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

[Tim Deegan <tim@xxxxxxx>]

At 15:47 +0100 on 18 Aug (1503071247), Tim Deegan wrote:
> At 01:48 -0600 on 17 Aug (1502934495), Jan Beulich wrote:
> > >>> On 16.08.17 at 18:47, <andrew.cooper3@xxxxxxxxxx> wrote:
> > > 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).
> 
> FWIW, I don't think that's quite right.  The GCC docs I have say that
> "volatile" will stop the compiler from omitting an asm altogether, or
> hoisting it out of a loop (on the assumption that it will always
> produce the same output for the same inputs).  And that "the compiler
> can move even volatile 'asm' instructions relative to other code,
> including across jump instructions."

...and indeed: https://godbolt.org/g/KW19QR

Tim.

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