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

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Date:	Mon, 17 Mar 2014 10:40:20 -0400
From:	Tejun Heo <tj@...nel.org>
To:	"dbasehore ." <dbasehore@...omium.org>
Cc:	Jan Kara <jack@...e.cz>, Alexander Viro <viro@...to.linux.org.uk>,
	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

Hello,

On Sun, Mar 16, 2014 at 12:13:55PM -0700, dbasehore . 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.

It is different.  mod_delayed_work() *guarantees* that the target work
item will become pending for execution at least after the specified
time has passed.  What you're suggesting removes any semantically
consistent meaning of the API.

> 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.

No, mod_delayed_work() does *one* thing - the work item is queued for
the specified delay no matter the current state of the work item.  It
is *guaranteed* that the work item will go pending after the specified
time.  That is the sole meaning of the API.

> 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.

You're completely misunderstanding the API.  Plesae re-read it and
understand what it does first.

Thanks.

-- 
tejun
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