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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Tue, 11 Sep 2018 13:11:50 +0200
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Subject: Re: [Qemu-devel] [PATCH 1/6] qsp: drop atomics when using the
 seqlock
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To: "Emilio G. Cota" <cota@braap.org>
Cc: qemu-devel@nongnu.org, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Richard Henderson <rth@twiddle.net>, Eduardo Habkost <ehabkost@redhat.com>, =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>

On 10/09/2018 17:44, Emilio G. Cota wrote:
> On Mon, Sep 10, 2018 at 01:32:15 +0200, Paolo Bonzini wrote:
>> On 03/09/2018 19:18, Emilio G. Cota wrote:
>>> Using atomics here is a mistake since they're not guaranteed
>>> to compile.
>>
>> But isn't it technically a C11 data race if you don't use atomics?
> 
> Yes, it's undefined behaviour.
> 
>> Could we make nocheck read/set degrade to just a volatile access when
>> used on a variable that is bigger than pointers, or perhaps always
>> except when using tsan?
> 
> But volatile wouldn't save you from undefined behaviour, would it?

Yeah, but 1) only on those hosts that cannot do CONFIG_ATOMIC64 2) we
pretty much already define what we expect from volatile.

Paolo

> A simpler and definitely correct alternative is to just use a
> spinlock instead of the seqlock also for reads when !CONFIG_ATOMIC64.
> We don't care about scalability on those rare hosts anyway, so
> I'd go with that.
> 
> Thanks,
> 
> 		Emilio
>