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

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Date:	Mon, 31 Aug 2015 13:22:26 +0800
From:	yalin wang <yalin.wang2010@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	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 Aug 30, 2015, at 01:08, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> 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
> —
why not provide API like:
fput()
fput_nosync() ?

because synchronous version are reasonable and safe in most time,
let the user to select which version to use is more feasible, no matter if it is kthread or not.

Thanks


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