Re: Soft lockup issue in Linux 4.1.9

[Eric Dumazet]

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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