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From: Sergey Fedorov <serge.fdrv@gmail.com>
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Date: Mon, 18 Jul 2016 20:07:34 +0300
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Subject: Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set
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To: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel@nongnu.org
Cc: sergey.fedorov@linaro.org, alex.bennee@linaro.org

On 18/07/16 20:00, Paolo Bonzini wrote:
>
> On 18/07/2016 18:57, Sergey Fedorov wrote:
>> On 18/07/16 19:53, Paolo Bonzini wrote:
>>> On 18/07/2016 18:52, Sergey Fedorov wrote:
>>>> So how are we going to use them?
>>> Instead of atomic_read/atomic_set when marking invalid TBs.
>> But shouldn't they be atomic to avoid reading torn writes?
> A torn write would probably fail to match anyway, but even if it doesn't
> it is indistinguishable from a race, isn't it?

I'm afraid, torn write can happen to be a false match against a wrong
TB. In case of a race with atomic access we either get the right TB or
an invalid one which couldn't match any valid CPU state. Probably, we
have to make sure (and document this) that TB invalidation process
cannot make a partially invalidated TB which can match any meaningful
CPU state.

>
> By the way, tb_cmp should also use volatile_read.

You are right, we must user the save type of access in tb_cmp().

Thanks,
Sergey