Re: [Xen-devel] [PATCH v2 2/4] xen/x86: Drop unnecessary barriers

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.08.17 at 18:47, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 16/08/17 16:23, Jan Beulich wrote:
>>>>> On 16.08.17 at 13:22, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> --- a/xen/arch/x86/mm/shadow/multi.c
>>> +++ b/xen/arch/x86/mm/shadow/multi.c
>>> @@ -3112,7 +3112,6 @@ static int sh_page_fault(struct vcpu *v,
>>>       * will make sure no inconsistent mapping being translated into
>>>       * shadow page table. */
>>>      version = atomic_read(&d->arch.paging.shadow.gtable_dirty_version);
>>> -    rmb();
>>>      walk_ok = sh_walk_guest_tables(v, va, &gw, error_code);
>> Isn't this supposed to make sure version is being read first? I.e.
>> doesn't this at least need to be barrier()?
> 
> atomic_read() is not free to be reordered by the compiler.  It is an asm
> volatile with a volatile memory reference.

Oh, right - I did forget about the volatiles there (since generally,
like in Linux, we appear to try to avoid volatile).

Jan


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