From: Waiman Long <Waiman.Long@hpe.com>
To: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>
Cc: linux-kernel@vger.kernel.org, Davidlohr Bueso <dave@stgolabs.net>,
Jason Low <jason.low2@hp.com>, Dave Chinner <david@fromorbit.com>,
Peter Hurley <peter@hurleysoftware.com>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Scott J Norton <scott.norton@hpe.com>,
Douglas Hatch <doug.hatch@hpe.com>,
Waiman Long <Waiman.Long@hpe.com>
Subject: [PATCH v4 4/5] locking/rwsem: Improve reader wakeup code
Date: Tue, 17 May 2016 21:26:22 -0400 [thread overview]
Message-ID: <1463534783-38814-5-git-send-email-Waiman.Long@hpe.com> (raw)
In-Reply-To: <1463534783-38814-1-git-send-email-Waiman.Long@hpe.com>
In __rwsem_do_wake(), the reader wakeup code will assume a writer
has stolen the lock if the active reader/writer count is not 0.
However, this is not as reliable an indicator as the original
"< RWSEM_WAITING_BIAS" check. If another reader is present, the code
will still break out and exit even if the writer is gone. This patch
changes it to check the same "< RWSEM_WAITING_BIAS" condition to
reduce the chance of false positive.
Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
---
kernel/locking/rwsem-xadd.c | 11 ++++++++---
1 files changed, 8 insertions(+), 3 deletions(-)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 007814f..e3a7e06 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -148,9 +148,14 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
try_reader_grant:
oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
- /* A writer stole the lock. Undo our reader grant. */
- if (rwsem_atomic_update(-adjustment, sem) &
- RWSEM_ACTIVE_MASK)
+ /*
+ * If the count is still less than RWSEM_WAITING_BIAS
+ * after removing the adjustment, it is assumed that
+ * a writer has stolen the lock. We have to undo our
+ * reader grant.
+ */
+ if (rwsem_atomic_update(-adjustment, sem)
+ < RWSEM_WAITING_BIAS)
goto out;
/* Last active locker left. Retry waking readers. */
goto try_reader_grant;
--
1.7.1
next prev parent reply other threads:[~2016-05-18 1:27 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-18 1:26 [PATCH v4 0/5] [PATCH v3 0/4] locking/rwsem: Add reader-owned state to the owner field Waiman Long
2016-05-18 1:26 ` [PATCH v4 1/5] " Waiman Long
2016-06-06 17:18 ` Davidlohr Bueso
2016-06-06 20:03 ` Waiman Long
2016-06-06 21:02 ` Peter Zijlstra
2016-06-06 21:49 ` Waiman Long
2016-06-08 14:25 ` [tip:locking/core] " tip-bot for Waiman Long
2016-05-18 1:26 ` [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE() Waiman Long
2016-05-18 14:04 ` Davidlohr Bueso
2016-05-18 17:21 ` [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE Jason Low
2016-05-18 18:29 ` Waiman Long
2016-05-18 19:58 ` Jason Low
2016-05-19 22:21 ` Jason Low
2016-05-20 20:26 ` Waiman Long
2016-05-21 16:04 ` Peter Hurley
2016-05-22 10:42 ` Peter Zijlstra
2016-05-23 18:46 ` Jason Low
2016-05-23 19:44 ` Davidlohr Bueso
2016-05-23 20:15 ` Paul E. McKenney
2016-05-23 21:04 ` Davidlohr Bueso
2016-05-25 1:25 ` Waiman Long
2016-05-18 17:23 ` [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE() Jason Low
2016-06-08 14:25 ` [tip:locking/core] " tip-bot for Waiman Long
2016-05-18 1:26 ` [PATCH v4 3/5] locking/rwsem: Don't wake up one's own task Waiman Long
2016-05-18 10:30 ` Peter Zijlstra
2016-05-18 16:04 ` Waiman Long
2016-05-18 1:26 ` Waiman Long [this message]
2016-06-08 14:25 ` [tip:locking/core] locking/rwsem: Improve reader wakeup code tip-bot for Waiman Long
2016-05-18 1:26 ` [PATCH v4 5/5] locking/rwsem: Streamline the rwsem_optimistic_spin() code Waiman Long
2016-06-08 14:26 ` [tip:locking/core] " tip-bot for Waiman Long
2016-05-18 10:52 ` [PATCH v4 0/5] [PATCH v3 0/4] locking/rwsem: Add reader-owned state to the owner field Peter Zijlstra
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