Re: panic with tcp timers

On Mon, Jun 20, 2016 at 11:55:55AM +0200, Julien Charbon wrote:
> 
>  Hi,
> 
> On 6/20/16 9:39 AM, Gleb Smirnoff wrote:
> > On Fri, Jun 17, 2016 at 11:27:39AM +0200, Julien Charbon wrote:
> > J> > Comparing stable/10 and head, I see two changes that could
> > J> > affect that:
> > J> > 
> > J> > - callout_async_drain
> > J> > - switch to READ lock for inp info in tcp timers
> > J> > 
> > J> > That's why you are in To, Julien and Hans :)
> > J> > 
> > J> > We continue investigating, and I will keep you updated.
> > J> > However, any help is welcome. I can share cores.
> > 
> > Now, spending some time with cores and adding a bunch of
> > extra CTRs, I have a sequence of events that lead to the
> > panic. In short, the bug is in the callout system. It seems
> > to be not relevant to the callout_async_drain, at least for
> > now. The transition to READ lock unmasked the problem, that's
> > why NetflixBSD 10 doesn't panic.
> > 
> > The panic requires heavy contention on the TCP info lock.
> > 
> > [CPU 1] the callout fires, tcp_timer_keep entered
> > [CPU 1] blocks on INP_INFO_RLOCK(&V_tcbinfo);
> > [CPU 2] schedules the callout
> > [CPU 2] tcp_discardcb called
> > [CPU 2] callout successfully canceled
> > [CPU 2] tcpcb freed
> > [CPU 1] unblocks... panic
> > 
> > When the lock was WLOCK, all contenders were resumed in a
> > sequence they came to the lock. Now, that they are readers,
> > once the lock is released, readers are resumed in a "random"
> > order, and this allows tcp_discardcb to go before the old
> > running callout, and this unmasks the panic.
> 
>  Highly interesting.  I should be able to reproduce that (will be useful
> for testing the corresponding fix).
> 
>  Fix proposal:  If callout_async_drain() returns 0 (fail) (instead of 1
> (success) here) when the callout cancellation is a success _but_ the
> callout is current running, that should fix it.
> 
>  For the history:  It comes back to my old callout question:
> 
>  Does _callout_stop_safe() is allowed to return 1 (success) even if the
> callout is still currently running;  a.k.a. it is not because you
> successfully cancelled a callout that the callout is not currently running.
> 
>  We did propose a patch to make _callout_stop_safe() returns 0 (fail)
> when the callout is currently running:
> 
> callout_stop() should return 0 when the callout is currently being
> serviced and indeed unstoppable
> https://reviews.freebsd.org/differential/changeset/?ref=62513&whitespace=ignore-most
> 
>  But this change impacted too many old code paths and was interesting
> only for TCP timers and thus was abandoned.

Look at callout_stop CS_MIGRBLOCK flag and the fix in sleepq_check_timeout().
Or, at least, do not allow this use of callout_stop() to rot again, after
previous dozen regressions and fixes there.