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

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Date:	Sat, 29 Aug 2015 10:08:30 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Al Viro <viro@...iv.linux.org.uk>,
	"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, Aug 29, 2015 at 7:11 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> If this needs to be kept, maybe then add following, to make sure
> we flush the list at most every BITS_PER_LONG files

Hmm.

I'm wondering if we should just make close_files() (or maybe even
filp_close()) use a synchronous fput().

Iirc, the reason we delay fput() is that we had some nasty issues for
the generic fput case. It was called from interrupt context by the aio
code, and just in general there's a lot of nasty cases that can cause
the final fput to happen (so there are lockdep issues with the mmap
locks because the last fput being from munmap etc).

Maybe I forget some detail - it's been several years by now - but I
think we could make the regular "close()" and "exit()" cases just use
the synchronous fput (it's called "__fput_sync()" and currently
explicitly limited to just kernel threads).

Al?

Because it feels all kinds of stupid to add things to the task-work
queue just to then remove it almost immediately again. And
close_files() is also called from various contexts. but the whole "put
the final 'files_struct' case is certainly not at all as special as
the 'put the final file'.

                    Linus
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