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

[Jan Kara <jack@suse.cz>]

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@redhat.com>
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > CC: Nick Piggin <npiggin@kernel.dk>
> > 
> > ---
> > 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.
  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)
  wait for reqs_active to get to 0

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)
  check ctx->dead

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
> 
> > +	if (ctx->dead) {
> > +		spin_unlock_irq(&ctx->ctx_lock);
> > +		ret = -EINVAL;
> > +		goto out_put_req;
> > +	}
> >  	aio_run_iocb(req);
> >  	if (!list_empty(&ctx->run_list)) {
> >  		/* drain the run list */
> 
> thanks,
> milton
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR