linux-kernel - Re: [fs] inode_lru_isolate(): Move counter increment into spinlock section

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Date:	Thu, 19 Dec 2013 09:26:12 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Dave Chinner <david@...morbit.com>
cc:	Dave Chinner <dchinner@...hat.com>, linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [fs] inode_lru_isolate(): Move counter increment into spinlock
 section

On Thu, 19 Dec 2013, Dave Chinner wrote:

> On Wed, Dec 18, 2013 at 07:24:46PM +0000, Christoph Lameter wrote:
> > The counter increment in inode_lru_isolate is happening after
> > spinlocks have been dropped with preemption on using __count_vm_events
> > making counter increment races possible.
>
> That's a nasty, undocumented problem that __count_vm_events() has.

AFACIT that is a pretty well established and known issue. It only
affects cases where the fallback code for the counter increments is used.

> Nobody who is modifying the fs/inode.c code is likely to know about
> this, so just moving the code under an unrelated lock is not
> sufficient to prevent this from happening again. Hence I'd prefer
> that you just change it to use count_vm_events() rather than try to
> be tricksy by replacing the landmine in the code that we've already
> stepped on once.

I have a patchset here that is supposed to be merged soon that will detect
these cases.

Moving the code is IMHO the simplest solution. count_vm_events
will have to disable interrupts on platforms that do not support fast RMV
operations otherwise.

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