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

FreeBSD Manual Pages


home | help
MUTEX(9)		 BSD 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);

     mtx_assert(struct mtx *mutex, int what);

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

     Mutexes are the most basic	and primary method of process 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 process 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 a machine dependent optimization	they may spin for some amount
     of	time before context switching.	It is important	to remember that since
     a process may be preempted	at any time, the possible context switch in-
     troduced by acquiring a mutex is guaranteed to not	break anything that
     isn't already broken.

     Mutexes which do not context switch are MTX_SPIN mutexes.	These should
     only be used to protect data shared with any devices that require non-
     preemptive	interrupts, and	low level scheduling code.  In most/all	archi-
     tectures both acquiring and releasing of a	uncontested spin mutex is more
     expensive than the	same operation on a non	spin mutex.  In	order to pro-
     tect an interrupt service routine from blocking against itself all	inter-
     rupts are blocked on a processor while holding a spin lock.  It is	per-
     missible to hold multiple spin mutexes.  In this case it is a required
     that they be released in the opposite order to that which they were ac-

     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 mtx_lock().  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	per-
     missible to pass the same mutex to	mtx_init() multiple times without in-
     tervening calls to	mtx_destroy().

     The mtx_lock() function acquires a	MTX_DEF	mutual exclusion lock on be-
     half 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 sleep).

     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() function	is used	to acquire exclusive access to those
     objects protected by the 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() and
     mtx_lock_spin() above.

     The mtx_unlock() function releases	a MTX_DEF mutual exclusion lock; if a
     higher priority thread is waiting for the mutex, the releasing thread
     will be disconnected to allow the higher priority thread to acquire the
     mutex and run unless the current thread is	executing in a critical	sec-

     The mtx_unlock_spin() function releases a MTX_SPIN	mutual exclusion lock;
     interrupt state prior to the acquiring of the lock	is restored.

     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
     process 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 process holds
     mutex.  If	the current process 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 to be made about mutex.  If
     the assertions are	not true and the kernel	is compiled with INVARIANTS
     then the kernel will panic.  Currently the	following assertions are sup-

     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	it
     cannot get	the lock.  The machine dependent 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	re-
     leased by another CPU.  This could	result in deadlock if a	thread inter-
     rupted the	thread which held a mutex and then tried to acquire the	mutex;
     for this reason spin locks	will 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	default	(i.e. sleep) locks.

   Initialization Options
     The options passed	in the opts argument of	mtx_init() specify the mutex
     type.  The	possibilities are:

     MTX_DEF	    Default lock type; will always allow the current thread to
		    be suspended to avoid deadlock conditions against inter-
		    rupt threads.  The machine dependent implementation	of
		    this lock type may spin for	a while	before suspending the
		    current thread.  If	this flag is specified,	clearly
		    MTX_SPIN must NOT be specified.

     MTX_SPIN	    Spin lock type; will never relinquish the CPU.  All	inter-
		    rupts are disabled on the local CPU	while any spin lock is

     MTX_RECURSE    Recursion option bit; specifies that the initialized mutex
		    is allowed to recurse.  This bit must be present if	the
		    mutex is going to be 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 be-
		    ing	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 un-
     locking should be performed with the mtx_lock(), mtx_lock_spin(),
     mtx_unlock(), and mtx_unlock_spin() functions.  Only if a flag is re-
     quired 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 mu-
     texes 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 almost 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 ac-
     cess 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), sema(9), sx(9)

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

BSD			       February	12, 2001			   BSD


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

home | help