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ad 2007-12-05 05:20:38 +00:00
parent 6a7a863131
commit 55bfbff8a0
1 changed files with 35 additions and 35 deletions
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70
share/man/man9/mutex.9
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@ -1,4 +1,4 @@
.\" $NetBSD: mutex.9,v 1.11 2007/12/05 05:17:53 ad Exp $
.\" $NetBSD: mutex.9,v 1.12 2007/12/05 05:20:38 ad Exp $
.\"
.\" Copyright (c) 2007 The NetBSD Foundation, Inc.
.\" All rights reserved.
@ -43,10 +43,10 @@
.Nm mutex_destroy ,
.Nm mutex_enter ,
.Nm mutex_exit ,
.Nm mutex_tryenter ,
.Nm mutex_owned ,
.Nm mutex_spin_enter ,
.Nm mutex_spin_exit
.Nm mutex_spin_exit ,
.Nm mutex_tryenter
.Nd mutual exclusion primitives
.Sh SYNOPSIS
.In sys/mutex.h
@ -59,13 +59,13 @@
.Ft void
.Fn mutex_exit "kmutex_t *mtx"
.Ft int
.Fn mutex_tryenter "kmutex_t *mtx"
.Ft int
.Fn mutex_owned "kmutex_t *mtx"
.Ft void
.Fn mutex_spin_enter "kmutex_t *mtx"
.Ft void
.Fn mutex_spin_exit "kmutex_t *mtx"
.Ft int
.Fn mutex_tryenter "kmutex_t *mtx"
.Pp
.Cd "options DIAGNOSTIC"
.Cd "options LOCKDEBUG"
@ -172,36 +172,6 @@ not be acquired once a spin mutex is held by the caller.
Release a mutex.
The mutex must have been previously acquired by the caller.
Mutexes may be released out of order as needed.
.It Fn mutex_tryenter "mtx"
.Pp
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.
.Pp
.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:
.Bd -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
.It Fn mutex_owned "mtx"
.Pp
For adaptive mutexes, return non-zero if the current LWP holds the mutex.
@ -236,6 +206,36 @@ but may only be used when it is known that
is a spin mutex.
On some architectures, this can substantially reduce the cost of releasing
an unheld spin mutex.
.It Fn mutex_tryenter "mtx"
.Pp
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.
.Pp
.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:
.Bd -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
.El
.Sh CODE REFERENCES
This section describes places within the