From: Oleg Nesterov <oleg@tv-sign.ru>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
Ingo Molnar <mingo@elte.hu>,
Christoph Hellwig <hch@infradead.org>,
Andrew Morton <akpm@osdl.org>,
linux-kernel@vger.kernel.org,
Alan Stern <stern@rowland.harvard.edu>
Subject: Re: [PATCH 3/7] barrier: a scalable synchonisation barrier
Date: Sat, 3 Feb 2007 19:38:50 +0300 [thread overview]
Message-ID: <20070203163850.GA675@tv-sign.ru> (raw)
In-Reply-To: <20070201004849.GS2574@linux.vnet.ibm.com>
On 01/31, Paul E. McKenney wrote:
>
> QRCU as currently written (http://lkml.org/lkml/2006/11/29/330) doesn't
> do what you want, as it acquires the lock unconditionally. I am proposing
> that synchronize_qrcu() change to something like the following:
>
> void synchronize_qrcu(struct qrcu_struct *qp)
> {
> int idx;
>
> smp_mb();
>
> if (atomic_read(qp->ctr[0]) + atomic_read(qp->ctr[1]) <= 1) {
> smp_rmb();
> if (atomic_read(qp->ctr[0]) +
> atomic_read(qp->ctr[1]) <= 1)
> goto out;
> }
>
> mutex_lock(&qp->mutex);
> idx = qp->completed & 0x1;
> atomic_inc(qp->ctr + (idx ^ 0x1));
> /* Reduce the likelihood that qrcu_read_lock() will loop */
> smp_mb__after_atomic_inc();
I almost forgot. Currently this smp_mb__after_atomic_inc() is not strictly
needed, and the comment is correct. However, it becomes mandatory with your
optimization. Without this barrier, it is possible that both checks above
mutex_lock() will see the result of atomic_dec(), but not the atomic_inc().
So, may I ask you to also update this comment?
/*
* Reduce the likelihood that qrcu_read_lock() will loop
* AND
* make sure the second re-check above will see the result
* of atomic_inc() if it sees the result of atomic_dec()
*/
Something like this, I hope you will make it better.
And another note: this all assumes that STORE-MB-LOAD works "correctly", yes?
We have other code which relies on that, should not be a problem.
(Alan Stern cc'ed).
Oleg.
next prev parent reply other threads:[~2007-02-03 16:39 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-01-28 11:51 [PATCH 0/7] breaking the global file_list_lock Peter Zijlstra
2007-01-28 11:51 ` [PATCH 1/7] lockdep: lock_set_subclass - reset a held locks subclass Peter Zijlstra
2007-01-28 11:51 ` [PATCH 3/7] barrier: a scalable synchonisation barrier Peter Zijlstra
2007-01-28 14:39 ` Christoph Hellwig
2007-01-28 15:24 ` Ingo Molnar
2007-01-28 15:34 ` Christoph Hellwig
2007-01-31 19:12 ` Paul E. McKenney
2007-01-31 21:13 ` Oleg Nesterov
2007-01-31 21:30 ` Oleg Nesterov
2007-01-31 21:48 ` Peter Zijlstra
2007-01-31 23:32 ` Paul E. McKenney
2007-02-01 0:03 ` Peter Zijlstra
2007-02-01 0:48 ` Paul E. McKenney
2007-02-01 16:00 ` Oleg Nesterov
2007-02-01 21:38 ` Paul E. McKenney
2007-02-02 11:56 ` Oleg Nesterov
2007-02-02 12:01 ` Peter Zijlstra
2007-02-02 17:13 ` Paul E. McKenney
2007-02-03 16:38 ` Oleg Nesterov [this message]
2007-02-04 0:23 ` Paul E. McKenney
2007-02-04 3:24 ` Alan Stern
2007-02-04 5:46 ` Paul E. McKenney
2007-01-28 11:51 ` [PATCH 4/7] fs: break the file_list_lock for sb->s_files Peter Zijlstra
2007-01-28 14:43 ` Christoph Hellwig
2007-01-28 15:21 ` Ingo Molnar
2007-01-28 15:30 ` Christoph Hellwig
2007-01-28 15:32 ` Peter Zijlstra
2007-01-28 15:36 ` Christoph Hellwig
2007-01-28 15:44 ` Ingo Molnar
2007-01-28 16:25 ` Bill Huey
2007-01-28 11:51 ` [PATCH 5/7] fs: restore previous sb->s_files iteration semantics Peter Zijlstra
2007-01-28 11:51 ` [PATCH 6/7] schedule_on_each_cpu_wq() Peter Zijlstra
2007-01-28 11:51 ` [PATCH 7/7] fs: fixup filevec_add_drain_all Peter Zijlstra
2007-01-28 12:16 ` [PATCH 8/7] fs: free_write_pipe() fix Peter Zijlstra
2007-01-28 14:43 ` [PATCH 0/7] breaking the global file_list_lock Christoph Hellwig
2007-01-28 15:11 ` Christoph Hellwig
2007-01-28 15:29 ` Peter Zijlstra
2007-01-28 15:33 ` Christoph Hellwig
2007-01-29 13:32 ` Stephen Smalley
2007-01-29 18:02 ` Christoph Hellwig
2007-01-28 15:24 ` Ingo Molnar
2007-01-28 16:52 ` Martin J. Bligh
2007-01-28 17:04 ` lockmeter Christoph Hellwig
2007-01-28 17:38 ` lockmeter Martin J. Bligh
2007-01-28 18:01 ` lockmeter Bill Huey
2007-01-28 19:26 ` lockmeter Ingo Molnar
2007-01-28 21:17 ` lockmeter Ingo Molnar
2007-01-29 5:27 ` lockmeter Bill Huey
2007-01-29 10:26 ` lockmeter Bill Huey
2007-01-29 1:08 ` lockmeter Arjan van de Ven
2007-01-29 1:12 ` lockmeter Martin J. Bligh
2007-01-28 18:41 ` [PATCH 0/7] breaking the global file_list_lock Ingo Molnar
2007-01-28 20:38 ` Christoph Hellwig
2007-01-28 21:05 ` Ingo Molnar
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