linux-kernel - Re: [PATCH] Export current_is

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Message-Id: <20061207024211.be739a4a.akpm@osdl.org>
Date:	Thu, 7 Dec 2006 02:42:11 -0800
From:	Andrew Morton <akpm@...l.org>
To:	David Howells <dhowells@...hat.com>
Cc:	torvalds@...l.org, dhowells@...hat.com, macro@...ux-mips.org,
	rdreier@...co.com, afleming@...escale.com, ben.collins@...ntu.com,
	linux-kernel@...r.kernel.org, jeff@...zik.org
Subject: Re: [PATCH] Export current_is_keventd() for libphy

> On Thu, 07 Dec 2006 10:29:39 +0000 David Howells <dhowells@...hat.com> wrote:
> Andrew Morton <akpm@...l.org> wrote:
> 
> > I guess I don't understand exactly what problem the noautorel stuff is
> > trying to solve.  It _seems_ to me that in all cases we can simply stuff
> > the old `data' field in alongside the controlling work_struct or
> > delayed_work which wants to operate on it.
> 
> The problem is that you have to be able to guarantee that the data is still
> accessible once you clear the pending bit.  The pending bit is your only
> guaranteed protection, and once it is clear, the containing structure might be
> deallocated.
> 
> I would like to be able to get rid of the NAR bit too, but I'm not confident
> that in all cases I can.  It'll take a bit more study of the code to be able
> to do that.
> 

But anyone who is going to free the structure which contains the
work_struct would need to run flush_workqueue() beforehand, after having
ensured that the work won't reschedule itself.  So the
struct-which-contains-the-work_struct is safe during the callback's
execution.

If that's not being done then the code was buggy in 2.6.19, too..
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