[PATCH 2/2] locking/rwsem: Don't hog RCU read lock while optimistically spinning

[Sultan Alsawaf <sultan@kerneltoast.com>]

From: Sultan Alsawaf <sultan@kerneltoast.com>

There's no reason to hold an RCU read lock the entire time while
optimistically spinning for a rwsem. This can needlessly lengthen RCU
grace periods and slow down synchronize_rcu() when it doesn't brute
force the RCU grace period via rcupdate.rcu_expedited=1.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
---
 kernel/locking/rwsem.c | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index f11b9bd3431d..a1e3ceb254d1 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -723,8 +723,10 @@ rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
 	if (state != OWNER_WRITER)
 		return state;
 
-	rcu_read_lock();
 	for (;;) {
+		bool same_owner;
+
+		rcu_read_lock();
 		/*
 		 * When a waiting writer set the handoff flag, it may spin
 		 * on the owner as well. Once that writer acquires the lock,
@@ -732,27 +734,32 @@ rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
 		 * handoff bit is set.
 		 */
 		new = rwsem_owner_flags(sem, &new_flags);
-		if ((new != owner) || (new_flags != flags)) {
-			state = rwsem_owner_state(new, new_flags, nonspinnable);
-			break;
-		}
 
 		/*
-		 * Ensure we emit the owner->on_cpu, dereference _after_
-		 * checking sem->owner still matches owner, if that fails,
+		 * Ensure 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();
+		same_owner = new == owner && new_flags == flags;
+		if (same_owner && !owner_on_cpu(owner))
+			state = OWNER_NONSPINNABLE;
+		rcu_read_unlock();
 
-		if (need_resched() || !owner_on_cpu(owner)) {
+		if (!same_owner) {
+			state = rwsem_owner_state(new, new_flags, nonspinnable);
+			break;
+		}
+
+		if (state == OWNER_NONSPINNABLE)
+			break;
+
+		if (need_resched()) {
 			state = OWNER_NONSPINNABLE;
 			break;
 		}
 
 		cpu_relax();
 	}
-	rcu_read_unlock();
 
 	return state;
 }
-- 
2.28.0