linux-kernel - Re: [PATCH] task_work: remove fifo ordering guarantee

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Date:	Sat, 5 Sep 2015 06:42:47 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Maciej Żenczykowski <maze@...gle.com>
Subject: Re: [PATCH] task_work: remove fifo ordering guarantee

On Sat, Sep 05, 2015 at 06:12:34AM +0100, Al Viro wrote:

> First of all, we'd better not count on e.g. delayed fput() *NOT* doing
> task_work_add() - we still need to check if any new work had been added.
> After all, final close() might very well have done a final mntput()
> on a lazy-unmounted filesystem, possibly leaving us with fs shutdown via
> task_work_add().  And if that sucker e.g. closes a socket, well, we are
> back to closing an opened struct file, with task_work_add() etc.
> 
> I'm a bit nervious about filp_close() (that sucker is exported and widely
> abused), but close_files()... sure, shouldn't be a problem.  And yes,
> we can teach __close_fd() to do the same.  I really don't understand what's
> the benefit, though - it's about the case when we are closing the last
> descriptor for given opened file, so I would be rather surprised if slower
> path taken on the way out to userland was not lost in noise...

OK, having found the beginning of the thread, I understand what is being
attempted, but... why the hell bother with FIFO in the first place?  AFAICS,
task_work_add() uses in VFS (final fput() and final mntput() alike)
do not care about the FIFO at all.

Sure, some out-of-tree mer^H^Hodule might rely on that.  So what?

IMO, unless we have a good in-tree reason for insisting on FIFO, dropping it
is the most obvious solution...
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