Messages in this thread | | | Date | Mon, 17 Feb 2014 16:45:11 -0800 | From | Josh Triplett <> | Subject | Re: [PATCH tip/core/rcu 5/6] Documentation/memory-barriers.txt: Need barriers() for some control dependencies |
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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" <paulmck@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 <paulmck@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.
> 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".)
- Josh Triplett
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