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Add some general comments about the mutex implementation.

This commit is contained in:
ad 2008-05-25 12:29:59 +00:00
parent 52c485503f
commit d5afa113a1
1 changed files with 19 additions and 2 deletions
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21
lib/libpthread/pthread_mutex.c
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@ -1,4 +1,4 @@
/* $NetBSD: pthread_mutex.c,v 1.48 2008/04/28 20:23:01 martin Exp $ */
/* $NetBSD: pthread_mutex.c,v 1.49 2008/05/25 12:29:59 ad Exp $ */
/*-
* Copyright (c) 2001, 2003, 2006, 2007, 2008 The NetBSD Foundation, Inc.
@ -29,8 +29,25 @@
* POSSIBILITY OF SUCH DAMAGE.
*/
/*
* To track threads waiting for mutexes to be released, we use lockless
* lists built on atomic operations and memory barriers.
*
* A simple spinlock would be faster and make the code easier to
* follow, but spinlocks are problematic in userspace. If a thread is
* preempted by the kernel while holding a spinlock, any other thread
* attempting to acquire that spinlock will needlessly busy wait.
*
* There is no good way to know that the holding thread is no longer
* running, nor to request a wake-up once it has begun running again.
* Of more concern, threads in the SCHED_FIFO class do not have a
* limited time quantum and so could spin forever, preventing the
* thread holding the spinlock from getting CPU time: it would never
* be released.
*/
#include <sys/cdefs.h>
__RCSID("$NetBSD: pthread_mutex.c,v 1.48 2008/04/28 20:23:01 martin Exp $");
__RCSID("$NetBSD: pthread_mutex.c,v 1.49 2008/05/25 12:29:59 ad Exp $");
#include <sys/types.h>
#include <sys/lwpctl.h>