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@linux-foundation.org>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lkml.kernel.org/r/20210426185017.19815-1-longman@redhat.com

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 {