linux-kernel - Re: [PATCH] backing_dev: Fix hung task on sync

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Date:	Sun, 16 Mar 2014 13:20:58 -0700
From:	"dbasehore ." <dbasehore@...omium.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Darrick J. Wong" <darrick.wong@...cle.com>,
	Kees Cook <keescook@...omium.org>,
	linux-fsdevel@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	bleung@...omium.org, sonnyrao@...omium.org,
	Luigi Semenzato <semenzato@...omium.org>
Subject: Re: [PATCH] backing_dev: Fix hung task on sync

Also, the difference it would make is fix the issue for when a
delayed_work is used for both immediate work (mod_delayed_work(0)) and
delayed work.

On Sun, Mar 16, 2014 at 12:13 PM, dbasehore . <dbasehore@...omium.org> wrote:
> There's already behavior that is somewhat like that with the current
> implementation. If there's an item on a workqueue, it could run at any
> time. From the perspective of the driver/etc. that is using the
> workqueue, there should be no difference between work being on the
> workqueue and the kernel triggering a schedule right after the work is
> removed from the workqueue, but before the work function has done
> anything.
>
> So to reiterate, calling mod_delayed_work on something that is already
> in the workqueue has two behaviors. One, the work is dispatched before
> mod_delayed_work can remove it from the workqueue. Two,
> mod_delayed_work removes it from the workqueue and sets the timer (or
> not in the case of 0). The behavior of the proposed change should be
> no different than the first behavior.
>
> This should not introduce new behavior from the perspective of the
> code using delayed_work. It is true that there is a larger window of
> time between when you call mod_delayed_work and when an already queued
> work item will run, but I don't believe that matters.
>
> The API will still make sense since we will only ever mod delayed work
> but not work that is no longer delayed (on the workqueue).
>
> On Sun, Mar 16, 2014 at 7:59 AM, Tejun Heo <tj@...nel.org> wrote:
>> On Sat, Mar 15, 2014 at 01:22:53PM -0700, dbasehore . wrote:
>>> mod_delayed_work currently removes a work item from a workqueue if it
>>> is on it. Correct me if I'm wrong, but I don't think that this is
>>> necessarily required for mod_delayed_work to have the current
>>> behavior. We should be able to set the timer while a delayed_work is
>>> currently on a workqueue. If the delayed_work is still on the
>>> workqueue when the timer goes off, everything is fine. If it has left
>>> the workqueue, we can queue it again.
>>
>> What different would that make w.r.t. this issue?  Plus, please note
>> that a work item may wait non-insignificant amount of time pending if
>> the workqueue is saturated to max_active.  Doing the above would make
>> mod_delayed_work()'s behavior quite fuzzy - the work item is modified
>> or queued to the specified time but if the timer has already expired,
>> the work item may execute after unspecified amount of time which may
>> be shorter than the new timeout.  What kind of interface would that
>> be?
>>
>> Thanks.
>>
>> --
>> tejun
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