Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
MUTEX(9)               FreeBSD Kernel Developer's Manual              MUTEX(9)

     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 -- ker-
     nel synchronization primitives

     #include <sys/param.h>
     #include <sys/lock.h>
     #include <sys/mutex.h>

     mtx_init(struct mtx *mutex, const char *name, const char *type,
         int opts);

     mtx_lock(struct mtx *mutex);

     mtx_lock_spin(struct mtx *mutex);

     mtx_lock_flags(struct mtx *mutex, int flags);

     mtx_lock_spin_flags(struct mtx *mutex, int flags);

     mtx_trylock(struct mtx *mutex);

     mtx_trylock_flags(struct mtx *mutex, int flags);

     mtx_unlock(struct mtx *mutex);

     mtx_unlock_spin(struct mtx *mutex);

     mtx_unlock_flags(struct mtx *mutex, int flags);

     mtx_unlock_spin_flags(struct mtx *mutex, int flags);

     mtx_destroy(struct mtx *mutex);

     mtx_initialized(struct mtx *mutex);

     mtx_owned(struct mtx *mutex);

     mtx_recursed(struct mtx *mutex);

     options INVARIANTS

     mtx_assert(struct mtx *mutex, int what);

     #include <sys/kernel.h>

     MTX_SYSINIT(name, struct mtx *mtx, const char *description, int opts);

     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

     2.   They must have the information and storage space to support priority

     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

     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() mul-
     tiple 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 avail-
     able.  Interrupts are disabled during the spin and remain disabled fol-
     lowing 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

     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 rou-

     The mtx_destroy() function is used to destroy mutex so the data associ-
     ated 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_initialized() function returns non-zero if mutex has been ini-
     tialized 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

     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 inter-
                    rupts are disabled on the local CPU while any spin lock is

     MTX_RECURSE    Specifies that the initialized mutex is allowed to
                    recurse.  This bit must be present if the mutex is permit-
                    ted 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.

   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 cir-
     cumstances 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.

     If Giant must be acquired, it must be acquired prior to acquiring other
     mutexes.  Put another way: it is impossible to acquire Giant non-recur-
     sively while holding another mutex.  It is possible to acquire other
     mutexes while holding Giant, and it is possible to acquire Giant recur-
     sively while holding other mutexes.

     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.

     condvar(9), msleep(9), mtx_pool(9), MUTEX_PROFILING(9), panic(9),
     sema(9), sx(9)

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

FreeBSD 6.2                    February 16, 2005                   FreeBSD 6.2


Want to link to this manual page? Use this URL:

home | help