linux-kernel - Re: [PATCH] Export current_is

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Date:	Tue, 05 Dec 2006 13:37:37 -0800
From:	Roland Dreier <rdreier@...co.com>
To:	Andrew Morton <akpm@...l.org>
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

 > 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.

 >    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.

 - R.
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