Date: Mon, 20 Jun 2016 02:58:22 -0700 From: Gleb Smirnoff <glebius@FreeBSD.org> To: Julien Charbon <jch@freebsd.org> Cc: rrs@FreeBSD.org, hselasky@FreeBSD.org, net@FreeBSD.org, current@FreeBSD.org Subject: Re: panic with tcp timers Message-ID: <20160620095822.GJ1076@FreeBSD.org> In-Reply-To: <1d18d0e2-3e42-cb26-928c-2989d0751884@freebsd.org> References: <20160617045319.GE1076@FreeBSD.org> <1f28844b-b4ea-b544-3892-811f2be327b9@freebsd.org> <20160620073917.GI1076@FreeBSD.org> <1d18d0e2-3e42-cb26-928c-2989d0751884@freebsd.org>
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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> J> Highly interesting. I should be able to reproduce that (will be useful J> for testing the corresponding fix). J> 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> J> For the history: It comes back to my old callout question: J> 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> J> We did propose a patch to make _callout_stop_safe() returns 0 (fail) J> when the callout is currently running: J> J> callout_stop() should return 0 when the callout is currently being J> serviced and indeed unstoppable J> https://reviews.freebsd.org/differential/changeset/?ref=62513&whitespace=ignore-most J> 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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