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

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Date:	Mon, 3 Aug 2015 15:33:08 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	"Peter Zijlstra (Intel)" <peterz@...radead.org>
CC:	lkml <linux-kernel@...r.kernel.org>, <arc-linux-dev@...opsys.com>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Subject: [PATCH 6/6] ARCv2: spinlock/rwlock: Reset retry delay when starting a new spin-wait cycle

A spin lock could be available momentarily, but the SCOND to actually
acquire it might still fail due to concurrent update from other core(s).
To elide hardware lock, the sequence is retried after a "delay" which is
increased expoenntially to get a nice backoff behaviour.

However, this could cause the delay counter to get to a high value. Thus
when the next spin cycle restarts, reset the counter back to starting
value of 1.

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 297c54484013..dce6e3d8583c 100644
--- a/arch/arc/include/asm/spinlock.h
+++ b/arch/arc/include/asm/spinlock.h
@@ -291,7 +291,7 @@ 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"
@@ -370,7 +370,7 @@ 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 */
@@ -439,7 +439,7 @@ 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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