linux-kernel - [tip:locking/core] locking/spinlocks: Remove an instruction from spin and write locks

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Message-ID: <tip-27df89689e257cccb604fdf56c91a75a25aa554a@git.kernel.org>
Date:   Tue, 2 Oct 2018 03:07:56 -0700
From:   tip-bot for Matthew Wilcox <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     peterz@...radead.org, will.deacon@....com, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, longman@...hat.com,
        torvalds@...ux-foundation.org, mingo@...nel.org, hpa@...or.com,
        willy@...radead.org, arnd@...db.de
Subject: [tip:locking/core] locking/spinlocks: Remove an instruction from
 spin and write locks

Commit-ID:  27df89689e257cccb604fdf56c91a75a25aa554a
Gitweb:     https://git.kernel.org/tip/27df89689e257cccb604fdf56c91a75a25aa554a
Author:     Matthew Wilcox <willy@...radead.org>
AuthorDate: Mon, 20 Aug 2018 10:19:14 -0400
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 2 Oct 2018 09:49:42 +0200

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@...radead.org>
Signed-off-by: Matthew Wilcox <willy@...radead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Will Deacon <will.deacon@....com>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Waiman Long <longman@...hat.com>
Link: http://lkml.kernel.org/r/20180820162639.GC25153@bombadil.infradead.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 include/asm-generic/qrwlock.h   |  7 ++++---
 include/asm-generic/qspinlock.h | 16 +++++++++-------
 2 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
index 0f7062bd55e5..36254d2da8e0 100644
--- a/include/asm-generic/qrwlock.h
+++ b/include/asm-generic/qrwlock.h
@@ -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);
diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
index 9cc457597ddf..7541fa707f5b 100644
--- a/include/asm-generic/qspinlock.h
+++ b/include/asm-generic/qspinlock.h
@@ -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);
 }