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SubjectRe: [PATCH] rwsem: fix missed wakeup due to reordering of load
On Thu, 07 Sep 2017, Prateek Sood wrote:
> /*
>+ * __rwsem_down_write_failed_common(sem)
>+ * rwsem_optimistic_spin(sem)
>+ * osq_unlock(sem->osq)
>+ * ...
>+ * atomic_long_add_return(&sem->count)
>+ *
>+ * - VS -
>+ *
>+ * __up_write()
>+ * if (atomic_long_sub_return_release(&sem->count) < 0)
>+ * rwsem_wake(sem)
>+ * osq_is_locked(&sem->osq)
>+ *
>+ * And __up_write() must observe !osq_is_locked() when it observes the
>+ * atomic_long_add_return() in order to not miss a wakeup.
>+ *
>+ * This boils down to:
>+ *
>+ * [S.rel] X = 1 [RmW] r0 = (Y += 0)
>+ * MB RMB
>+ * [RmW] Y += 1 [L] r1 = X
>+ *
>+ * exists (r0=1 /\ r1=0)
>+ */
>+ smp_rmb();

Instead, how about just removing the release from atomic_long_sub_return_release()
such that the osq load is not hoisted over the atomic compound (along with Peter's
comment):

diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
index 6c6a2141f271..487ce31078ff 100644
--- a/include/asm-generic/rwsem.h
+++ b/include/asm-generic/rwsem.h
@@ -101,7 +101,7 @@ static inline void __up_read(struct rw_semaphore *sem)
*/
static inline void __up_write(struct rw_semaphore *sem)
{
- if (unlikely(atomic_long_sub_return_release(RWSEM_ACTIVE_WRITE_BIAS,
+ if (unlikely(atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
&sem->count) < 0))
rwsem_wake(sem);
}
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