Re: [PATCH] aio: ensure access to ctx->ring_pages is correctly serialised

[Sasha Levin]

On 03/24/2014 03:07 PM, Benjamin LaHaise wrote:
> On Mon, Mar 24, 2014 at 02:22:06PM -0400, Sasha Levin wrote:
> > On 03/21/2014 02:35 PM, Benjamin LaHaise wrote:
> > > Hi all,
> > >
> > > Based on the issues reported by Tang and Gu, I've come up with the an
> > > alternative fix that avoids adding additional locking in the event read
> > > code path.  The fix is to take the ring_lock mutex during page migration,
> > > which is already used to syncronize event readers and thus does not add
> > > any new locking requirements in aio_read_events_ring().  I've dropped
> > > the patches from Tang and Gu as a result.  This patch is now in my
> > > git://git.kvack.org/~bcrl/aio-next.git tree and will be sent to Linus
> > > once a few other people chime in with their reviews of this change.
> > > Please review Tang, Gu.  Thanks!
> >
> > Hi Benjamin,
> >
> > This patch seems to trigger:
> >
> > [  433.476216] ======================================================
> > [  433.478468] [ INFO: possible circular locking dependency detected ]
> ...
>
> Yeah, that's a problem -- thanks for the report.  The ring_lock mutex can't
> be nested inside of mmap_sem, as aio_read_events_ring() can take a page
> fault while holding ring_mutex.  That makes the following change required.
> I'll fold this change into the patch that caused this issue.

Yup, that does the trick.

Could you please add something to document why this is a trylock instead of a lock? If
I were reading the code there's no way I'd understand what's the reason behind it
without knowing of this bug report.


Thanks,
Sasha

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