Module Name: src
Committed By: riastradh
Date: Fri Feb 24 11:21:29 UTC 2023
Modified Files:
src/sys/kern: kern_mutex.c
Log Message:
mutex(9): Simplify membars.
- Elide macro indirection for membar_acquire.
- Use atomic_store_release instead of membar_release and store.
No functional change intended. Possible very very very very minor
performance gain on architectures with a native store-release
instruction.
Note: It is possible there are some code paths that worked by
accident with this change which could, in theory, break now, such as
the logic I recently fixed in kern_descrip.c that assumed a
mutex_enter/exit cycle would serve as a store-before-store barrier:
fp->f_... = ...; // A
/* fd_affix */
mutex_enter(&fp->f_lock);
fp->f_count++;
mutex_exit(&fp->f_lock);
...
ff->ff_file = fp; // B
This logic was never correct, and is likely already broken in
practice on aarch64 because the mutex_exit stub already uses STLXR
instead of DMB ISH(ST); STXR. This change only affects the slow path
of mutex_exit, so it doesn't change even the accidental guarantees
mutex_exit makes in all paths.
To generate a diff of this commit:
cvs rdiff -u -r1.103 -r1.104 src/sys/kern/kern_mutex.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/kern/kern_mutex.c
diff -u src/sys/kern/kern_mutex.c:1.103 src/sys/kern/kern_mutex.c:1.104
--- src/sys/kern/kern_mutex.c:1.103 Thu Feb 23 14:57:29 2023
+++ src/sys/kern/kern_mutex.c Fri Feb 24 11:21:28 2023
@@ -1,4 +1,4 @@
-/* $NetBSD: kern_mutex.c,v 1.103 2023/02/23 14:57:29 riastradh Exp $ */
+/* $NetBSD: kern_mutex.c,v 1.104 2023/02/24 11:21:28 riastradh Exp $ */
/*-
* Copyright (c) 2002, 2006, 2007, 2008, 2019 The NetBSD Foundation, Inc.
@@ -40,7 +40,7 @@
#define __MUTEX_PRIVATE
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_mutex.c,v 1.103 2023/02/23 14:57:29 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_mutex.c,v 1.104 2023/02/24 11:21:28 riastradh Exp $");
#include <sys/param.h>
#include <sys/atomic.h>
@@ -167,12 +167,8 @@ do { \
*/
#ifdef __HAVE_ATOMIC_AS_MEMBAR
#define MUTEX_MEMBAR_ENTER()
-#define MUTEX_MEMBAR_ACQUIRE()
-#define MUTEX_MEMBAR_RELEASE()
#else
#define MUTEX_MEMBAR_ENTER() membar_enter()
-#define MUTEX_MEMBAR_ACQUIRE() membar_acquire()
-#define MUTEX_MEMBAR_RELEASE() membar_release()
#endif
/*
@@ -238,7 +234,7 @@ MUTEX_ACQUIRE(kmutex_t *mtx, uintptr_t c
MUTEX_INHERITDEBUG(oldown, mtx->mtx_owner);
MUTEX_INHERITDEBUG(newown, oldown);
rv = MUTEX_CAS(&mtx->mtx_owner, oldown, newown);
- MUTEX_MEMBAR_ACQUIRE();
+ membar_acquire();
return rv;
}
@@ -256,10 +252,9 @@ MUTEX_RELEASE(kmutex_t *mtx)
{
uintptr_t newown;
- MUTEX_MEMBAR_RELEASE();
newown = 0;
MUTEX_INHERITDEBUG(newown, mtx->mtx_owner);
- mtx->mtx_owner = newown;
+ atomic_store_release(&mtx->mtx_owner, newown);
}
#endif /* __HAVE_SIMPLE_MUTEXES */
