linux-kernel - (BUG?) round_jiffies() is non-monotonic on SMP

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Message-ID: <Pine.LNX.4.44L0.0811011205030.21703-100000@netrider.rowland.org>
Date:	Sat, 1 Nov 2008 12:14:41 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Kernel development list <linux-kernel@...r.kernel.org>
Subject: (BUG?) round_jiffies() is non-monotonic on SMP

Is it generally recognized that round_jiffies() can be non-monotonic on 
SMP systems?  By this, I mean that if cpu-a and cpu-b respectively do:

	ra = round_jiffies(ja);

		and

	rb = round_jiffies(jb);

then the ordering of ra and rb can be opposite the ordering of ja and 
jb.

If this is known, is it regarded as a potential problem?  It certainly
seems likely that some code somewhere depends on timeouts expiring in
the correct order.

Alan Stern

P.S.: As a related matter, it seems very odd that we don't have a 
round_jiffies_up() routine.  Surely there are plenty of places where it 
doesn't matter if an event is a little late but where the event must 
not be early.  (I know two such places offhand.)  Any objection to 
having one added?

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