[PATCH] rwsem: fix missed wakeup due to reordering of load

[Prateek Sood]

If a spinner is present, there is a chance that the load of
rwsem_has_spinner() in rwsem_wake() can be reordered with
respect to decrement of rwsem count in __up_write() leading
to wakeup being missed.
 spinning writer                  up_write caller
 ---------------                  -----------------------
 [S] osq_unlock()                 [L] osq
  spin_lock(wait_lock)
  sem->count=0xFFFFFFFF00000001
            +0xFFFFFFFF00000000
  count=sem->count
  MB
                                   sem->count=0xFFFFFFFE00000001
                                             -0xFFFFFFFF00000001
                                   RMB
                                   spin_trylock(wait_lock)
                                   return
 rwsem_try_write_lock(count)
 spin_unlock(wait_lock)
 schedule()

Reordering of atomic_long_sub_return_release() in __up_write()
and rwsem_has_spinner() in rwsem_wake() can cause missing of
wakeup in up_write() context. In spinning writer, sem->count
and local variable count is 0XFFFFFFFE00000001. It would result
in rwsem_try_write_lock() failing to acquire rwsem and spinning
writer going to sleep in rwsem_down_write_failed().

The smp_rmb() will make sure that the spinner state is
consulted after sem->count is updated in up_write context.

Signed-off-by: Prateek Sood <prsood@xxxxxxxxxxxxxx>
---
 kernel/locking/rwsem-xadd.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 34e727f..5c687f6 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -586,6 +586,51 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
 	DEFINE_WAKE_Q(wake_q);
 
 	/*
+	 * If a spinner is present, there is a chance that the load of
+	 * rwsem_has_spinner() in rwsem_wake() can be reordered with
+	 * respect to decrement of rwsem count in __up_write() leading
+	 * to wakeup being missed.
+	 *
+	 * spinning writer                  up_write caller
+	 * ---------------                  -----------------------
+	 * [S] osq_unlock()                 [L] osq
+	 *  spin_lock(wait_lock)
+	 *  sem->count=0xFFFFFFFF00000001
+	 *            +0xFFFFFFFF00000000
+	 *  count=sem->count
+	 *  MB
+	 *                                   sem->count=0xFFFFFFFE00000001
+	 *                                             -0xFFFFFFFF00000001
+	 *                                   RMB
+	 *                                   spin_trylock(wait_lock)
+	 *                                   return
+	 * rwsem_try_write_lock(count)
+	 * spin_unlock(wait_lock)
+	 * schedule()
+	 *
+	 * Reordering of atomic_long_sub_return_release() in __up_write()
+	 * and rwsem_has_spinner() in rwsem_wake() can cause missing of
+	 * wakeup in up_write() context. In spinning writer, sem->count
+	 * and local variable count is 0XFFFFFFFE00000001. It would result
+	 * in rwsem_try_write_lock() failing to acquire rwsem and spinning
+	 * writer going to sleep in rwsem_down_write_failed().
+	 *
+	 *
+	 * The RMB in below example is to make sure that the spinner state is
+	 * consulted after sem->count is updated in up_write context.
+	 * This would guarantee trylock on rwsem is successful
+	 * in rwsem_down_write_failed().
+	 * spinning writer                  up_write caller
+	 * ---------------                  -----------------------
+	 * [S] osq_unlock()                 atomic_update(sem->count)
+	 *                                  RMB
+	 * atomic_update(sem->count)        [L] osq
+	 * MB
+	 * rwsem_try_write_lock(count)
+	 */
+	smp_rmb();
+
+	/*
 	 * If a spinner is present, it is not necessary to do the wakeup.
 	 * Try to do wakeup only if the trylock succeeds to minimize
 	 * spinlock contention which may introduce too much delay in the
-- 
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