.\" $OpenBSD: mutex.9,v 1.14 2010/08/08 12:39:05 jmc Exp $
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.\" Copyright (c) 2005 Pedro Martelletto <pedro@ambientworks.net>
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.Dd $Mdocdate: August 8 2010 $
.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
.Fd #include <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"
.Sh DESCRIPTION
The
.Nm
set of functions provides a non-recursive, interrupt-aware spinning mechanism
to ensure mutual exclusion between different CPUs.
.Pp
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.
.Pp
The
.Fn mtx_enter
function acquires a mutex, spinning if necessary.
.Pp
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.
.Pp
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.
.Pp
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.
.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 art@openbsd.org .
.Sh CAVEATS
As these are spinning locks, don't sleep while holding one.
.Pp
If using a mutex in an interrupt handler, locks must be initialised
with the correct IPL or deadlock will occur.
.Pp
Multiple mutexes may be nested, but not interleaved.
This is okay:
.Bd -literal -offset indent
mtx_enter(foo);
mtx_enter(bar);
mtx_leave(bar);
mtx_leave(foo);
.Ed
.Pp
While this is
.Fa not :
.Bd -literal -offset indent
mtx_enter(foo);
mtx_enter(bar);
mtx_leave(foo);
mtx_leave(bar);
.Ed