netdev - Re: Soft lockup issue in Linux 4.1.9

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Message-ID: <1443821560.32531.60.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Fri, 02 Oct 2015 14:32:40 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Eric Dumazet <edumazet@...gle.com>,
	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 Fri, 2015-10-02 at 23:04 +0200, Thomas Gleixner wrote:
> On Fri, 2 Oct 2015, Eric Dumazet wrote:
> > On Fri, 2015-10-02 at 22:04 +0200, Thomas Gleixner wrote:
> > 
> > > What makes sure, that the timer cannot be readded while that timer
> > > callback is running?
> > 
> > What is exactly your question ?
> 
> CPU0   	  	       		CPU1
> 
> timer expires
>   callback
> 				add timer
>   timer_pending() == true
>   ===> del_timer_sync()
> 
> I was just curious how this is prevented as I got lost in the
> networking code as usual :)

Sure ;)

I believe this can not happen for following reasons :

mod_timer_pinned() is used only when req is created, while timer cannot
possibly be running on the same req. The _pinned part is critical
because we set the req->refcnt _after_ starting the timer,
to avoid being visible and caught from rcu lookups in hash tables.

Then, timer might be modified only by mod_timer_pending() from
tcp_check_req() : This should not re-start timer if another cpu is in
the timer callback.

Thanks



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