linux-kernel - Re: [PATCHSET] FUSE: extend FUSE to support more operations

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Date:	Mon, 17 Nov 2008 18:17:28 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	fuse-devel@...ts.sourceforge.net, greg@...ah.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHSET] FUSE: extend FUSE to support more operations

Hello, Miklos.

I tried to implement poll as you suggested but it doesn't work because
poll actually is synchronous.  Please consider the following scenario.
A file system implements a file which supports poll and other file
operations and there's a single threaded client which does the
following.

1. open the file
2. do polling (timeout == 0) poll on the fd
3-1. if POLLIN, consume data and goto #2
3-2. if ! POLLIN, do a ioctl (or whatever) on the fd and goto #2

For a client with single stream of syscalls (single threaded), it's
generally guaranteed that the attempt to consume data is successful
after POLLIN unless the fd dies inbetween.  I don't think this is
something guaranteed in POSIX but for most in-kernel poll
implementations, this holds and I've seen good amount of code
depending on it.

To satisfy the above assumption, if ->poll is always asynchronous,
FUSE has to cache revents from previous ->poll attempts and clear it
when something which could have consumed data has occurred.
Unfortunately, in the above case, FUSE has no idea what constitutes
"consume data" but, double unfortunately, it can't take big hammer
approach (clearing on any access) either, because intervening non-data
consuming call like 3-2 above would mean that poll() will never
succeed.

Because data availability should be determined atomically && only the
filesystem knows when or how data availability changes, revents return
from ->poll() must be synchronous.

We can still use req -> reply approach where there's a flag telling
the FUSE server whether the request is synchronous or not but at that
point it seems just obfuscating to me.

So, ->poll() needs to be the combination of synchronous data
availability check + asynchronous notification which can be spurious
to implement the required semantics and I think the original interface
was much more natural for such functionality.

What do you think?

Thanks.

-- 
tejun
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