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Re: [Qemu-devel] [PATCH 1/6] qsp: drop atomics when using the seqlock

From: Emilio G. Cota
Subject: Re: [Qemu-devel] [PATCH 1/6] qsp: drop atomics when using the seqlock
Date: Mon, 10 Sep 2018 11:44:42 -0400
User-agent: Mutt/1.9.4 (2018-02-28)

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?

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.



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