The patch titled
x86 rwsem: stay on fast path when count>0 in __up_write()
has been added to the -mm tree. Its filename is
x86-rwsem-stay-on-fast-path-when-count0-in-__up_write.patch
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------------------------------------------------------
Subject: x86 rwsem: stay on fast path when count>0 in __up_write()
From: Michel Lespinasse <walken@xxxxxxxxxx>
When count>0 there is no need to take the call_rwsem_wake path. If we did
take that path, it would just return without doing anything due to the
active count not being zero.
Signed-off-by: Michel Lespinasse <walken@xxxxxxxxxx>
Acked-by: David Howells <dhowells@xxxxxxxxxx>
Cc: Mike Waychison <mikew@xxxxxxxxxx>
Cc: Suleiman Souhlal <suleiman@xxxxxxxxxx>
Cc: Ying Han <yinghan@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
arch/x86/include/asm/rwsem.h | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff -puN arch/x86/include/asm/rwsem.h~x86-rwsem-stay-on-fast-path-when-count0-in-__up_write arch/x86/include/asm/rwsem.h
--- a/arch/x86/include/asm/rwsem.h~x86-rwsem-stay-on-fast-path-when-count0-in-__up_write
+++ a/arch/x86/include/asm/rwsem.h
@@ -216,9 +216,8 @@ static inline void __up_write(struct rw_
rwsem_count_t tmp;
asm volatile("# beginning __up_write\n\t"
LOCK_PREFIX " xadd %1,(%2)\n\t"
- /* tries to transition
- 0xffff0001 -> 0x00000000 */
- " jz 1f\n"
+ /* substracts 0xffff0001, returns the old value */
+ " jns 1f\n\t"
" call call_rwsem_wake\n"
"1:\n\t"
"# ending __up_write\n"
_
Patches currently in -mm which might be from walken@xxxxxxxxxx are
x86-rwsem-stay-on-fast-path-when-count0-in-__up_write.patch
x86-rwsem-minor-cleanups.patch
rwsem-fully-separate-code-pathes-to-wake-writers-vs-readers.patch
rwsem-lighter-active-count-checks-when-waking-up-readers.patch
rwsem-let-rwsem_waiting_bias-represent-any-number-of-waiting-threads.patch
rwsem-wake-queued-readers-when-writer-blocks-on-active-read-lock.patch
rwsem-smaller-wrappers-around-rwsem_down_failed_common.patch
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