linux-kernel - [tip:locking/core] locking/qspinlock: Re-order code

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Date:   Tue, 16 Oct 2018 09:04:25 -0700
From:   tip-bot for Peter Zijlstra <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     mingo@...nel.org, peterz@...radead.org,
        torvalds@...ux-foundation.org, hpa@...or.com, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, will.deacon@....com
Subject: [tip:locking/core] locking/qspinlock: Re-order code

Commit-ID:  53bf57fab7321fb42b703056a4c80fc9d986d170
Gitweb:     https://git.kernel.org/tip/53bf57fab7321fb42b703056a4c80fc9d986d170
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Wed, 26 Sep 2018 13:01:18 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 16 Oct 2018 17:33:53 +0200

locking/qspinlock: Re-order code

Flip the branch condition after atomic_fetch_or_acquire(_Q_PENDING_VAL)
such that we loose the indent. This also result in a more natural code
flow IMO.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Will Deacon <will.deacon@....com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: andrea.parri@...rulasolutions.com
Cc: longman@...hat.com
Link: https://lkml.kernel.org/r/20181003130257.156322446@infradead.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/locking/qspinlock.c | 56 ++++++++++++++++++++++------------------------
 1 file changed, 27 insertions(+), 29 deletions(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index bfaeb05123ff..ec343276f975 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -330,39 +330,37 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	 * 0,0,1 -> 0,1,1 ; pending
 	 */
 	val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
-	if (!(val & ~_Q_LOCKED_MASK)) {
-		/*
-		 * We're pending, wait for the owner to go away.
-		 *
-		 * *,1,1 -> *,1,0
-		 *
-		 * this wait loop must be a load-acquire such that we match the
-		 * store-release that clears the locked bit and create lock
-		 * sequentiality; this is because not all
-		 * clear_pending_set_locked() implementations imply full
-		 * barriers.
-		 */
-		if (val & _Q_LOCKED_MASK) {
-			atomic_cond_read_acquire(&lock->val,
-						 !(VAL & _Q_LOCKED_MASK));
-		}
-
-		/*
-		 * take ownership and clear the pending bit.
-		 *
-		 * *,1,0 -> *,0,1
-		 */
-		clear_pending_set_locked(lock);
-		qstat_inc(qstat_lock_pending, true);
-		return;
+	/*
+	 * If we observe any contention; undo and queue.
+	 */
+	if (unlikely(val & ~_Q_LOCKED_MASK)) {
+		if (!(val & _Q_PENDING_MASK))
+			clear_pending(lock);
+		goto queue;
 	}
 
 	/*
-	 * If pending was clear but there are waiters in the queue, then
-	 * we need to undo our setting of pending before we queue ourselves.
+	 * We're pending, wait for the owner to go away.
+	 *
+	 * 0,1,1 -> 0,1,0
+	 *
+	 * this wait loop must be a load-acquire such that we match the
+	 * store-release that clears the locked bit and create lock
+	 * sequentiality; this is because not all
+	 * clear_pending_set_locked() implementations imply full
+	 * barriers.
+	 */
+	if (val & _Q_LOCKED_MASK)
+		atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_MASK));
+
+	/*
+	 * take ownership and clear the pending bit.
+	 *
+	 * 0,1,0 -> 0,0,1
 	 */
-	if (!(val & _Q_PENDING_MASK))
-		clear_pending(lock);
+	clear_pending_set_locked(lock);
+	qstat_inc(qstat_lock_pending, true);
+	return;
 
 	/*
 	 * End of pending bit optimistic spinning and beginning of MCS