linux-kernel - [PATCH v2 4/8] locking/rwsem: Simplify rwsem_writer

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Date:   Mon, 27 Mar 2023 16:24:09 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org, Waiman Long <longman@...hat.com>
Subject: [PATCH v2 4/8] locking/rwsem: Simplify rwsem_writer_wake()

From: Peter Zijlstra <peterz@...radead.org>

Since @waiter := rwsem_first_waiter() simplify things.

Suggested-by: Waiman Long <longman@...hat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/locking/rwsem.c | 21 ++++-----------------
 1 file changed, 4 insertions(+), 17 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 0cc0aa566a6b..225e86326ea4 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -426,26 +426,12 @@ rwsem_waiter_wake(struct rwsem_waiter *waiter, struct wake_q_head *wake_q)
 static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
 					struct rwsem_waiter *waiter)
 {
-
-	struct rwsem_waiter *first = rwsem_first_waiter(sem);
 	long count, new;
 
 	lockdep_assert_held(&sem->wait_lock);
 
 	count = atomic_long_read(&sem->count);
 	do {
-		bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
-
-		if (has_handoff) {
-			/*
-			 * Honor handoff bit and yield only when the first
-			 * waiter is the one that set it. Otherwisee, we
-			 * still try to acquire the rwsem.
-			 */
-			if (first->handoff_set && (waiter != first))
-				return false;
-		}
-
 		new = count;
 
 		if (count & RWSEM_LOCK_MASK) {
@@ -454,8 +440,9 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
 			 * if it is an RT task or wait in the wait queue
 			 * for too long.
 			 */
-			if (has_handoff || (!rt_task(waiter->task) &&
-					    !time_after(jiffies, waiter->timeout)))
+			if ((count & RWSEM_FLAG_HANDOFF) ||
+			    (!rt_task(waiter->task) &&
+			     !time_after(jiffies, waiter->timeout)))
 				return false;
 
 			new |= RWSEM_FLAG_HANDOFF;
@@ -474,7 +461,7 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
 	 * set here to enable optimistic spinning in slowpath loop.
 	 */
 	if (new & RWSEM_FLAG_HANDOFF) {
-		first->handoff_set = true;
+		waiter->handoff_set = true;
 		lockevent_inc(rwsem_wlock_handoff);
 		return false;
 	}
-- 
2.31.1