s390/rwlock: improve writer fairness

Set the write-lock bit in the out-of-line rwlock code to indicate that
a writer is waiting. Additional readers will no be able to get the lock
until at least one writer got the lock. Additional writers have to wait
for the first writer to release the lock again.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 9 insertions, 5 deletions

diff --git a/arch/s390/lib/spinlock.c b/arch/s390/lib/spinlock.c
index c717f4a..01f29bb 100644
--- a/arch/s390/lib/spinlock.c
+++ b/arch/s390/lib/spinlock.c
@@ -149,9 +149,10 @@ EXPORT_SYMBOL(_raw_read_trylock_retry);
 
 void _raw_write_lock_wait(arch_rwlock_t *rw)
 {
-	unsigned int owner, old;
+	unsigned int owner, old, prev;
 	int count = spin_retry;
 
+	prev = 0x80000000;
 	owner = 0;
 	while (1) {
 		if (count-- <= 0) {
@@ -161,10 +162,13 @@ void _raw_write_lock_wait(arch_rwlock_t *rw)
 		}
 		old = ACCESS_ONCE(rw->lock);
 		owner = ACCESS_ONCE(rw->owner);
-		if (old)
-			continue;
-		if (_raw_compare_and_swap(&rw->lock, 0, 0x80000000))
-			return;
+		if ((int) old >= 0 &&
+		    _raw_compare_and_swap(&rw->lock, old, old | 0x80000000))
+			prev = old;
+		else
+			smp_rmb();
+		if ((old & 0x7fffffff) == 0 && (int) prev >= 0)
+			break;
 	}
 }
 EXPORT_SYMBOL(_raw_write_lock_wait);