[RFC][PATCH] mutex: Optimize mutex_trylock() fast-path

[Peter Zijlstra <peterz@infradead.org>]

A while back Viro posted a number of 'interesting' mutex_is_locked()
users on IRC, one of those was RCU.

RCU seems to use mutex_is_locked() to avoid doing mutex_trylock(), the
regular load before modify pattern.

While the use isn't wrong per se, its curious in that its needed at all,
mutex_trylock() should be good enough on its own to avoid the pointless
cacheline bounces.

So fix those and remove the mutex_is_locked() (ab)use from RCU.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/ia64/include/asm/mutex.h    | 2 +-
 arch/powerpc/include/asm/mutex.h | 2 +-
 arch/x86/include/asm/mutex_32.h  | 2 +-
 arch/x86/include/asm/mutex_64.h  | 6 +++---
 include/asm-generic/mutex-dec.h  | 2 +-
 include/asm-generic/mutex-xchg.h | 6 +++++-
 kernel/rcu/tree.c                | 1 -
 7 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/arch/ia64/include/asm/mutex.h b/arch/ia64/include/asm/mutex.h
index f41e66d65e31..28cb819e0ff9 100644
--- a/arch/ia64/include/asm/mutex.h
+++ b/arch/ia64/include/asm/mutex.h
@@ -82,7 +82,7 @@ __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
 static inline int
 __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
 {
-	if (cmpxchg_acq(count, 1, 0) == 1)
+	if (atomic_read(count) == 1 && cmpxchg_acq(count, 1, 0) == 1)
 		return 1;
 	return 0;
 }
diff --git a/arch/powerpc/include/asm/mutex.h b/arch/powerpc/include/asm/mutex.h
index 127ab23e1f6c..078155fa1189 100644
--- a/arch/powerpc/include/asm/mutex.h
+++ b/arch/powerpc/include/asm/mutex.h
@@ -124,7 +124,7 @@ __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
 static inline int
 __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
 {
-	if (likely(__mutex_cmpxchg_lock(count, 1, 0) == 1))
+	if (likely(atomic_read(count) == 1 && __mutex_cmpxchg_lock(count, 1, 0) == 1))
 		return 1;
 	return 0;
 }
diff --git a/arch/x86/include/asm/mutex_32.h b/arch/x86/include/asm/mutex_32.h
index 85e6cda45a02..e9355a84fc67 100644
--- a/arch/x86/include/asm/mutex_32.h
+++ b/arch/x86/include/asm/mutex_32.h
@@ -101,7 +101,7 @@ static inline int __mutex_fastpath_trylock(atomic_t *count,
 					   int (*fail_fn)(atomic_t *))
 {
 	/* cmpxchg because it never induces a false contention state. */
-	if (likely(atomic_cmpxchg(count, 1, 0) == 1))
+	if (likely(atomic_read(count) == 1 && atomic_cmpxchg(count, 1, 0) == 1))
 		return 1;
 
 	return 0;
diff --git a/arch/x86/include/asm/mutex_64.h b/arch/x86/include/asm/mutex_64.h
index 07537a44216e..d9850758464e 100644
--- a/arch/x86/include/asm/mutex_64.h
+++ b/arch/x86/include/asm/mutex_64.h
@@ -118,10 +118,10 @@ do {								\
 static inline int __mutex_fastpath_trylock(atomic_t *count,
 					   int (*fail_fn)(atomic_t *))
 {
-	if (likely(atomic_cmpxchg(count, 1, 0) == 1))
+	if (likely(atomic_read(count) == 1 && atomic_cmpxchg(count, 1, 0) == 1))
 		return 1;
-	else
-		return 0;
+
+	return 0;
 }
 
 #endif /* _ASM_X86_MUTEX_64_H */
diff --git a/include/asm-generic/mutex-dec.h b/include/asm-generic/mutex-dec.h
index fd694cfd678a..c54829d3de37 100644
--- a/include/asm-generic/mutex-dec.h
+++ b/include/asm-generic/mutex-dec.h
@@ -80,7 +80,7 @@ __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
 static inline int
 __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
 {
-	if (likely(atomic_cmpxchg_acquire(count, 1, 0) == 1))
+	if (likely(atomic_read(count) == 1 && atomic_cmpxchg_acquire(count, 1, 0) == 1))
 		return 1;
 	return 0;
 }
diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h
index a6b4a7bd6ac9..3269ec4e195f 100644
--- a/include/asm-generic/mutex-xchg.h
+++ b/include/asm-generic/mutex-xchg.h
@@ -91,8 +91,12 @@ __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
 static inline int
 __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
 {
-	int prev = atomic_xchg_acquire(count, 0);
+	int prev;
 
+	if (atomic_read(count) != 1)
+		return 0;
+
+	prev = atomic_xchg_acquire(count, 0);
 	if (unlikely(prev < 0)) {
 		/*
 		 * The lock was marked contended so we must restore that
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c7f1bc4f817c..b7326893221f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3681,7 +3681,6 @@ static bool exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
 	if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) &&
 	    (rnp == rnp_root ||
 	     ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) &&
-	    !mutex_is_locked(&rsp->exp_mutex) &&
 	    mutex_trylock(&rsp->exp_mutex))
 		goto fastpath;