Re: Soft lockup issue in Linux 4.1.9

From	Eric Dumazet <eric.dumazet@gmail.com>
Newsgroups	linux.kernel
Subject	Re: Soft lockup issue in Linux 4.1.9
Date	2015-10-02 23:40 +0200
Message-ID	<qfgdA-78j-3@gated-at.bofh.it> (permalink)
References	(3 earlier) <qeKx5-2WG-27@gated-at.bofh.it> <qeKGL-383-33@gated-at.bofh.it> <qfeOu-5fB-7@gated-at.bofh.it> <qffAS-6a6-17@gated-at.bofh.it> <qffKx-6Av-5@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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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Thread

Re: Soft lockup issue in Linux 4.1.9 Holger Hoffstätte   <holger.hoffstaette@googlemail.com> - 2015-10-01 13:50 +0200
  Re: Soft lockup issue in Linux 4.1.9 Eric Dumazet <edumazet@google.com> - 2015-10-01 14:00 +0200
    Re: Soft lockup issue in Linux 4.1.9 Andre Tomt <andre@tomt.net> - 2015-10-02 09:00 +0200
      Re: Soft lockup issue in Linux 4.1.9 Holger Hoffstätte   <holger.hoffstaette@googlemail.com> - 2015-10-02 09:20 +0200
        Re: Soft lockup issue in Linux 4.1.9 Wolfgang Walter <linux@stwm.de> - 2015-10-02 21:30 +0200
        Re: Soft lockup issue in Linux 4.1.9 "Thomas D." <whissi@whissi.de> - 2015-10-03 21:30 +0200
    Re: Soft lockup issue in Linux 4.1.9 Thomas Gleixner <tglx@linutronix.de> - 2015-10-02 22:10 +0200
      Re: Soft lockup issue in Linux 4.1.9 Eric Dumazet <eric.dumazet@gmail.com> - 2015-10-02 23:00 +0200
        Re: Soft lockup issue in Linux 4.1.9 Thomas Gleixner <tglx@linutronix.de> - 2015-10-02 23:10 +0200
          Re: Soft lockup issue in Linux 4.1.9 Eric Dumazet <eric.dumazet@gmail.com> - 2015-10-02 23:40 +0200

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