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

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Message-ID: <20140930191933.GC5011@tucsk.piliscsaba.szeredi.hu>
Date:	Tue, 30 Sep 2014 21:19:33 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Anand Avati <avati@...ster.org>,
	Maxim Patlasov <MPatlasov@...allels.com>,
	"open list:FUSE: FILESYSTEM..." <fuse-devel@...ts.sourceforge.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/5] fuse: handle release synchronously (v4)

On Tue, Sep 30, 2014 at 12:04:08PM -0700, Linus Torvalds wrote:
> On Mon, Sep 29, 2014 at 10:33 PM, Anand Avati <avati@...ster.org> wrote:
> >
> > In general that sounds reasonable. The problem (as described in the original
> > thread, at http://sourceforge.net/p/fuse/mailman/message/29889055/)
> 
> From a quick look at that thread, the solution is clear: you *must*
> flush your write buffers in the "flush" function.
> 
> The fact that you must flush write buffers multiple times if people
> have done "dup()" is a complete non-issue. Just flush them each time.
> There is no "how do I differentiate the first flush and
> the second flush?" The answer is that you don't, and that you MUST
> NOT. You need to flush on both (or more). Trying to distinguish first
> vs second is broken, and would be wrogn *anyway*. There is no possible
> situation where it could validaly matter, and you simply cannot tell.
> 
> Don't do any data structure cleanups, that's for "release()". But yes,
> you do have to flush write buffers at flush time (and return IO errors
> if they happen). That's very much the point of flush.

What about flock(2), FL_SETLEASE, etc semantics (which are the sane ones,
compared to the POSIX locks shit which mandates release of lock on each close(2)
instead of "when all [duplicate] descriptors have been closed")?

You have to do that from ->release(), there's no question about that.  And while
I haven't looked at the wording of the standards, doing that synchronously with
the last close is a pretty decent thing to expect.

Thanks,
Miklos
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