linux-kernel - [tip: locking/urgent] locking/qrwlock: Cleanup queued_write_lock

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Message-ID: <162030889600.29796.9867411555566955804.tip-bot2@tip-bot2>
Date:   Thu, 06 May 2021 13:48:16 -0000
From:   "tip-bot2 for Waiman Long" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Waiman Long <longman@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: locking/urgent] locking/qrwlock: Cleanup queued_write_lock_slowpath()

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     28ce0e70ecc30cc7d558a0304e6b816d70848f9a
Gitweb:        https://git.kernel.org/tip/28ce0e70ecc30cc7d558a0304e6b816d70848f9a
Author:        Waiman Long <longman@...hat.com>
AuthorDate:    Mon, 26 Apr 2021 14:50:17 -04:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 06 May 2021 15:33:49 +02:00

locking/qrwlock: Cleanup queued_write_lock_slowpath()

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().

Note that the previous use of atomic_add() isn't wrong as only one
writer that is the wait_lock owner can set the waiting flag and the
flag will be cleared later on when acquiring the write lock.

Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Waiman Long <longman@...hat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Will Deacon <will@...nel.org>
Link: https://lkml.kernel.org/r/20210426185017.19815-1-longman@redhat.com
---
 kernel/locking/qrwlock.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index b94f383..ec36b73 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -66,12 +66,12 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
 	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))
+	if (!(cnts = atomic_read(&lock->cnts)) &&
+	    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);
+	atomic_or(_QW_WAITING, &lock->cnts);
 
 	/* When no more readers or writers, set the locked flag */
 	do {