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>

diff --git a/lib/lockref.c b/lib/lockref.c
index 3d468b53d4c931de6d76d0f51be6a07b3a066ecf..5b34bbd3eba818563db89437eb267782e8c3a6c9 100644 (file)
--- 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)