linux-kernel - [PATCH] locking/qrwlock: queued_write_lock

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Date:   Sun, 25 Apr 2021 16:06:37 -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, Borislav Petkov <bp@...e.de>,
        Ali Saidi <alisaidi@...zon.com>,
        Steve Capper <steve.capper@....com>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH] locking/qrwlock: queued_write_lock_slowpath() cleanup

Make the code more readable by replacing the atomic_cmpxchg_acquire()
by an equivalent atomic_try_cmpxchg_acquire() and change atomic_add()
to atomic_or().

For architectures that use qrwlock, I do not find one that has an
atomic_add() defined but not an atomic_or().  I guess it should be fine
by changing atomic_add() to atomic_or(). I add a comment to state that
we can change it back to atomic_add() if there is an architecture that
has a more performant atomic_add() than an atomic_or().

Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/locking/qrwlock.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index b94f3831e963..a1fa0f636b07 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -60,18 +60,26 @@ EXPORT_SYMBOL(queued_read_lock_slowpath);
  */
 void queued_write_lock_slowpath(struct qrwlock *lock)
 {
-	int cnts;
+	int cnts = 0;
 
 	/* Put the writer into the wait queue */
 	arch_spin_lock(&lock->wait_lock);
 
 	/* Try to acquire the lock directly if no reader is present */
 	if (!atomic_read(&lock->cnts) &&
-	    (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
+	    atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED))
 		goto unlock;
 
-	/* Set the waiting flag to notify readers that a writer is pending */
-	atomic_add(_QW_WAITING, &lock->cnts);
+	/*
+	 * Set the waiting flag to notify readers that a writer is pending
+	 *
+	 * As only one writer who is the wait_lock owner can set the waiting
+	 * flag which will be cleared later on when acquiring the write lock,
+	 * we can easily replace atomic_or() by an atomic_add() if there is
+	 * an architecture where an atomic_add() performs better than an
+	 * atomic_or().
+	 */
+	atomic_or(_QW_WAITING, &lock->cnts);
 
 	/* When no more readers or writers, set the locked flag */
 	do {
-- 
2.18.1