Re: [2/2] fs: Fix race between io_destroy() and io_submit() in AIO

[Jan Kara]

  Hi,

On Tue 15-02-11 12:50:32, Milton Miller wrote:
> On Tue, 15 Feb 2011 about 11:16:16 -0600, Jan Kara wrote:
> > On Tue 15-02-11 12:59:24, Milton Miller wrote:
> > > > A race can occur when io_submit() races with io_destroy():
> > > > 
> > > >  CPU1						CPU2
> > > > io_submit()
> > > >   do_io_submit()
> > > >     ...
> > > >     ctx = lookup_ioctx(ctx_id);
> > > > 						io_destroy()
> > > >     Now do_io_submit() holds the last reference to ctx.
> > > >     ...
> > > >     queue new AIO
> > > >     put_ioctx(ctx) - frees ctx with active AIOs
> > > > 
> > > > We solve this issue by checking whether ctx is being destroyed
> > > > in AIO submission path after adding new AIO to ctx. Then we
> > > > are guaranteed that either io_destroy() waits for new AIO or
> > > > we see that ctx is being destroyed and bail out.
> > > > 
> > > > Reviewed-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
> > > > Signed-off-by: Jan Kara <jack@xxxxxxx>
> > > > CC: Nick Piggin <npiggin@xxxxxxxxx>
> > > > 
> > > > ---
> > > > fs/aio.c |   15 +++++++++++++++
> > > >  1 files changed, 15 insertions(+), 0 deletions(-)
> > > > 
> > > > diff --git a/fs/aio.c b/fs/aio.c
> > > > index b4dd668..0244c04 100644
> > > > --- a/fs/aio.c
> > > > +++ b/fs/aio.c
> > > > @@ -1642,6 +1642,21 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
> > > >  		goto out_put_req;
> > > >  
> > > >  	spin_lock_irq(&ctx->ctx_lock);
> > > > +	/*
> > > > +	 * We could have raced with io_destroy() and are currently holding a
> > > > +	 * reference to ctx which should be destroyed. We cannot submit IO
> > > > +	 * since ctx gets freed as soon as io_submit() puts its reference.
> > > > +	 * The check here is reliable since io_destroy() sets ctx->dead before
> > > > +	 * waiting for outstanding IO. Thus if we don't see ctx->dead set here,
> > > > +	 * io_destroy() waits for our IO to finish.
> > > > +	 * The check is inside ctx->ctx_lock to avoid extra memory barrier
> > > > +	 * in this fast path...
> > > > +	 */
> > > 
> > > When reading this comment, and with all of the recient discussions I
> > > had with Paul in the smp ipi thread (especially with resepect to third
> > > party writes), I looked to see that the spinlock was paired with the
> > > spinlock to set dead in io_destroy.  It is not.   It took me some time
> > > to find that the paired lock is actually in wait_for_all_aios.  Also,
> > > dead is also set in aio_cancel_all which is under the same spinlock.
> > > 
> > > Please update this lack of memory barrier comment to reflect the locking.
> 
> This locking description is wrong:
> 
> >   Hum, sorry but I don't understand. The above message wants to say that
> > io_destroy() does
> >   ctx->dead = 1
> >   barrier (implied by a spin_unlock)
> 
> no spin_unlock only does a release barrier.
> 
> >   wait for reqs_active to get to 0
> 
> This read can move up into the spinlocked region (up to the lock acquire).
> 
> > 
> > while io_submit() does
> >   increment reqs_active
> >   barrier (implied by a spin_lock - on a different lock but that does not
> > matter as we only need the barrier semantics)
> 
> No only an acquire barrier, old writes can move into the spinlock region
> 
> >   check ctx->dead
> 
> the increment can move down past this check to the unlock here.
  Ah OK, you're right. I was typing too fast and thinking too slow ;).

> > So if io_submit() gets past ctx->dead check, io_destroy() will certainly
> > wait for our reference in reqs_active to be released.
> > 
> > I don't see any lock pairing needed here... But maybe I miss something.
> > 
> > 								Honza
> 
> spin lock and unlock are only half barriers as described in 
> Documentation/memory-barriers.txt
> 
> 
> Now, as I said, the code is ok because the active count is read and
> written under ctx->ctx_lock, and aio_cancel_all sets dead under
> that lock.
> 
> But the comment needs to reflect that and not just the the code is
> under in some random spin_lock region instead of a memory barrier,
> which is not sufficient.   Bad lock descriptions leads to making bad
> code in the future, either through copying it to another context or
> though future work removing the additional constraints not mentioned.
> 
> So please, comment which locks are being used here, as what
> you described is not enough.
  Yep, I'll improve the comment. Thanks for explanation.

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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