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MUTEX(9) Kernel Developer's Manual MUTEX(9)

mutex, mtx_init, mtx_init_flags, mtx_enter, mtx_enter_try, mtx_leave, MUTEX_ASSERT_LOCKED, MUTEX_ASSERT_UNLOCKED, MUTEX_INITIALIZER, MUTEX_INITIALIZER_FLAGS
interface to CPU mutexes

#include <sys/mutex.h>
void
mtx_init(struct mutex *mtxp, int wantipl);
void
mtx_init_flags(struct mutex *mtxp, int wantipl, const char *name, int flags);
void
mtx_enter(struct mutex *mtxp);
int
mtx_enter_try(struct mutex *mtxp);
void
mtx_leave(struct mutex *mtxp);
MUTEX_ASSERT_LOCKED(struct mutex *mtxp);
MUTEX_ASSERT_UNLOCKED(struct mutex *mtxp);
MUTEX_INITIALIZER(int wantipl);
MUTEX_INITIALIZER_FLAGS(int wantipl, const char *name, int flags);

The mutex set of functions provides a non-recursive, interrupt-aware spinning mechanism to ensure mutual exclusion between different CPUs.
The mtx_init() function is used to initiate the mutex pointed to by mtxp. When acquired, the mutex will cause the processor interrupt level to be raised to wantipl if necessary.
The mtx_init_flags() macro is similar to mtx_init(), but it additionally accepts parameters for witness(4). The pointer name differentiates a lock type. Two mutexes have the same lock type only if they have been created by the same occurrence of mtx_init_flags() with the same pointer name. The flags parameter is a bitwise OR of the following options:
 
 
Prevents witness(4) from logging when a CPU acquires more than one lock of this lock type.
 
 
Instructs witness(4) to ignore this lock.
The mtx_enter() function acquires a mutex, spinning if necessary.
The mtx_enter_try() function attempts to acquire a mutex.
The mtx_leave() function releases a mutex. In case the acquisition of the mutex caused the interrupt level to be changed, it is then restored.
The MUTEX_ASSERT_LOCKED() and MUTEX_ASSERT_UNLOCKED() macros may be used to assert that a mutex is held locked or unlocked by the current CPU.
A mutex declaration may be initialised with the MUTEX_INITIALIZER() macro. When acquired, the mutex will cause the processor interrupt level to be raised to wantipl if necessary.
The MUTEX_INITIALIZER_FLAGS() macro is similar to MUTEX_INITIALIZER(), but it additionally accepts parameters for witness(4). See the mtx_init_flags() macro for details.

mtx_init() and mtx_init_flags() can be called during autoconf, from process context, or from interrupt context.
mtx_enter(), mtx_enter_try(), and mtx_leave() can be called during autoconf, from process context, or from any interrupt context at or below the interrupt level mtxp was initialised with.

The mtx_enter_try() function will return non-zero if it succeeds in acquiring the mutex mtxp, otherwise it will return 0.

msleep(9), rwlock(9), spl(9)

The mutex functions first appeared in OpenBSD 3.6.

The mutex functions were written by Artur Grabowski <art@openbsd.org>.

As these are spinning locks, don't sleep while holding one.
Multiple mutexes may be nested, but not interleaved. This is okay:
mtx_enter(foo); 
mtx_enter(bar); 
mtx_leave(bar); 
mtx_leave(foo);
While this is not:
mtx_enter(foo); 
mtx_enter(bar); 
mtx_leave(foo); 
mtx_leave(bar);
October 30, 2017 OpenBSD-current