linux-kernel - Re: Soft lockup issue in Linux 4.1.9

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Date:	Fri, 2 Oct 2015 22:04:44 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Eric Dumazet <edumazet@...gle.com>
cc:	Holger Hoffstätte 
	<holger.hoffstaette@...glemail.com>,
	"David S. Miller" <davem@...emloft.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org,
	netdev <netdev@...r.kernel.org>
Subject: Re: Soft lockup issue in Linux 4.1.9

On Thu, 1 Oct 2015, Eric Dumazet wrote:
> On Thu, Oct 1, 2015 at 4:43 AM, Holger Hoffstätte
> <holger.hoffstaette@...glemail.com> wrote:
> > On 10/01/15 13:29, Eric Dumazet wrote:
> 
> >> commit 83fccfc3940c4a2db90fd7e7079f5b465cd8c6af
> >> Author: Eric Dumazet <edumazet@...gle.com>
> >> Date:   Thu Aug 13 15:44:51 2015 -0700
> >>
> >>     inet: fix potential deadlock in reqsk_queue_unlink()
> >>
> >>     When replacing del_timer() with del_timer_sync(), I introduced
> >>     a deadlock condition :
> >>
> >>     reqsk_queue_unlink() is called from inet_csk_reqsk_queue_drop()
> >>
> >>     inet_csk_reqsk_queue_drop() can be called from many contexts,
> >>     one being the timer handler itself (reqsk_timer_handler()).
> >>
> >>     In this case, del_timer_sync() loops forever.
> >>
> >>     Simple fix is to test if timer is pending.
> >>
> >>     Fixes: 2235f2ac75fd ("inet: fix races with reqsk timers")
> >>     Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> >>     Signed-off-by: David S. Miller <davem@...emloft.net>
> >
> > Whohoo! It applies/builds cleanly to 4.1.10-rc1 and is running as
> > we speak. Let's hope that this fixes the lockups.
> >
> 
> It definitely should help !

What makes sure, that the timer cannot be readded while that timer
callback is running?

Thanks,

	tglx