From: Dave Chinner <david@fromorbit.com>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: mengcong <mc@linux.vnet.ibm.com>,
linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
Nick Piggin <npiggin@kernel.dk>
Subject: Re: [PATCH] VFS: br_write_lock locks on possible CPUs other than online CPUs
Date: Mon, 19 Dec 2011 16:00:38 +1100 [thread overview]
Message-ID: <20111219050037.GO23662@dastard> (raw)
In-Reply-To: <20111219041142.GH2203@ZenIV.linux.org.uk>
On Mon, Dec 19, 2011 at 04:11:42AM +0000, Al Viro wrote:
> On Mon, Dec 19, 2011 at 11:36:15AM +0800, mengcong wrote:
> > In a heavily loaded system, when frequently turning on and off CPUs, the
> > kernel will detect soft-lockups on multiple CPUs. The detailed bug report
> > is at https://lkml.org/lkml/2011/8/24/185.
> >
> > The root cause is that brlock functions, i.e. br_write_lock() and
> > br_write_unlock(), only locks/unlocks the per-CPU spinlock of CPUs that
> > are online, which means, if one online CPU is locked and then goes
> > offline, any later unlocking operation happens during its offline state
> > will not touch it; and when it goes online again, it has the incorrect
> > brlock state. This has been verified in current kernel.
> >
> > I can reproduce this bug on the intact 3.1 kernel. After my patch applied,
> > I've ran an 8-hours long test(test script provided by the bug reporter),
> > and no soft lockup happened again.
>
> Argh... OK, that's seriously nasty. I agree that this is broken, but
> your patch makes br_write_lock() very costly on kernels build with
> huge number of possible CPUs, even when it's run on a box with few
> CPUs ;-/
I fixed this problem with the XFS per-cpu superblock counters
(bit lock + counters in per-cpu structs) back in 2006. It basically
uses a lglock-like local/global locking structure and iterates them
using for_each_online_cpu().
I fixed it simply by registering a hotplug notifier and
draining/reinitialising counters on the appropriate event under a
global lock context. i.e. make CPU hotplug serialise against
concurrent lock operations. See commit e8234a68 ("[XFS] Add support
for hotplug CPUs...")
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
next prev parent reply other threads:[~2011-12-19 5:00 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-12-19 3:36 [PATCH] VFS: br_write_lock locks on possible CPUs other than online CPUs mengcong
2011-12-19 4:11 ` Al Viro
2011-12-19 5:00 ` Dave Chinner [this message]
2011-12-19 6:07 ` mengcong
2011-12-19 7:31 ` Srivatsa S. Bhat
2011-12-19 9:12 ` Stephen Boyd
2011-12-19 11:03 ` Srivatsa S. Bhat
2011-12-19 12:11 ` Al Viro
2011-12-19 20:23 ` Srivatsa S. Bhat
2011-12-19 20:52 ` Al Viro
2011-12-20 4:56 ` Srivatsa S. Bhat
2011-12-20 6:27 ` Al Viro
2011-12-20 7:28 ` Srivatsa S. Bhat
2011-12-20 9:37 ` mengcong
2011-12-20 10:36 ` Srivatsa S. Bhat
2011-12-20 11:08 ` Srivatsa S. Bhat
2011-12-20 12:50 ` Srivatsa S. Bhat
2011-12-20 14:06 ` Al Viro
2011-12-20 14:35 ` Srivatsa S. Bhat
2011-12-20 17:59 ` Al Viro
2011-12-20 19:12 ` Srivatsa S. Bhat
2011-12-20 19:58 ` Al Viro
2011-12-20 22:27 ` Dave Chinner
2011-12-20 23:31 ` Al Viro
2011-12-21 21:15 ` Srivatsa S. Bhat
2011-12-21 22:02 ` Al Viro
2011-12-21 22:12 ` Andrew Morton
2011-12-22 7:02 ` Al Viro
2011-12-22 7:20 ` Andrew Morton
2011-12-22 8:08 ` Al Viro
2011-12-22 8:17 ` Andi Kleen
2011-12-22 8:39 ` Al Viro
2011-12-22 8:22 ` Andi Kleen
2011-12-20 7:30 ` mengcong
2011-12-20 7:37 ` Srivatsa S. Bhat
2011-12-19 23:56 ` Dave Chinner
2011-12-20 4:05 ` Al Viro
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