Commit 893a7d32 authored Jun 05, 2019 by Jan Glauber Committed by Linus Torvalds Jun 07, 2019

lockref: Limit number of cmpxchg loop retries



The lockref cmpxchg loop is unbound as long as the spinlock is not
taken. Depending on the hardware implementation of compare-and-swap
a high number of loop retries might happen.

Add an upper bound to the loop to force the fallback to spinlocks
after some time. A retry value of 100 should not impact any hardware
that does not have this issue.

With the retry limit the performance of an open-close testcase
improved between 60-70% on ThunderX2.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jan Glauber <jglauber@marvell.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent d9344522

lib/lockref.c

+3 −0

Original line number	Diff line number	Diff line
		@@ -9,6 +9,7 @@
		* failure case.
		*/
		#define CMPXCHG_LOOP(CODE, SUCCESS) do { \
		int retry = 100; \
		struct lockref old; \
		BUILD_BUG_ON(sizeof(old) != 8); \
		old.lock_count = READ_ONCE(lockref->lock_count); \
		@@ -21,6 +22,8 @@
		if (likely(old.lock_count == prev.lock_count)) { \
		SUCCESS; \
		} \
		if (!--retry) \
		break; \
		cpu_relax(); \
		} \
		} while (0)