linux-kernel - Re: [PATCH] Export current_is

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Date:	Tue, 5 Dec 2006 13:57:53 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Roland Dreier <rdreier@...co.com>
Cc:	Andy Fleming <afleming@...escale.com>,
	"Maciej W. Rozycki" <macro@...ux-mips.org>,
	Ben Collins <ben.collins@...ntu.com>,
	linux-kernel@...r.kernel.org, Linus Torvalds <torvalds@...l.org>,
	Jeff Garzik <jeff@...zik.org>
Subject: Re: [PATCH] Export current_is_keventd() for libphy

On Tue, 05 Dec 2006 13:37:37 -0800
Roland Dreier <rdreier@...co.com> wrote:

>  > a) Ban the calling of flush_scheduled_work() from under rtnl_lock(). 
>  >    Sounds hard.
> 
> Unfortunate if this is happening a lot.  It seems like the most
> sensible fix -- flush_scheduled_work() is in effect calling into
> an unknown and changeable in the future set of functions (since it
> waits for them to finish), and it seems error-prone to hold a lock
> across such a call.

yes, I agree.  It's really bad to be calling flush_scheduled_work() with
any locks held at all.  Fragile, hard-to-maintain, source of
once-in-a-blue-moon failures, etc.  I guess lockdep will help.

But running flush_scheduled_work() from within dev_close() is a very
sensible thing to do, and dev_close is called under rtnl_lock().
davem is -> thattaway ;)

>  >    This will almost work, as long as it's done in workqueue.c with
>  >    appropriate locking.  The bug occurs when some other CPU is running
>  >    phy_change() right now - we'll end up freeing data which that CPU is
>  >    presently playing with.
>  > 
>  >    But perhaps we can take care of this within workqueue.c.  We need a
>  >    cancel function which will cancel the work and, if its callback is
>  >    presently executing it will block until that execution has completed.
> 
> I may be misunderstanding you, but this seems to deadlock in exactly
> the same way: if someone calls this cancel routine holding rtnl_lock,
> and the work function that will also take rtnl_lock has just started,
> it will get stuck when the work function tries to take rtnl_lock.

Ah.  The point is that the phy code doesn't want to flush _all_ pending
callbacks.  It only wants to flush its own one.  And its own one doesn't
take rtnl_lock().

IOW, the phy code has no interest in running some random other subsystem's
callback - it just wants to run its own.  Hence no deadlock.

Maybe the lesson here is that flush_scheduled_work() is a bad function.
It should really be flush_this_work(struct work_struct *w).  That is in
fact what approximately 100% of the flush_scheduled_work() callers actually
want to do.

hmm.
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