linux-kernel - Re: [GIT PULL] First batch of KVM changes for 4.1

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Message-ID: <20150423115117.GA32371@amt.cnet>
Date:	Thu, 23 Apr 2015 08:51:17 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Gleb Natapov <gleb@...nel.org>, kvm list <kvm@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Andrew Lutomirski <luto@...nel.org>
Subject: Re: [GIT PULL] First batch of KVM changes for 4.1

On Thu, Apr 23, 2015 at 11:13:23AM +0200, Paolo Bonzini wrote:
> 
> 
> On 22/04/2015 23:21, Marcelo Tosatti wrote:
> > On Mon, Apr 20, 2015 at 01:27:58PM -0700, Andy Lutomirski wrote:
> >> On Mon, Apr 20, 2015 at 9:59 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> >>>
> >>>
> >>> On 17/04/2015 22:18, Marcelo Tosatti wrote:
> >>>> The bug which this is fixing is very rare, have no memory of a report.
> >>>>
> >>>> In fact, its even difficult to create a synthetic reproducer.
> >>>
> >>> But then why was the task migration notifier even in Jeremy's original
> >>> code for Xen?  Was it supposed to work even on non-synchronized TSC?
> >>>
> >>> If that's the case, then it could be reverted indeed; but then why did
> >>> you commit this patch to 4.1?  Did you think of something that would
> >>> cause the seqcount-like protocol to fail, and that turned out not to be
> >>> the case later?  I was only following the mailing list sparsely in March.
> >>
> >> I don't think anyone ever tried that hard to test this stuff.  There
> >> was an infinte loop that Firefox was triggering as a KVM guest
> >> somewhat reliably until a couple months ago in the same vdso code.  :(
> > 
> > https://bugzilla.redhat.com/show_bug.cgi?id=1174664
> 
> That was the missing volatile in an asm.  Older compilers didn't catch
> it. :(

How do you know that? It looks like memory corruption (look at the
pattern at the end).


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