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

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Message-ID: <491BC87F.4050108@kernel.org>
Date:	Thu, 13 Nov 2008 15:26:07 +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.

Miklos Szeredi wrote:
>   0006-FUSE-implement-unsolicited-notification.patch
>   0007-FUSE-implement-poll-support.patch
> 
> This would be nice, but...  I don't really like the fact that it uses
> the file handle.  Could we have a separate "poll handle" that is
> returned in the POLL reply?

Eh... I replied too early for this.  I'm now trying to convert it to its
own handle but there is a rather serious problem.  It's usually much
easier to have the entity to be waken up registered before calling
->poll so that ->poll can use the same notification path from ->poll ans
for later.

However, if we allocate poll handle from ->poll and tell it to kernel
via reply, it creates two problem.  1. the entity which is to be waken
up can't be registered prior to calling ->poll as there's nothing to
identify it, 2. the interval from reply write and in-kernel polled
entity registration must be made atomic so that no notification can come
through inbetween.  #1 means that ->poll can't call the same
notification path from ->poll itself and #2 means that there needs to be
special provision from dev.c::fuse_dev_write() to
file.c::fuse_file_poll() so that atomicity can be guaranteed.  Both of
which can be done but I'm not really sure whether using a separate
handle would be a good idea even with the involved cost.

Why do you think using separate poll handle would be better?  And do you
still think the overhead is justifiable?

Thanks.

-- 
tejun
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