linux-kernel - Re: [PATCH 0/5] fuse: handle release synchronously (v4)

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Message-ID: <CAJfpegvS+rmFqGgvHq183Z-MxLAwcgEB57LQxAwy3QAe5CaVwg@mail.gmail.com>
Date:	Thu, 16 Oct 2014 15:43:52 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Maxim Patlasov <mpatlasov@...allels.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Anand Avati <avati@...ster.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Michael j Theall <mtheall@...ibm.com>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>
Subject: Re: [PATCH 0/5] fuse: handle release synchronously (v4)

On Thu, Oct 16, 2014 at 12:31 PM, Maxim Patlasov
<mpatlasov@...allels.com> wrote:

> Something as simple as:
>
> int main(int argc, char *argv[])
> {
>     int fd = open(argv[1], O_RDWR);
>     fork();
> }
>
> may easily dive into fuse_try_sync_release() concurrently and both observe
> file->f_count == 2. Then both return falling back to sending the release
> asynchronously. This makes sync/async behaviour unpredictable even for
> well-behaved applications which don't do any esoteric things like racing i/o
> with close or exiting while a descriptor is in-flight in a unix domain
> socket.
>
> I cannot see any way to recognise last flush without help of VFS layer, can
> you?

No.

One idea is to change ->flush() so it's responsible for fput()-ing the
file.  That way we could take control of the actual refcount
decrement.  There are only 20 flush instances in the tree, so it
wouldn't be a huge change.

Thanks,
Miklos
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