2024-01-24 11:25:24

by Uros Bizjak

[permalink] [raw]
Subject: [PATCH] locking/rtmutex: Use try_cmpxchg_relaxed in mark_rt_mutex_waiters()

Use try_cmpxchg() instead of cmpxchg(*ptr, old, new) == old.

X86 CMPXCHG instruction returns success in ZF flag, so this change saves a
compare after CMPXCHG (and related move instruction in front of CMPXCHG).

Also, try_cmpxchg() implicitly assigns old *ptr value to "old" when CMPXCHG
fails. There is no need to re-read the value in the loop.

Note that the value from *ptr should be read using READ_ONCE to prevent
the compiler from merging, refetching or reordering the read.

No functional change intended.

Signed-off-by: Uros Bizjak <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Waiman Long <[email protected]>
Cc: Boqun Feng <[email protected]>
---
kernel/locking/rtmutex.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 4a10e8c16fd2..88d08eeb8bc0 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -237,12 +237,13 @@ static __always_inline bool rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,
*/
static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)
{
- unsigned long owner, *p = (unsigned long *) &lock->owner;
+ unsigned long *p = (unsigned long *) &lock->owner;
+ unsigned long owner, new;

+ owner = READ_ONCE(*p);
do {
- owner = *p;
- } while (cmpxchg_relaxed(p, owner,
- owner | RT_MUTEX_HAS_WAITERS) != owner);
+ new = owner | RT_MUTEX_HAS_WAITERS;
+ } while (!try_cmpxchg_relaxed(p, &owner, new));

/*
* The cmpxchg loop above is relaxed to avoid back-to-back ACQUIRE
--
2.31.1



2024-03-01 12:39:03

by tip-bot2 for Jacob Pan

[permalink] [raw]
Subject: [tip: locking/core] locking/rtmutex: Use try_cmpxchg_relaxed() in mark_rt_mutex_waiters()

The following commit has been merged into the locking/core branch of tip:

Commit-ID: ce3576ebd62d99f79c1dc98824e2ef6d6ab68434
Gitweb: https://git.kernel.org/tip/ce3576ebd62d99f79c1dc98824e2ef6d6ab68434
Author: Uros Bizjak <[email protected]>
AuthorDate: Wed, 24 Jan 2024 11:49:53 +01:00
Committer: Ingo Molnar <[email protected]>
CommitterDate: Fri, 01 Mar 2024 13:02:05 +01:00

locking/rtmutex: Use try_cmpxchg_relaxed() in mark_rt_mutex_waiters()

Use try_cmpxchg() instead of cmpxchg(*ptr, old, new) == old.

The x86 CMPXCHG instruction returns success in the ZF flag, so this change
saves a compare after CMPXCHG (and related move instruction in front of CMPXCHG).

Also, try_cmpxchg() implicitly assigns old *ptr value to "old" when CMPXCHG
fails. There is no need to re-read the value in the loop.

Note that the value from *ptr should be read using READ_ONCE() to prevent
the compiler from merging, refetching or reordering the read.

No functional change intended.

Signed-off-by: Uros Bizjak <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Waiman Long <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
---
kernel/locking/rtmutex.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 4a10e8c..88d08ee 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -237,12 +237,13 @@ static __always_inline bool rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,
*/
static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)
{
- unsigned long owner, *p = (unsigned long *) &lock->owner;
+ unsigned long *p = (unsigned long *) &lock->owner;
+ unsigned long owner, new;

+ owner = READ_ONCE(*p);
do {
- owner = *p;
- } while (cmpxchg_relaxed(p, owner,
- owner | RT_MUTEX_HAS_WAITERS) != owner);
+ new = owner | RT_MUTEX_HAS_WAITERS;
+ } while (!try_cmpxchg_relaxed(p, &owner, new));

/*
* The cmpxchg loop above is relaxed to avoid back-to-back ACQUIRE