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

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Message-ID: <20180802085938.GA13473@lst.de>
Date:   Thu, 2 Aug 2018 10:59:38 +0200
From:   Christoph Hellwig <hch@....de>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>, Avi Kivity <avi@...lladb.com>,
        linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] aio: add a iocb refcount

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.