Fix sx_try_slock(), so it only fails when there is an exclusive owner.

Before that fix, it was possible for the function to fail if number
of sharers changes between 'x = sx->sx_lock' step and atomic_cmpset_acq_ptr()
call.

This fixes ZFS problem when ZFS returns strange EIO errors under load.
In ZFS there is a code that depends on the fact that sx_try_slock() can
only fail if there is an exclusive owner.

Discussed with:	attilio
Reviewed by:	jhb
Approved by:	re (kensmith)

1 files changed, 12 insertions, 9 deletions

diff --git a/sys/kern/kern_sx.c b/sys/kern/kern_sx.c
index 03ffd0e..23dd4b6 100644
--- a/sys/kern/kern_sx.c
+++ b/sys/kern/kern_sx.c
@@ -217,15 +217,18 @@ _sx_try_slock(struct sx *sx, const char *file, int line)
 {
 	uintptr_t x;
 
-	x = sx->sx_lock;
-	KASSERT(x != SX_LOCK_DESTROYED,
-	    ("sx_try_slock() of destroyed sx @ %s:%d", file, line));
-	if ((x & SX_LOCK_SHARED) && atomic_cmpset_acq_ptr(&sx->sx_lock, x,
-	    x + SX_ONE_SHARER)) {
-		LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 1, file, line);
-		WITNESS_LOCK(&sx->lock_object, LOP_TRYLOCK, file, line);
-		curthread->td_locks++;
-		return (1);
+	for (;;) {
+		x = sx->sx_lock;
+		KASSERT(x != SX_LOCK_DESTROYED,
+		    ("sx_try_slock() of destroyed sx @ %s:%d", file, line));
+		if (!(x & SX_LOCK_SHARED))
+			break;
+		if (atomic_cmpset_acq_ptr(&sx->sx_lock, x, x + SX_ONE_SHARER)) {
+			LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 1, file, line);
+			WITNESS_LOCK(&sx->lock_object, LOP_TRYLOCK, file, line);
+			curthread->td_locks++;
+			return (1);
+		}
 	}
 
 	LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 0, file, line);