netdev - Re: [PATCH 1/3] io_uring: add support for async work inheriting files table

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Message-ID: <CAG48ez1akvnVpK3dMH4H=C2CsNGDZkDaxZEF2stGAPCnUcaa+g@mail.gmail.com>
Date:   Fri, 18 Oct 2019 20:50:12 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/3] io_uring: add support for async work inheriting files table

On Fri, Oct 18, 2019 at 8:16 PM Jens Axboe <axboe@...nel.dk> wrote:
> On 10/18/19 12:06 PM, Jann Horn wrote:
> > But actually, by the way: Is this whole files_struct thing creating a
> > reference loop? The files_struct has a reference to the uring file,
> > and the uring file has ACCEPT work that has a reference to the
> > files_struct. If the task gets killed and the accept work blocks, the
> > entire files_struct will stay alive, right?
>
> Yes, for the lifetime of the request, it does create a loop. So if the
> application goes away, I think you're right, the files_struct will stay.
> And so will the io_uring, for that matter, as we depend on the closing
> of the files to do the final reap.
>
> Hmm, not sure how best to handle that, to be honest. We need some way to
> break the loop, if the request never finishes.

A wacky and dubious approach would be to, instead of taking a
reference to the files_struct, abuse f_op->flush() to synchronously
flush out pending requests with references to the files_struct... But
it's probably a bad idea, given that in f_op->flush(), you can't
easily tell which files_struct the close is coming from. I suppose you
could keep a list of (fdtable, fd) pairs through which ACCEPT requests
have come in and then let f_op->flush() probe whether the file
pointers are gone from them...