locking/rwsem: Fix lock optimistic spinning when owner is not running

Ming reported soft lockups occurring when running xfstest due to
the following tip:locking/core commit:

  b3fd4f03ca0b ("locking/rwsem: Avoid deceiving lock spinners")

When doing optimistic spinning in rwsem, threads should stop
spinning when the lock owner is not running. While a thread is
spinning on owner, if the owner reschedules, owner->on_cpu
returns false and we stop spinning.

However, this commit essentially caused the check to get
ignored because when we break out of the spin loop due to
!on_cpu, we continue spinning if sem->owner != NULL.

This patch fixes this by making sure we stop spinning if the
owner is not running. Furthermore, just like with mutexes,
refactor the code such that we don't have separate checks for
owner_running(). This makes it more straightforward in terms of
why we exit the spin on owner loop and we would also avoid
needing to "guess" why we broke out of the loop to make this
more readable.

Reported-and-tested-by: Ming Lei <[email protected]>
Signed-off-by: Jason Low <[email protected]>
Acked-by: Davidlohr Bueso <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Dave Jones <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Sasha Levin <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Tim Chen <[email protected]>
Link: http://lkml.kernel.org/r/1425714331.2475.388.camel@j-VirtualBox
Signed-off-by: Ingo Molnar <[email protected]>

1 files changed, 11 insertions, 20 deletions

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 06e2214edf98..3417d0172a5d 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -324,32 +324,23 @@ done:
 	return ret;
 }
 
-static inline bool owner_running(struct rw_semaphore *sem,
-				 struct task_struct *owner)
-{
-	if (sem->owner != owner)
-		return false;
-
-	/*
-	 * Ensure we emit the owner->on_cpu, dereference _after_ checking
-	 * sem->owner still matches owner, if that fails, owner might
-	 * point to free()d memory, if it still matches, the rcu_read_lock()
-	 * ensures the memory stays valid.
-	 */
-	barrier();
-
-	return owner->on_cpu;
-}
-
 static noinline
 bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
 {
 	long count;
 
 	rcu_read_lock();
-	while (owner_running(sem, owner)) {
-		/* abort spinning when need_resched */
-		if (need_resched()) {
+	while (sem->owner == owner) {
+		/*
+		 * Ensure we emit the owner->on_cpu, dereference _after_
+		 * checking sem->owner still matches owner, if that fails,
+		 * owner might point to free()d memory, if it still matches,
+		 * the rcu_read_lock() ensures the memory stays valid.
+		 */
+		barrier();
+
+		/* abort spinning when need_resched or owner is not running */
+		if (!owner->on_cpu || need_resched()) {
 			rcu_read_unlock();
 			return false;
 		}