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

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Date:	Thu, 23 Apr 2015 14:02:29 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Marcelo Tosatti <mtosatti@...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 23/04/2015 13:51, Marcelo Tosatti wrote:
>>> > > 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).

I suspect some kind of operator error there, it makes no sense.

On the other hand, bug 1178975 is much clearer and the symptoms are the
same.  In that bug, you can see that the same kernel source works on f20
(package version 3.17.7-200.fc20.x86_64) and fails on f21 (package
version 3.17.7-300.fc21.x86_64).  Of course the compiler is different.
The newer one hoists the lsl out of the loop; if you get a CPU migration
at the wrong time, the cpu != cpu1 condition will always be true the
loop will never exit.

Paolo
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