linux-kernel - Re: [RFC 2/4] fs: Add IOCB_NOIO flag for generic_file_read

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Date:   Fri, 3 Jul 2020 11:45:29 +0200
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 2/4] fs: Add IOCB_NOIO flag for generic_file_read_iter

On Thu, Jul 2, 2020 at 10:18 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Jul 2, 2020 at 12:58 PM Andreas Gruenbacher <agruenba@...hat.com> wrote:
> > > Of course, if you want to avoid both new reads to be submitted _and_
> > > avoid waiting for existing pending reads, you should just set both
> > > flags, and you get the semantics you want. So for your case, this may
> > > not make any difference.
> >
> > Indeed, in the gfs2 case, waiting for existing pending reads should be
> > fine. I'll send an update after some testing.
>
> Do note that "wait for pending reads" very much does imply "wait for
> those reads to _complete_".
>
> And maybe the IO completion handler itself ends up having to finalize
> something and take the lock to do that?
>
> So in that case, even just "waiting" will cause a deadlock. Not
> because the waiter itself needs the lock, but because the thing it
> waits for might possibly need it.
>
> But in many simple cases, IO completion shouldn't need any filesystem
> locks. I just don't know the gfs2 code at all, so I'm not even going
> to guess. I just wanted to mention it.

Yes, that makes sense. Luckily gfs2 doesn't do any such locking on IO
completion.

Thanks,
Andreas