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| author | Uros Bizjak <ubizjak@gmail.com> | 2024-03-21 22:52:47 +0300 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2024-04-11 16:14:54 +0300 |
| commit | 79a34e3d8411050c3c7550c5163d6f9dc41e8f66 (patch) | |
| tree | ddc759da727ab424638b4ea0cfa5200201e0a787 | |
| parent | 21689e4bfb9ae8f8b45279c53faecaa5a056ffa5 (diff) | |
| download | linux-79a34e3d8411050c3c7550c5163d6f9dc41e8f66.tar.xz | |
locking/qspinlock: Use atomic_try_cmpxchg_relaxed() in xchg_tail()
Use atomic_try_cmpxchg_relaxed(*ptr, &old, new) instead of
atomic_cmpxchg_relaxed (*ptr, old, new) == old in xchg_tail().
x86 CMPXCHG instruction returns success in ZF flag,
so this change saves a compare after CMPXCHG.
No functional change intended.
Since this code requires NR_CPUS >= 16k, I have tested it
by unconditionally setting _Q_PENDING_BITS to 1 in
<asm-generic/qspinlock_types.h>.
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Waiman Long <longman@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240321195309.484275-1-ubizjak@gmail.com
| -rw-r--r-- | kernel/locking/qspinlock.c | 13 |
1 files changed, 5 insertions, 8 deletions
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index ebe6b8ec7cb3..1df5fef8a656 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -220,21 +220,18 @@ static __always_inline void clear_pending_set_locked(struct qspinlock *lock) */ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail) { - u32 old, new, val = atomic_read(&lock->val); + u32 old, new; - for (;;) { - new = (val & _Q_LOCKED_PENDING_MASK) | tail; + old = atomic_read(&lock->val); + do { + new = (old & _Q_LOCKED_PENDING_MASK) | tail; /* * We can use relaxed semantics since the caller ensures that * the MCS node is properly initialized before updating the * tail. */ - old = atomic_cmpxchg_relaxed(&lock->val, val, new); - if (old == val) - break; + } while (!atomic_try_cmpxchg_relaxed(&lock->val, &old, new)); - val = old; - } return old; } #endif /* _Q_PENDING_BITS == 8 */ |