linux-kernel - [PATCH v2 6/6] ARCv2: spinlock/rwlock: Reset retry delay when starting a new spin-wait cycle

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Date:	Mon, 3 Aug 2015 20:06:08 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	<linux-kernel@...r.kernel.org>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Subject: [PATCH v2 6/6] ARCv2: spinlock/rwlock: Reset retry delay when starting a new spin-wait cycle

The previous commit for delayed retry of SCOND needs some fine tuning
for spin locks.

The backoff from delayed retry in conjunction with spin looping of lock
itself can potentially cause the delay counter to reach high values.
So to provide fairness to any lock operation, after a lock "seems"
available (i.e. just before first SCOND try0, reset the delay counter
back to starting value of 1

Essentially reset delay to 1 for a new spin-wait-loop-acquire cycle.

Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
---
 arch/arc/include/asm/spinlock.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arc/include/asm/spinlock.h b/arch/arc/include/asm/spinlock.h
index 2a84525b5aa4..7071fc0da56a 100644
--- a/arch/arc/include/asm/spinlock.h
+++ b/arch/arc/include/asm/spinlock.h
@@ -279,7 +279,7 @@ static inline void arch_spin_lock(arch_spinlock_t *lock)
 	__asm__ __volatile__(
 	"0:	mov	%[delay], 1		\n"
 	"1:	llock	%[val], [%[slock]]	\n"
-	"	breq	%[val], %[LOCKED], 1b	\n"	/* spin while LOCKED */
+	"	breq	%[val], %[LOCKED], 0b	\n"	/* spin while LOCKED */
 	"	scond	%[LOCKED], [%[slock]]	\n"	/* acquire */
 	"	bz	4f			\n"	/* done */
 	"					\n"
@@ -358,7 +358,7 @@ static inline void arch_read_lock(arch_rwlock_t *rw)
 	__asm__ __volatile__(
 	"0:	mov	%[delay], 1		\n"
 	"1:	llock	%[val], [%[rwlock]]	\n"
-	"	brls	%[val], %[WR_LOCKED], 1b\n"	/* <= 0: spin while write locked */
+	"	brls	%[val], %[WR_LOCKED], 0b\n"	/* <= 0: spin while write locked */
 	"	sub	%[val], %[val], 1	\n"	/* reader lock */
 	"	scond	%[val], [%[rwlock]]	\n"
 	"	bz	4f			\n"	/* done */
@@ -427,7 +427,7 @@ static inline void arch_write_lock(arch_rwlock_t *rw)
 	__asm__ __volatile__(
 	"0:	mov	%[delay], 1		\n"
 	"1:	llock	%[val], [%[rwlock]]	\n"
-	"	brne	%[val], %[UNLOCKED], 1b	\n"	/* while !UNLOCKED spin */
+	"	brne	%[val], %[UNLOCKED], 0b	\n"	/* while !UNLOCKED spin */
 	"	mov	%[val], %[WR_LOCKED]	\n"
 	"	scond	%[val], [%[rwlock]]	\n"
 	"	bz	4f			\n"
-- 
1.9.1

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