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.\" $OpenBSD: mutex.9,v 1.12 2009/09/08 13:36:20 jmc Exp $
.\"
.\" Copyright (c) 2005 Pedro Martelletto <pedro@ambientworks.net>
.\" All rights reserved.
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.Dd $Mdocdate: September 8 2009 $
.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"
.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.
.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
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