On Mon, Feb 17, 2014 at 07:29:55PM -0800, Josh Triplett wrote:
> On Mon, Feb 17, 2014 at 05:21:37PM -0800, Paul E. McKenney wrote:
> > On Mon, Feb 17, 2014 at 04:45:11PM -0800, Josh Triplett wrote:
> > > On Mon, Feb 17, 2014 at 04:17:40PM -0800, Paul E. McKenney wrote:
> > > > On Mon, Feb 17, 2014 at 04:02:47PM -0800, Josh Triplett wrote:
> > > > > On Mon, Feb 17, 2014 at 02:58:16PM -0800, Paul E. McKenney wrote:
> > > > > > On Mon, Feb 17, 2014 at 01:46:06PM -0800, Josh Triplett wrote:
> > > > > > > On Mon, Feb 17, 2014 at 01:26:52PM -0800, Paul E. McKenney wrote:
> > > > > > > > From: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
> > > > > > > >
> > > > > > > > Current compilers can "speculate" stores in the case where both
> > > > > > > > legs
> > > > > > > > of the "if" statement start with identical stores. Because the
> > > > > > > > stores
> > > > > > > > are identical, the compiler knows that the store will
> > > > > > > > unconditionally
> > > > > > > > execute regardless of the "if" condition, and so the compiler
> > > > > > > > is within
> > > > > > > > its rights to hoist the store to precede the condition. Such
> > > > > > > > hoisting
> > > > > > > > destroys the control-dependency ordering. This ordering can be
> > > > > > > > restored
> > > > > > > > by placing a barrier() at the beginning of each leg of the "if"
> > > > > > > > statement.
> > > > > > > > This commit adds this requirement to the control-dependencies
> > > > > > > > section.
> > > > > > > >
> > > > > > > > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> > > > > > >
> > > > > > > This is starting to become a rather unreasonable level of
> > > > > > > fighting the
> > > > > > > compiler. ("Threads cannot be implemented as a library" indeed.)
> > > > > > > This
> > > > > > > doesn't seem like a reasonable thing to require users to do. Is
> > > > > > > there
> > > > > > > really no other way to cope with this particular bit of "help"
> > > > > > > from the
> > > > > > > compiler?
> > > > > >
> > > > > > Well, we could use smp_mb() instead of barrier(), but that was the
> > > > > > sort of thing that Peter Zijlstra was trying to avoid.
> > > > >
> > > > > Yeah, that's not an improvement. The goal would be to make the code
> > > > > no
> > > > > more complex than it already needs to be with ACCESS_ONCE; changing
> > > > > "barrier()" to something else doesn't help (quite apart from smp_mb()
> > > > > being suboptimal).
> > > > >
> > > > > > That said, I do sympathize completely with your position here -- it
> > > > > > is
> > > > > > just that it is better to have our compiler-fights documented that
> > > > > > not, right?
> > > > >
> > > > > Sure, better to document them, but better still to not have them. Is
> > > > > there some other way we could avoid this one entirely?
> > > >
> > > > We could try change the standard so as to outlaw pulling common code
> > > > from
> > > > both legs of an "if" statement, but that will be a serious uphill
> > > > battle.
> > >
> > > And insufficient given widespread use of existing compilers.
> >
> > Fair point...
> >
> > > > Or perhaps do something to warn the developer about the possibility of
> > > > this happening.
> > > >
> > > > Other thoughts?
> > >
> > > Might be worth bringing this up with the GCC folks to find out if
> > > there's something obvious we're missing. (For non-obvious values of
> > > "obvious".)
> >
> > Non-obvious values of "obvious" -- I have no idea what that means, but
> > it does have a nice counter-intuitive sound to it, doesn't it? ;-)
>
> I'm hoping for something that we'll consider obvious in hindsight.
>
> > This conversation has started, albeit with much more noise and smoke
> > than signal or light.
>
> Whereabouts is that conversation taking place?
Here you go: https://lkml.org/lkml/2014/2/6/193
Not too bad, only about 80 messages thus far. Spirited at times, though. ;-)
Thanx, Paul
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