netdev - Re: [PATCH 2/2] ipv4 igmp: use del_timer_sync instead of del_timer in ip_mc

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Date:	Sat, 28 Sep 2013 03:15:32 +0400
From:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
To:	Salam Noureddine <noureddine@...stanetworks.com>
Cc:	Stephen Hemminger <stephen@...workplumber.org>,
	"David S. Miller" <davem@...emloft.net>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 2/2] ipv4 igmp: use del_timer_sync instead of del_timer in ip_mc_down

On Fri, Sep 27, 2013 at 04:04:00PM -0700, Salam Noureddine wrote:
> 				I don't quite understand the reason
> for having __in_dev_put decrement the refcnt without destroying the
> in_device in case it reaches 0. If the timer handler assumes it cannot
> be the last one to hold a reference then that would mean it doesn't
> need the reference in the first place.

I would like to explain this, because this can result in a big mistake.

Timer takes reference, when it _can_ be the last sometimes.

When we do del_timer() and know for sure that someone holds refcnt, we can just
decrese it. In this cae it is obvious: we sit in context whcih deals with in_dev,
so that reference from timer acannot be the last: caller of the function holds refcnt.

But in another places timer _expires_ and that last reference is dropped by in_dev_put().

Alexey
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