linux-kernel - Re: Regression on todays tip/master (commit 16f70beccf43)

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Date:   Sat, 25 Jul 2020 12:38:50 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: Regression on todays tip/master (commit 16f70beccf43)


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Joerg Roedel <joro@...tes.org> wrote:
> 
> > On Thu, Jul 23, 2020 at 04:46:04PM +0200, Thomas Gleixner wrote:
> > > Joerg Roedel <joro@...tes.org> writes:
> > > > while testing the SEV-ES patches on todays tip/master I triggered the BUG
> > > > below:
> > > >
> > > > [  137.629660] ------------[ cut here ]------------
> > > > [  137.630769] kernel BUG at kernel/signal.c:1917!
> > > > [  137.631796] invalid opcode: 0000 [#1] SMP NOPTI
> > > > [  137.632822] CPU: 3 PID: 28596 Comm: test_syscall_vd Not tainted 5.8.0-rc6-tip+ #3
> > > > [  137.634495] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
> > > > [  137.636236] RIP: 0010:do_notify_parent+0x25c/0x290
> > > > The guest had 4 VCPUs and ran 4 instances of the in-kernel x86-selftests
> > > > in a loop, together with 'perf top -e cycles:k'. As you can see in the
> > > > time-stamps, the issue triggered pretty quickly.
> > > >
> > > > Please let me know if you need more information or testing from my side.
> > > 
> > > Any chance to bisect this?
> > 
> > Yes, will try. I am currently testing plain -rc6, it seems to be fine.
> > Bisecting is next.
> 
> Given that you are perf stress-testing the box, some recent perf 
> commit would be the primary suspect - before doing a full bisect you 
> might want to try current perf/core (2ac5413e5edc) and its upstream 
> base: v5.8-rc3, to narrow it down.
> 
> But in principle any other commit could be the cause as well, the 
> assert suggests memory corruption - I don't think we changed anything 
> in the signal code.

On a second thought, I think this recent bug might have been the 
culprit:

  d136122f5845: ("sched: Fix race against ptrace_freeze_trace()")

Fixed in tip:sched/urgent - this is why it went away in your testing 
perhaps?

I'm sending this fix to Linus today.

Thanks,

	Ingo