linux-kernel - Re: [PATCH 3.10] aio: restore locking of ioctx list on removal

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Date:	Fri, 6 Dec 2013 09:53:43 -0500
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Mateusz Guzik <mguzik@...hat.com>
Cc:	Greg KH <gregkh@...uxfoundation.org>, stable@...r.kernel.org,
	Eryu Guan <eguan@...hat.com>, Jeff Moyer <jmoyer@...hat.com>,
	Kent Overstreet <kmo@...erainc.com>, linux-aio@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3.10] aio: restore locking of ioctx list on removal

Hi Greg, Mateusz,

On Fri, Dec 06, 2013 at 09:51:37AM +0100, Mateusz Guzik wrote:
> On Thu, Dec 05, 2013 at 05:03:47PM -0800, Greg KH wrote:
> > On Thu, Dec 05, 2013 at 11:09:02AM +0100, Mateusz Guzik wrote:
> > > Commit 36f5588905c10a8c4568a210d601fe8c3c27e0f0
> > > "aio: refcounting cleanup" resulted in ioctx_lock not being held
> > > during ctx removal, leaving the list susceptible to corruptions.
> > > 
> > > In mainline kernel the issue went away as a side effect of
> > > db446a08c23d5475e6b08c87acca79ebb20f283c "aio: convert the ioctx list to
> > > table lookup v3".
> > > 
> > > Fix the problem by restoring appropriate locking.
> > 
> > Why can't I just take db446a08c23d5475e6b08c87acca79ebb20f283c instead?
> > Does it not work well enough, or is there other issues involved in it
> > that would keep it out of stable?
> > 
> > Also, it seems like the performance increase of that patch would be good
> > to have backported, right?
> > 
> 
> Sorry, should have noted this in my original message:
> db446a08c23d5475e6b08c87acca79ebb20f283c is not trivial and applying it
> results in some conflicts, in addition to that the patch itself had bugs
> which were fixed in:
> da90382c2ec367aac88ff6aa76afb659ee0e4235
> f30d704fe1244c44a984d3d1f47bc648bcc6c9f7
> 77d30b14d24e557f89c41980011d72428514d729
> d9b2c8714aef102dea95544a8cd9372b21af463f
> 
> It may be that the most convienent way to deal with this backport would
> be to just sync the file with mainline.
> 
> As such, I think backporting is too risky at this stage.

Mateusz's analysis is correct.  Backporting db446a08c23d5475e6b08c87acca79ebb20f283c
would essentially mean redoing the entire patch and would end up being a  
bunch more untested code.  Please take Mateusz's patch which fixes the bug 
within a narrow scope.  It is applicable to 3.10 and 3.11.  For 3.12 the 
issue is fixed by other changes.  Feel free to add my Acked-by.

                -ben


> Additionally my understanding of Documentation/stable_kernel_rules.txt
> was that somewhat simpler patches are preferred.
> 
> So in the end I decided to fix the problem just by adding locking.
> 
> Unfortunately at this time I can't volunteer to do the work if
> backporting is preferred.
> 
> -- 
> Mateusz Guzik
> 
> --
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