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locking/spinlocks: Remove an instruction from spin and write locks 

Both spin locks and write locks currently do:

 f0 0f b1 17             lock cmpxchg %edx,(%rdi)
 85 c0                   test   %eax,%eax
 75 05                   jne    [slowpath]

This 'test' insn is superfluous; the cmpxchg insn sets the Z flag
appropriately.  Peter pointed out that using atomic_try_cmpxchg_acquire()
will let the compiler know this is true.  Comparing before/after
disassemblies show the only effect is to remove this insn.

Take this opportunity to make the spin & write lock code resemble each
other more closely and have similar likely() hints.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <longman@redhat.com>
Link: http://lkml.kernel.org/r/20180820162639.GC25153@bombadil.infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Matthew Wilcox 2018-08-20 10:19:14 -04:00 committed by Ingo Molnar
parent cb92173d1f
commit 27df89689e
2 changed files with 13 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
include/asm-generic/qrwlock.h
View File

@ -71,8 +71,8 @@ static inline int queued_write_trylock(struct qrwlock *lock)
if (unlikely(cnts))
return 0;
return likely(atomic_cmpxchg_acquire(&lock->cnts,
cnts, cnts | _QW_LOCKED) == cnts);
return likely(atomic_try_cmpxchg_acquire(&lock->cnts, &cnts,
_QW_LOCKED));
}
/**
* queued_read_lock - acquire read lock of a queue rwlock
@ -96,8 +96,9 @@ static inline void queued_read_lock(struct qrwlock *lock)
*/
static inline void queued_write_lock(struct qrwlock *lock)
{
u32 cnts = 0;
/* Optimize for the unfair lock case where the fair flag is 0. */
if (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0)
if (likely(atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)))
return;
queued_write_lock_slowpath(lock);

16
include/asm-generic/qspinlock.h
View File

@ -66,10 +66,12 @@ static __always_inline int queued_spin_is_contended(struct qspinlock *lock)
*/
static __always_inline int queued_spin_trylock(struct qspinlock *lock)
{
if (!atomic_read(&lock->val) &&
(atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL) == 0))
return 1;
return 0;
u32 val = atomic_read(&lock->val);
if (unlikely(val))
return 0;
return likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL));
}
extern void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
@ -80,11 +82,11 @@ extern void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
*/
static __always_inline void queued_spin_lock(struct qspinlock *lock)
{
u32 val;
u32 val = 0;
val = atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL);
if (likely(val == 0))
if (likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL)))
return;
queued_spin_lock_slowpath(lock, val);
}