From: Peter Zijlstra <peterz@infradead.org>
To: will.deacon@arm.com, mingo@kernel.org
Cc: linux-kernel@vger.kernel.org, longman@redhat.com,
andrea.parri@amarulasolutions.com, tglx@linutronix.de,
Peter Zijlstra <peterz@infradead.org>
Subject: [RFC][PATCH 2/3] locking/qspinlock: Rework some comments
Date: Wed, 26 Sep 2018 13:01:19 +0200 [thread overview]
Message-ID: <20180926111307.457488877@infradead.org> (raw)
In-Reply-To: 20180926110117.405325143@infradead.org
[-- Attachment #1: peterz-qspinlock-opt-1.patch --]
[-- Type: text/plain, Size: 2598 bytes --]
While working my way through the code again; I felt the comments could
use help.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/locking/qspinlock.c | 40 ++++++++++++++++++++++++++++------------
1 file changed, 28 insertions(+), 12 deletions(-)
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -326,16 +326,23 @@ void queued_spin_lock_slowpath(struct qs
/*
* trylock || pending
*
- * 0,0,0 -> 0,0,1 ; trylock
- * 0,0,1 -> 0,1,1 ; pending
+ * 0,0,* -> 0,1,* -> 0,0,1 pending, trylock
*/
val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
+
/*
- * If we observe any contention; undo and queue.
+ * If we observe contention, there was a concurrent lock.
+ *
+ * Undo and queue; our setting of PENDING might have made the
+ * n,0,0 -> 0,0,0 transition fail and it will now be waiting
+ * on @next to become !NULL.
*/
if (unlikely(val & ~_Q_LOCKED_MASK)) {
+
+ /* Undo PENDING if we set it. */
if (!(val & _Q_PENDING_MASK))
clear_pending(lock);
+
goto queue;
}
@@ -466,7 +473,7 @@ void queued_spin_lock_slowpath(struct qs
* claim the lock:
*
* n,0,0 -> 0,0,1 : lock, uncontended
- * *,*,0 -> *,*,1 : lock, contended
+ * *,0,0 -> *,0,1 : lock, contended
*
* If the queue head is the only one in the queue (lock value == tail)
* and nobody is pending, clear the tail code and grab the lock.
@@ -474,16 +481,25 @@ void queued_spin_lock_slowpath(struct qs
*/
/*
- * In the PV case we might already have _Q_LOCKED_VAL set.
+ * In the PV case we might already have _Q_LOCKED_VAL set, because
+ * of lock stealing; therefore we must also allow:
*
- * The atomic_cond_read_acquire() call above has provided the
- * necessary acquire semantics required for locking.
- */
- if (((val & _Q_TAIL_MASK) == tail) &&
- atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL))
- goto release; /* No contention */
+ * n,0,1 -> 0,0,1
+ *
+ * Note: at this point: (val & _Q_PENDING_MASK) == 0, because of the
+ * above wait condition, therefore any concurrent setting of
+ * PENDING will make the uncontended transition fail.
+ */
+ if ((val & _Q_TAIL_MASK) == tail) {
+ if (atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL))
+ goto release; /* No contention */
+ }
- /* Either somebody is queued behind us or _Q_PENDING_VAL is set */
+ /*
+ * Either somebody is queued behind us or _Q_PENDING_VAL got set
+ * which will then detect the remaining tail and queue behind us
+ * ensuring we'll see a @next.
+ */
set_locked(lock);
/*
next prev parent reply other threads:[~2018-09-26 11:30 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-26 11:01 [RFC][PATCH 0/3] locking/qspinlock: Improve determinism for x86 Peter Zijlstra
2018-09-26 11:01 ` [RFC][PATCH 1/3] locking/qspinlock: Re-order code Peter Zijlstra
2018-10-01 17:17 ` Will Deacon
2018-09-26 11:01 ` Peter Zijlstra [this message]
2018-10-01 17:17 ` [RFC][PATCH 2/3] locking/qspinlock: Rework some comments Will Deacon
2018-10-01 19:10 ` Peter Zijlstra
2018-10-02 13:20 ` Will Deacon
2018-10-02 13:43 ` Peter Zijlstra
2018-09-26 11:01 ` [RFC][PATCH 3/3] locking/qspinlock: Optimize for x86 Peter Zijlstra
2018-09-26 16:30 ` Waiman Long
2018-09-26 17:54 ` Peter Zijlstra
2018-09-27 7:29 ` Peter Zijlstra
2018-09-26 20:52 ` Andrea Parri
2018-09-27 7:17 ` Peter Zijlstra
2018-09-27 7:47 ` Andrea Parri
2018-09-27 7:59 ` Peter Zijlstra
2018-09-27 8:13 ` Andrea Parri
2018-09-27 8:57 ` Peter Zijlstra
2018-09-27 12:16 ` David Laight
2018-10-01 17:17 ` Will Deacon
2018-10-01 20:00 ` Peter Zijlstra
2018-10-02 13:19 ` Will Deacon
2018-10-02 14:14 ` Peter Zijlstra
2018-10-02 12:31 ` Andrea Parri
2018-10-02 13:22 ` Will Deacon
2018-10-02 13:44 ` Andrea Parri
2018-09-26 15:01 ` [RFC][PATCH 0/3] locking/qspinlock: Improve determinism " Sebastian Andrzej Siewior
2018-09-26 15:08 ` Thomas Gleixner
2018-09-26 15:38 ` Sebastian Andrzej Siewior
2018-09-26 16:20 ` Waiman Long
2018-09-26 17:51 ` Peter Zijlstra
2018-09-26 23:21 ` Waiman Long
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