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

The fix I am working on now is doing exactly that. callout_reset must
return 0 if the callout is currently running.

What are the old paths impacted?

Totus tuus, Glebius.
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