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

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Date:   Thu, 2 Jul 2020 13:17:51 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andreas Gruenbacher <agruenba@...hat.com>
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 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.

               Linus