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Date: Wed, 8 Nov 2017 11:40:02 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: David Howells <dhowells@redhat.com>
cc: torvalds@linux-foundation.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: Is there a race between __mod_timer() and del_timer()?
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On Wed, 8 Nov 2017, David Howells wrote:

> 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.

Correct, if two CPUs fiddle with the same timer concurrently then there is
no guaranteed outcome.

> A couple of possible alleviations:
> 
>  (1) Recheck timer_pending() before returning from __mod_timer().

That's just adding more instructions into that code path for a dubious
value.

>  (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.

Right.

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

Agreed.

Thanks,

	tglx