On Fri, May 18, 2018 at 08:30:27AM +1000, Benjamin Herrenschmidt wrote: > On Thu, 2018-05-17 at 14:23 -0500, Segher Boessenkool wrote: > > On Thu, May 17, 2018 at 01:06:10PM +1000, Benjamin Herrenschmidt wrote: > > > The current asm statement in __patch_instruction() for the cache flushes > > > doesn't have a "volatile" statement and no memory clobber. That means > > > gcc can potentially move it around (or move the store done by put_user > > > past the flush). > > > > volatile is completely superfluous here, except maybe as documentation: > > any asm without outputs is always volatile. > > I wasn't aware of that. I was drilled early on to always stick volatile > in my asm statements if they have any form of side effect :-)
If an asm without output was not marked automatically as having another side effect, every such asm would be immediately deleted ;-) Adding volatile as documentation for side effects can be good; it just doesn't do much (nothing, in fact) for asms without output as far as the compiler is concerned. > > (And the memory clobber does not prevent the compiler from moving the > > asm around, or duplicating it, etc., and neither does the volatile). > > It prevents load/stores from moving around doesn't it ? I wanted to > make sure the store of the instruction doesn't move in/pass the asm. If > you say that's not needed then ignore the patch. No, it's fine here, and you want either that or put exactly the memory you are touching in a constraint (probably overkill here). I just wanted to say that a "memory" clobber does nothing more than say the asm touches some unspecified memory; there is no magic other meaning to it. Your patch is correct, just the "volatile" part isn't needed, and the explanation was a bit cargo-culty ;-) Segher
