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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
