From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Tue, 26 Apr 2011 01:10:58 +0000 (-0700)
Subject: bit_spinlock: don't play preemption games inside the busy loop
X-Git-Tag: firefly_0821_release~7613^2~1555
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=3dd2ee4824b668a635d6d2bb6bc73f33708cab9f;p=firefly-linux-kernel-4.4.55.git

bit_spinlock: don't play preemption games inside the busy loop

When we are waiting for the bit-lock to be released, and are looping
over the 'cpu_relax()' should not be doing anything else - otherwise we
miss the point of trying to do the whole 'cpu_relax()'.

Do the preemption enable/disable around the loop, rather than inside of
it.

Noticed when I was looking at the code generation for the dcache
__d_drop usage, and the code just looked very odd.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---

diff --git a/include/linux/bit_spinlock.h b/include/linux/bit_spinlock.h
index e612575a2596..b4326bfa684f 100644
--- a/include/linux/bit_spinlock.h
+++ b/include/linux/bit_spinlock.h
@@ -23,11 +23,11 @@ static inline void bit_spin_lock(int bitnum, unsigned long *addr)
 	preempt_disable();
 #if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
 	while (unlikely(test_and_set_bit_lock(bitnum, addr))) {
-		while (test_bit(bitnum, addr)) {
-			preempt_enable();
+		preempt_enable();
+		do {
 			cpu_relax();
-			preempt_disable();
-		}
+		} while (test_bit(bitnum, addr));
+		preempt_disable();
 	}
 #endif
 	__acquire(bitlock);