Re: [PATCHSET] FUSE: extend FUSE to support more operations

[Tejun Heo <tj@kernel.org>]

Hello,

Miklos Szeredi wrote:
>> Why do you think using separate poll handle would be better?  And do you
>> still think the overhead is justifiable?
> 
> Because it would be a change in the semantics of the file handle.
> Previously it was just an opaque cookie that the kernel stored for the
> filesystem, not making any assumptions about it (like uniqueness).
> 
> OK, we can say that if the filesystems wants to implement poll, it has
> to make the file handle unique.  Also now the filesystem (or
> something) has to deal with races between poll notification and
> reuse of the file handle (release/open).
> 
> With a new poll handle we'd have more room to properly deal with these
> without overloading the file handle with extra requirements.
> 
> How about this: the poll handle is allocated by the kernel, not by the
> filesystem.  This guarantees uniqueness, so the filesystem cannot get
> this wrong.  Releasing the poll handle is still tricky, there could be
> various races... only the userspace filesystem knows if it has no
> outstanding notificiatons on a poll handle, so the release has to come
> after all outstanding notifications have been ack'ed.  Something like
> this:
> 
> (userspace <- kernel)
> 
> <- POLL-request(pollhandle) (alloc handle)
> -> POLL-reply
> ...
> -> POLL-notification(pollhandle)
> <- POLL-ack
> ...
> <- POLL_RELEASE(pollhandle)
> -> POLL_RELEASE-reply (free handle)

Hmm... yeah, allocating handle from kernel should work fine, but I
wouldn't worry about race here.  We can just use 64 bit and guarantee
that any handle won't be reused ever.

Thanks.

-- 
tejun