, mtx_init, mtx_lock, mtx_lock_spin, mtx_lock_flags, mtx_lock_spin_flags, mtx_trylock, mtx_trylock_flags,
mtx_unlock,
mtx_unlock_spin, mtx_unlock_flags, mtx_unlock_spin_flags,
mtx_destroy,
mtx_initialized, mtx_owned, mtx_recursed, mtx_assert,

MTX_SYSINIT

nel synchronization primitives

SYNOPSIS

#include <sys/param.h>
#include <sys/lock.h>
#include <sys/mutex.h>
void
mtx_init(struct  mtx  *mutex,  const  char *name, const char
*type,
        int opts);
void
mtx_lock(struct mtx *mutex);
void
mtx_lock_spin(struct mtx *mutex);
void
mtx_lock_flags(struct mtx *mutex, int flags);
void
mtx_lock_spin_flags(struct mtx *mutex, int flags);
int
mtx_trylock(struct mtx *mutex);
int
mtx_trylock_flags(struct mtx *mutex, int flags);
void
mtx_unlock(struct mtx *mutex);
void
mtx_unlock_spin(struct mtx *mutex);
void
mtx_unlock_flags(struct mtx *mutex, int flags);
void
mtx_unlock_spin_flags(struct mtx *mutex, int flags);
void
mtx_destroy(struct mtx *mutex);
int
mtx_initialized(struct mtx *mutex);
int
mtx_owned(struct mtx *mutex);
int
mtx_recursed(struct mtx *mutex);
options INVARIANTS
options INVARIANT_SUPPORT
void
mtx_assert(struct mtx *mutex, int what);
#include <sys/kernel.h>
MTX_SYSINIT(name,   struct   mutex    *mtx,    const    char
*description, int opts);

DESCRIPTION

Mutexes are the most basic and primary method of thread syn
chronization. The major design considerations for mutexes are:
1. Acquiring and releasing uncontested mutexes should be
as cheap as possible.
2. They must have the information and storage space to
support priority propagation.
3. A thread must be able to recursively acquire a mutex,
provided that the mutex is initialized to support recursion.
There are currently two flavors of mutexes, those that con
text switch when they block and those that do not.
By default, MTX_DEF mutexes will context switch when they
are already held. As an optimization, they may spin for some amount of
time before context switching. It is important to remember that since a
thread may be preempted at any time, the possible context switch intro
duced by acquiring a mutex is guaranteed to not break anything that
is not already broken.
Mutexes which do not context switch are MTX_SPIN mutexes.
These should only be used to protect data shared with primary interrupt
code. This includes INTR_FAST interrupt handlers and low level schedul
ing code. In all architectures both acquiring and releasing of a uncon
tested spin mutex is more expensive than the same operation on a non
spin mutex. In order to protect an interrupt service routine from blocking
against itself all interrupts are either blocked or deferred on a
processor while holding a spin lock. It is permissible to hold multiple
spin mutexes.
Once a spin mutex has been acquired it is not permissible to
acquire a blocking mutex.
The storage needed to implement a mutex is provided by a
struct mtx. In general this should be treated as an opaque object and ref
erenced only with the mutex primitives.
The mtx_init() function must be used to initialize a mutex
before it can be passed to any of the other mutex functions. The name op
tion is used to identify the lock in debugging output etc. The type op
tion is used by the witness code to classify a mutex when doing checks of
lock ordering. If type is NULL, name is used in its place. The pointer
passed in as name and type is saved rather than the data it points to.
The data pointed to must remain stable until the mutex is destroyed.
The opts argument is used to set the type of mutex. It may contain
either MTX_DEF or MTX_SPIN but not both. See below for additional initial
ization options. It is not permissible to pass the same mutex to
mtx_init() multiple times without intervening calls to mtx_destroy().
The mtx_lock() function acquires a MTX_DEF mutual exclusion
lock on behalf of the currently running kernel thread. If another
kernel thread is holding the mutex, the caller will be disconnected from
the CPU until the mutex is available (i.e., it will block).
The mtx_lock_spin() function acquires a MTX_SPIN mutual ex
clusion lock on behalf of the currently running kernel thread. If another
kernel thread is holding the mutex, the caller will spin until the mutex
becomes available. Interrupts are disabled during the spin and remain
disabled following the acquiring of the lock.
It is possible for the same thread to recursively acquire a
mutex with no ill effects, provided that the MTX_RECURSE bit was passed to
mtx_init()
during the initialization of the mutex.
The mtx_lock_flags() and mtx_lock_spin_flags() functions ac
quire a MTX_DEF or MTX_SPIN lock, respectively, and also accept a
flags argument. In both cases, the only flag presently available for lock
acquires is MTX_QUIET. If the MTX_QUIET bit is turned on in the flags
argument, then if KTR_LOCK tracing is being done, it will be silenced dur
ing the lock acquire.
The mtx_trylock() attempts to acquire the MTX_DEF mutex
pointed to by mutex. If the mutex cannot be immediately acquired
mtx_trylock() will
return 0, otherwise the mutex will be acquired and a non-ze
ro value will be returned.
The mtx_trylock_flags() function has the same behavior as
mtx_trylock()
but should be used when the caller desires to pass in a
flags value.
Presently, the only valid value in the mtx_trylock() case is
MTX_QUIET, and its effects are identical to those described for
mtx_lock() above.
The mtx_unlock() function releases a MTX_DEF mutual exclu
sion lock. The current thread may be preempted if a higher priority thread
is waiting for the mutex.
The mtx_unlock_spin() function releases a MTX_SPIN mutual
exclusion lock.
The mtx_unlock_flags() and mtx_unlock_spin_flags() functions
behave in exactly the same way as do the standard mutex unlock rou
tines above, while also allowing a flags argument which may specify
MTX_QUIET. The behavior of MTX_QUIET is identical to its behavior in the
mutex lock routines.
The mtx_destroy() function is used to destroy mutex so the
data associated with it may be freed or otherwise overwritten. Any mu
tex which is destroyed must previously have been initialized with
mtx_init(). It is
permissible to have a single hold count on a mutex when it
is destroyed. It is not permissible to hold the mutex recursively, or have
another thread blocked on the mutex when it is destroyed.
The mtx_initialized() function returns non-zero if mutex has
been initialized and zero otherwise.
The mtx_owned() function returns non-zero if the current
thread holds mutex. If the current thread does not hold mutex zero is
returned.
The mtx_recursed() function returns non-zero if the mutex is
recursed. This check should only be made if the running thread already
owns mutex.
The mtx_assert() function allows assertions specified in
what to be made about mutex. If the assertions are not true and the kernel
is compiled
with options INVARIANTS and options INVARIANT_SUPPORT, the
kernel will panic. Currently the following assertions are supported:
MA_OWNED Assert that the current thread holds the mu
tex pointed to by the first argument.
MA_NOTOWNED Assert that the current thread does not hold
the mutex pointed to by the first argument.
MA_RECURSED Assert that the current thread has recursed
on the mutexpointed to by the first argument. This as
sertion is only valid in conjunction with MA_OWNED.
MA_NOTRECURSED Assert that the current thread has not re
cursed on the mutex pointed to by the first argument.
This assertion is only valid in conjunction with MA_OWNED.
The MTX_SYSINIT() macro is used to generate a call to the
mtx_sysinit()
routine at system startup in order to initialize a given mu
tex lock. The
parameters are the same as mtx_init() but with an additional
argument, name, that is used in generating unique variable names for
the related structures associated with the lock and the sysinit routine.
The Default Mutex Type
Most kernel code should use the default lock type, MTX_DEF.
The default lock type will allow the thread to be disconnected from the
CPU if the lock is already held by another thread. The implementation
may treat the lock as a short term spin lock under some circumstances.
However, it is always safe to use these forms of locks in an interrupt
thread without fear of deadlock against an interrupted thread on the same
CPU.
The Spin Mutex Type
A MTX_SPIN mutex will not relinquish the CPU when it cannot
immediately get the requested lock, but will loop, waiting for the mutex
to be released by another CPU. This could result in deadlock if
another thread interrupted the thread which held a mutex and then tried to
acquire the mutex. For this reason spin locks disable all interrupts on
the local CPU.
Spin locks are fairly specialized locks that are intended to
be held for very short periods of time. Their primary purpose is to
protect portions of the code that implement other synchronization primitives
such as default mutexes, thread scheduling, and interrupt threads.
Initialization OptionsThe options passed in the opts argument of mtx_init() speci
fy the mutex type. One of the MTX_DEF or MTX_SPIN options is required
and only one of those two options may be specified. The possibilities are:
MTX_DEF Default mutexes will always allow the current
thread to be suspended to avoid deadlock conditions
against interrupt threads. The implementation of this lock
type may spin for a while before suspending the current
thread.
MTX_SPIN Spin mutexes will never relinquish the CPU.
All inter rupts are disabled on the local CPU while any
spin lock is held.
MTX_RECURSE Specifies that the initialized mutex is al
lowed torecurse. This bit must be present if the mu
tex is permitted to recurse.
MTX_QUIET Do not log any mutex operations for this
lock.
MTX_NOWITNESS Instruct witness(4) to ignore this lock.
MTX_DUPOK Witness should not log messages about dupli
cate locks being acquired.
Lock and Unlock Flags
The flags passed to the mtx_lock_flags(),
mtx_lock_spin_flags(),
mtx_unlock_flags(), and mtx_unlock_spin_flags() functions
provide some basic options to the caller, and are often used only under
special circumstances to modify lock or unlock behavior. Standard
locking and
unlocking should be performed with the mtx_lock(),
mtx_lock_spin(),
mtx_unlock(), and mtx_unlock_spin() functions. Only if a
flag is required should the corresponding flags-accepting routines
be used.
Options that modify mutex behavior:
MTX_QUIET This option is used to quiet logging messages
during individ ual mutex operations. This can be used to trim
superfluous logging messages for debugging purposes.
GiantIf Giant must be acquired, it must be acquired prior to ac
quiring other mutexes. Put another way: it is impossible to acquire Giant
non-recursively while holding another mutex. It is possible to ac
quire other mutexes while holding Giant, and it is possible to acquire
Giant recursively while holding other mutexes.
Sleeping
Sleeping while holding a mutex (except for Giant) is never
safe and should be avoided. There are numerous assertions which will
fail if this is attempted.
Functions Which Access Memory in UserspaceNo mutexes should be held (except for Giant) across func
tions which access memory in userspace, such as copyin(9), copyout(9),
uiomove(9), fuword(9), etc. No locks are needed when calling these
functions.

HISTORY

These functions appeared in BSD/OS 4.1 and FreeBSD 5.0.
BSD February 16, 2005

docs.sk

comprehensive documentation repository

See also

mutex(9)

NAME

MTX_SYSINIT

SYNOPSIS

DESCRIPTION

SEE ALSO

HISTORY