Copyright (c) 2005 Pedro Martelletto <pedro@ambientworks.net>
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.Dd $Mdocdate: July 17 2013 $ .Dt MUTEX 9 .Os .Sh NAME .Nm mutex , .Nm mtx_init , .Nm mtx_enter , .Nm mtx_enter_try , .Nm mtx_leave .Nd interface to CPU mutexes .Sh SYNOPSIS n sys/mutex.h .Ft void .Fn mtx_init "struct mutex *mtxp" "int wantipl" .Ft void .Fn mtx_enter "struct mutex *mtxp" .Ft int .Fn mtx_enter_try "struct mutex *mtxp" .Ft void .Fn mtx_leave "struct mutex *mtxp" .Fn MUTEX_ASSERT_LOCKED "struct mutex *mtxp" .Fn MUTEX_ASSERT_UNLOCKED "struct mutex *mtxp" .Fn MUTEX_INITIALIZER "int wantipl" .Sh DESCRIPTION The .Nm set of functions provides a non-recursive, interrupt-aware spinning mechanism to ensure mutual exclusion between different CPUs.
p The .Fn mtx_init function is used to initiate the mutex pointed to by .Fa mtxp . When acquired, the mutex will cause the processor interrupt level to be raised to .Fa wantipl if necessary.
p The .Fn mtx_enter function acquires a mutex, spinning if necessary.
p The .Fn mtx_enter_try function attempts to acquire a mutex. If it succeeds in acquiring the mutex it will return non-zero, otherwise it will return 0.
p The .Fn mtx_leave function releases a mutex. In case the acquisition of the mutex caused the interrupt level to be changed, it is then restored.
p The .Fn MUTEX_ASSERT_LOCKED and .Fn MUTEX_ASSERT_UNLOCKED macros may be used to assert that a mutex is held locked or unlocked by the current CPU.
p A mutex declaration may be initialised with the .Fn MUTEX_INITIALIZER macro. When acquired, the mutex will cause the processor interrupt level to be raised to .Fa wantipl if necessary. .Sh SEE ALSO .Xr lockmgr 9 , .Xr rwlock 9 , .Xr spl 9 .Sh HISTORY The .Nm functions first appeared in .Ox 3.6 . .Sh AUTHORS The .Nm functions were written by .An Artur Grabowski Aq Mt art@openbsd.org . .Sh CAVEATS As these are spinning locks, don't sleep while holding one.
p If using a mutex in an interrupt handler, locks must be initialised with the correct IPL or deadlock will occur.
p Multiple mutexes may be nested, but not interleaved. This is okay: d -literal -offset indent mtx_enter(foo); mtx_enter(bar); mtx_leave(bar); mtx_leave(foo); .Ed
p While this is .Fa not : d -literal -offset indent mtx_enter(foo); mtx_enter(bar); mtx_leave(foo); mtx_leave(bar); .Ed