linux-kernel - Re: [PATCH 2/7] revoke: add f

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Date:	Fri, 9 Mar 2007 11:08:58 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Pekka J Enberg <penberg@...helsinki.fi>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	hch@...radead.org, alan@...rguk.ukuu.org.uk, serue@...ibm.com
Subject: Re: [PATCH 2/7] revoke: add f_light flag for struct file

On Friday 09 March 2007 09:14, Pekka J Enberg wrote:
> From: Pekka Enberg <penberg@...helsinki.fi>
>
> This adds a f_light flag to struct file to indicate that the file was
> looked up with fget_light().  Needed by revoke to ensure we don't
> close a file pointer while someone is using it without actually
> holding a reference.
>
> These bits were taken from the forced unmount patches by Tigran
> Aivazian.

Well, I disagree very much with this patch.

One of the interest of fget_light() is not dirtying file structure (avoiding 
atomic changes to f_count).

You add a 'flag' (4 bytes !) at the end of the file structure (so in a 
different cache line than the parts that are usually accessed in a fd_related 
syscall) and dirty this part at syscall entry and exit. Thats really a heavy 
price for supporting an unlikely revoke() syscall.

Also, the thing is racy.

( BTW, the whole revoke() concept is evil, especially if we want to avoid 
using inodes/dentries for some kind of pseudo files like sockets / pipes)

Cannot we use a flag in 'struct files_struct', set to one when the task is 
mono-thread (at task creation in fact), and set to 0 when it creates a new 
thread  (or when someone remotely access to this "struct files_struct" 
in /proc/pid/fd/... )

No need to set back this flag to 1 when task revert to mono-threaded, since 
this case is probably unlikely. This way we can be non racy.

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