netdev - Is there a race between __mod_timer() and del

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Date:   Wed, 08 Nov 2017 10:15:41 +0000
From:   David Howells <dhowells@...hat.com>
To:     tglx@...utronix.de
cc:     dhowells@...hat.com, torvalds@...ux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Is there a race between __mod_timer() and del_timer()?

Is there a race between the optimisation for networking code in __mod_timer()
and del_timer() - or, at least, a race that matters?

Consider:

	CPU A				CPU B
	===============================	===============================
	[timer X is active]
	==>__mod_timer(X)
	if (timer_pending(timer))
		[Take the true path]
	-- IRQ --			==>del_timer(X)
					<==
	if (timer->expires == expires)
		[Take the true path]
	<==return 1
	[timer X is not active]

There's no locking to prevent this, but __mod_timer() returns without
restarting the timer.  I'm not sure this is a problem exactly, however, since
del_timer() *was* issued, and could've deleted the timer after __mod_timer()
returned.

A couple of possible alleviations:

 (1) Recheck timer_pending() before returning from __mod_timer().

 (2) Set timer->expires to jiffies in del_timer() - but since there's nothing
     preventing the optimisation in __mod_timer() from occurring concurrently
     with del_timer(), this probably won't help.

I think it might just be best to put a note in the comments in __mod_timer().

Thoughts?

David