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

[Emilio G. Cota]

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.

Thanks,

                Emilio