From 893a7d32e8e04ca4d6c882336b26ed660ca0a48d Mon Sep 17 00:00:00 2001 From: Jan Glauber Date: Wed, 5 Jun 2019 15:48:49 +0200 Subject: 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 Signed-off-by: Jan Glauber Signed-off-by: Linus Torvalds --- lib/lockref.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/lib/lockref.c b/lib/lockref.c index 3d468b53d4c9..5b34bbd3eba8 100644 --- a/lib/lockref.c +++ b/lib/lockref.c @@ -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) -- cgit v1.2.3-58-ga151