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.Dd April 9, 2007 .Dt MUTEX 9 .Os .Sh NAME .Nm mutex , .Nm mutex_init , .Nm mutex_destroy , .Nm mutex_enter , .Nm mutex_exit , .Nm mutex_tryenter , .Nm mutex_owned .Nd mutual exclusion primitives .Sh SYNOPSIS n sys/mutex.h .Ft void .Fn mutex_init "kmutex_t *mtx" "kmutex_type_t type" "int ipl" .Ft void .Fn mutex_destroy "kmutex_t *mtx" .Ft void .Fn mutex_enter "kmutex_t *mtx" .Ft void .Fn mutex_exit "kmutex_t *mtx" .Ft int .Fn mutex_tryenter "kmutex_t *mtx" .Ft int .Fn mutex_owned "kmutex_t *mtx"
p .Cd "options DIAGNOSTIC" .Cd "options LOCKDEBUG" .Sh DESCRIPTION Mutexes are used in the kernel to implement mutual exclusion among LWPs (lightweight processes) and interrupt handlers.
p The .Vt kmutex_t type provides storage for the mutex object. This should be treated as an opaque object and not examined directly by consumers.
p Mutexes replace the .Xr spl 9 system traditionally used to provide synchronization between interrupt handlers and LWPs, and in combination with reader / writer locks replace the .Xr lockmgr 9 facility. .Sh OPTIONS l -tag -width abcd t Cd "options DIAGNOSTIC"
p Kernels compiled with the .Dv DIAGNOSTIC option perform basic sanity checks on mutex operations. t Cd "options LOCKDEBUG"
p Kernels compiled with the .Dv LOCKDEBUG option perform potentially CPU intensive sanity checks on mutex operations. .El .Sh FUNCTIONS l -tag -width abcd t Fn mutex_init "mtx" "type" "ipl"
p Dynamically initialize a mutex for use. No other operations can be performed on a mutex until it has been initialized. .Fn mutex_init may block in order to allocate memory.
p The .Fa type argument specifies the kind of mutex to be initialized, controlling the behavior of the mutex. Once initialized, all types of mutex are manipulated using the same interface. Valid types are as follows: l -tag -width cdoscdosrunru t Dv MUTEX_DEFAULT Requests an adaptive mutex. Adaptive mutexes provide mutual exclusion between LWPs.
p When initializing an adaptive mutex, .Dv IPL_NONE must be specified as the .Fa ipl argument. Adaptive mutexes should not be acquired from an interrupt handler.
p An LWP may either sleep or busy-wait when attempting to acquire an adaptive mutex that is already held. t Dv MUTEX_SPIN Requests a spin mutex. Spin mutexes provide mutual exclusion between LWPs, and between LWPs and interrupt handlers.
p When initializing a spin mutex, the .Fa ipl argument is used to pass an system interrupt priority level (SPL) that will block all interrupt handlers that may try to acquire the mutex.
p LWPs that own spin mutexes may not sleep, and therefore must not try to acquire adaptive mutexes or other sleep locks.
p A processor will always busy-wait when attempting to acquire a spin mutex that is already held. .El
p In general, device drivers should not directly request spin or adaptive mutexes. For device drivers a third type of mutex is provided: l -tag -width cdoscdosrunru t Dv MUTEX_DRIVER Requests a device driver mutex.
p The .Fa ipl argument is used to determine whether a spin or adaptive mutex is returned, depending on how interrupt handling is implemented by the machine architecture.
p If .Dv IPL_NONE is specified, .Fn mutex_init is guaranteed to return an adaptive mutex that can be used to provide mutual exclusion for device driver code that does not run from an interrupt handler. Other levels return mutexes that can be used to synchronize with interrupt handlers. For example, to request a mutex for synchronizing with network interrupt handlers, specify .Dv IPL_NET . Beyond this, device drivers should not make assumptions about the type of mutex returned. .El t Fn mutex_destroy "mtx"
p Release resources used by a mutex. The mutex may not be used after it has been destroyed. .Fn mutex_destroy may block in order to free memory. t Fn mutex_enter "mtx"
p Acquire a mutex. If the mutex is already held, the caller will block and not return until the mutex is acquired.
p Mutexes and other types of locks must always be acquired in a consistent order with respect to each other. Otherwise, the potential for system deadlock exists.
p Adaptive mutexes and other types of lock that can sleep may not be acquired once a spin mutex is held by the caller. Note that device driver mutexes at a level other than .Dv IPL_NONE can be spin mutexes. t Fn mutex_exit "mtx"
p Release a mutex. The mutex must have been previously acquired by the caller. Mutexes may be released out of order as needed. t Fn mutex_tryenter "mtx"
p Try to acquire a mutex, but do not block if the mutex is already held. Returns non-zero if the mutex was acquired, or zero if the mutex was already held.
p .Fn mutex_tryenter can be used as an optimization when acquiring locks in the the wrong order. For example, in a setting where the convention is that .Dv first_lock must be acquired before .Dv second_lock , the following can be used to optimistically lock in reverse order: d -literal /* We hold second_lock, but not first_lock. */ KASSERT(mutex_owned(\*[Am]second_lock)); if (!mutex_tryenter(\*[Am]first_lock)) { /* Failed to get it - lock in the correct order. */ mutex_exit(\*[Am]second_lock); mutex_enter(\*[Am]first_lock); mutex_enter(\*[Am]second_lock); /* * We may need to recheck any conditions the code * path depends on, as we released second_lock * briefly. */ } .Ed t Fn mutex_owned "mtx"
p For adaptive mutexes, return non-zero if the current LWP holds the mutex. For spin mutexes, return non-zero if the mutex is held, potentially by the current processor. Otherwise, return zero.
p .Fn mutex_owned is provided for making diagnostic checks to verify that a lock is held. For example: d -literal KASSERT(mutex_owned(\*[Am]driver_lock)); .Ed
p It should not be used to make locking decisions at run time, or to verify that a lock is unheld. .El .Sh CODE REFERENCES This section describes places within the .Nx source tree where code implementing mutexes can be found. All pathnames are relative to
p The core of the mutex implementation is in
a sys/kern/kern_mutex.c .
p The header file
a sys/sys/mutex.h describes the public interface, and interfaces that machine-dependent code must provide to support mutexes. .Sh SEE ALSO .Xr condvar 9 , .Xr mb 9 , .Xr rwlock 9
p .Rs .%A Jim Mauro .%A Richard McDougall .%T Solaris Internals: Core Kernel Architecture , .%I Prentice Hall .%D 2001 .%O ISBN 0-13-022496-0 .Re .Sh HISTORY The mutex primitives first appeared in .Nx 5.0 .
Indexes created Thu Oct 02 14:10:14 GMT 2025