Re: [PATCH 2/4] aio: add a iocb refcount

From: Christoph Hellwig
Date: Thu Aug 02 2018 - 12:03:06 EST

On Thu, Aug 02, 2018 at 12:19:56AM +0100, Al Viro wrote:
> On Mon, Jul 30, 2018 at 09:15:42AM +0200, Christoph Hellwig wrote:
> > This is needed to prevent races caused by the way the ->poll API works.
> > To avoid introducing overhead for other users of the iocbs we initialize
> > it to zero and only do refcount operations if it is non-zero in the
> > completion path.
> refcount_t looks like a bad match - you, AFAICS, have count 0 for everything
> except poll, while for poll you start with 2. That looks like
> if (iocb->shared && test_and_clear_bit(0, &iocb->shared))
> return;
> kill the sucker
> in your iocb_put() and initializing it to 1 in poll. No?

For non-poll we don't need the recount, so we can ignore that.

For poll we have two reference - submitting and wakee context. We
could replace the refcount_t with an atomic bitops - but it would

a) make the code harder to read than plain old refcounting
b) make the iocb bigger - at least for now as there is nothing
else we can share that unsigned long with.

Your call - I can respin it either way.