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.
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@xxxxxxx>
SUSE Labs, CR
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