Commit-ID: debfab74e453f079cd8b12b0604387a8c510ef3a
Gitweb: http://git.kernel.org/tip/debfab74e453f079cd8b12b0604387a8c510ef3a
Author: Jason Low <jason.low2@xxxxxx>
AuthorDate: Tue, 16 Sep 2014 17:16:57 -0700
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 3 Oct 2014 06:09:29 +0200
locking/rwsem: Avoid double checking before try acquiring write lock
Commit 9b0fc9c09f1b ("rwsem: skip initial trylock in rwsem_down_write_failed")
checks for if there are known active lockers in order to avoid write trylocking
using expensive cmpxchg() when it likely wouldn't get the lock.
However, a subsequent patch was added such that we directly
check for sem->count == RWSEM_WAITING_BIAS right before trying
that cmpxchg().
Thus, commit 9b0fc9c09f1b now just adds overhead.
This patch modifies it so that we only do a check for if
count == RWSEM_WAITING_BIAS.
Also, add a comment on why we do an "extra check" of count
before the cmpxchg().
Signed-off-by: Jason Low <jason.low2@xxxxxx>
Acked-by: Davidlohr Bueso <dbueso@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Aswin Chandramouleeswaran <aswin@xxxxxx>
Cc: Chegu Vinod <chegu_vinod@xxxxxx>
Cc: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1410913017.2447.22.camel@j-VirtualBox
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
kernel/locking/rwsem-xadd.c | 20 +++++++++++---------
1 file changed, 11 insertions(+), 9 deletions(-)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 12166ec..7628c3f 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -250,16 +250,18 @@ EXPORT_SYMBOL(rwsem_down_read_failed);
static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
{
- if (!(count & RWSEM_ACTIVE_MASK)) {
- /* try acquiring the write lock */
- if (sem->count == RWSEM_WAITING_BIAS &&
- cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
- RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
- if (!list_is_singular(&sem->wait_list))
- rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
- return true;
- }
+ /*
+ * Try acquiring the write lock. Check count first in order
+ * to reduce unnecessary expensive cmpxchg() operations.
+ */
+ if (count == RWSEM_WAITING_BIAS &&
+ cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
+ RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
+ if (!list_is_singular(&sem->wait_list))
+ rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
+ return true;
}
+
return false;
}
--
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