From: Will Deacon <will.deacon@arm.com>
To: Waiman Long <waiman.long@hp.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
Thomas Gleixner <tglx@linutronix.de>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Scott J Norton <scott.norton@hp.com>,
Douglas Hatch <doug.hatch@hp.com>
Subject: Re: [PATCH 2/4] locking/qrwlock: Reduce reader/writer to reader lock transfer latency
Date: Tue, 7 Jul 2015 10:17:11 +0100 [thread overview]
Message-ID: <20150707091711.GA23879@arm.com> (raw)
In-Reply-To: <559ADBCD.6020803@hp.com>
On Mon, Jul 06, 2015 at 08:49:33PM +0100, Waiman Long wrote:
> On 07/06/2015 02:23 PM, Will Deacon wrote:
> > I've just finished rebasing my arm64 qrwlock stuff, but I think it will
> > conflict with these patches. Do you mind if I post them for review anyway,
> > so we can at least co-ordinate our efforts?
>
> Yes, sure. I would also like to coordinate my changes with yours to
> minimize conflict. BTW, I just got 2 tip-bot messages about the commits:
>
> locking/qrwlock: Better optimization for interrupt context readers
> locking/qrwlock: Rename functions to queued_*()
>
> So I need to rebase my patches also.
Yeah, I've been carrying those two on my branch as well, but everything
should rebase cleanly.
> >> ---
> >> kernel/locking/qrwlock.c | 12 ++++--------
> >> 1 files changed, 4 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> >> index 81bae99..ecd2d19 100644
> >> --- a/kernel/locking/qrwlock.c
> >> +++ b/kernel/locking/qrwlock.c
> >> @@ -88,15 +88,11 @@ void queue_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
> >> arch_spin_lock(&lock->lock);
> >>
> >> /*
> >> - * At the head of the wait queue now, wait until the writer state
> >> - * goes to 0 and then try to increment the reader count and get
> >> - * the lock. It is possible that an incoming writer may steal the
> >> - * lock in the interim, so it is necessary to check the writer byte
> >> - * to make sure that the write lock isn't taken.
> >> + * At the head of the wait queue now, increment the reader count
> >> + * and wait until the writer, if it has the lock, has gone away.
> >> + * At ths stage, it is not possible for a writer to remain in the
> >> + * waiting state (_QW_WAITING). So there won't be any deadlock.
> >> */
> >> - while (atomic_read(&lock->cnts)& _QW_WMASK)
> >> - cpu_relax_lowlatency();
> > Thinking about it, can we kill _QW_WAITING altogether and set (cmpxchg
> > from 0) wmode to _QW_LOCKED in the write_lock slowpath, polling (acquire)
> > rmode until it hits zero?
>
> No, this is how we make the lock fair so that an incoming streams of
> later readers won't block a writer from getting the lock.
But won't those readers effectively see that the lock is held for write
(because we set wmode to _QW_LOCKED before the existing reader had drained)
and therefore fall down the slow-path and get held up on the spinlock?
Will
next prev parent reply other threads:[~2015-07-07 9:17 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-06 15:43 [PATCH 0/4] locking/qrwlock: Improve qrwlock performance Waiman Long
2015-07-06 15:43 ` [PATCH 1/4] locking/qrwlock: Better optimization for interrupt context readers Waiman Long
2015-07-06 15:43 ` [PATCH 2/4] locking/qrwlock: Reduce reader/writer to reader lock transfer latency Waiman Long
2015-07-06 18:23 ` Will Deacon
2015-07-06 19:49 ` Waiman Long
2015-07-07 9:17 ` Will Deacon [this message]
2015-07-07 11:17 ` Peter Zijlstra
2015-07-07 11:49 ` Will Deacon
2015-07-07 14:30 ` Waiman Long
2015-07-07 17:27 ` Will Deacon
2015-07-07 18:10 ` Will Deacon
2015-07-07 21:29 ` Waiman Long
2015-07-08 9:52 ` Peter Zijlstra
2015-07-08 17:19 ` Will Deacon
2015-07-06 15:43 ` [PATCH 3/4] locking/qrwlock: Reduce writer to writer " Waiman Long
2015-07-06 15:43 ` [PATCH 4/4] locking/qrwlock: Use direct MCS lock/unlock in slowpath Waiman Long
2015-07-07 11:24 ` Peter Zijlstra
2015-07-07 21:59 ` Waiman Long
2015-07-07 22:13 ` Peter Zijlstra
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