Provided by: freebsd-manpages_8.2-1_all bug

NAME

     mutex, mtx_init, mtx_destroy, 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_sleep, mtx_initialized, mtx_owned, mtx_recursed, mtx_assert,
     MTX_SYSINIT — kernel 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_destroy(struct mtx *mutex);

     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);

     int
     mtx_sleep(void *chan, struct mtx *mtx, int priority, const char *wmesg, int timo);

     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 mtx *mtx, const char *description, int opts);

DESCRIPTION

     Mutexes are the most basic and primary method of thread synchronization.  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 context 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 introduced 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 scheduling code.  In all architectures both acquiring and releasing of a
     uncontested 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 referenced 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 option is used to identify the lock in debugging output
     etc.  The type option 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 initialization
     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 exclusion 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 acquire 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 during 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-zero 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 exclusion 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 routines 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 mutex 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_sleep() function is used to atomically release mtx while waiting for an event.  For
     more details on the parameters to this function, see sleep(9).

     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 mutex 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 mutex pointed to by the
                     first argument.  This assertion is only valid in conjunction with MA_OWNED.

     MA_NOTRECURSED  Assert that the current thread has not recursed 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 mutex 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 Options
     The options passed in the opts argument of mtx_init() specify 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 interrupts are disabled on
                    the local CPU while any spin lock is held.

     MTX_RECURSE    Specifies that the initialized mutex is allowed to recurse.  This bit must be
                    present if the mutex 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 duplicate locks being acquired.

     MTX_NOPROFILE  Do not profile this lock.

   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 individual mutex operations.
                This can be used to trim superfluous logging messages for debugging purposes.

   Giant
     If Giant must be acquired, it must be acquired prior to acquiring other mutexes.  Put
     another way: it is impossible to acquire Giant non-recursively while holding another mutex.
     It is possible to acquire 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 Userspace
     No mutexes should be held (except for Giant) across functions which access memory in
     userspace, such as copyin(9), copyout(9), uiomove(9), fuword(9), etc.  No locks are needed
     when calling these functions.

SEE ALSO

     condvar(9), LOCK_PROFILING(9), locking(9), mtx_pool(9), panic(9), rwlock(9), sema(9),
     sleep(9), sx(9)

HISTORY

     These functions appeared in BSD/OS 4.1 and FreeBSD 5.0.