On Thu, 2014-02-06 at 20:06 -0800, Paul E. McKenney wrote: > On Thu, Feb 06, 2014 at 11:58:22PM +0100, Torvald Riegel wrote: > > On Thu, 2014-02-06 at 13:55 -0800, Paul E. McKenney wrote: > > > On Thu, Feb 06, 2014 at 10:09:25PM +0100, Torvald Riegel wrote: > > > > On Thu, 2014-02-06 at 18:59 +0000, Will Deacon wrote: > > > > > To answer that question, you need to go and look at the definitions of > > > > > synchronises-with, happens-before, dependency_ordered_before and a whole > > > > > pile of vaguely written waffle to realise that you don't know. > > > > > > > > Are you familiar with the formalization of the C11/C++11 model by Batty > > > > et al.? > > > > http://www.cl.cam.ac.uk/~mjb220/popl085ap-sewell.pdf > > > > http://www.cl.cam.ac.uk/~mjb220/n3132.pdf > > > > > > > > They also have a nice tool that can run condensed examples and show you > > > > all allowed (and forbidden) executions (it runs in the browser, so is > > > > slow for larger examples), including nice annotated graphs for those: > > > > http://svr-pes20-cppmem.cl.cam.ac.uk/cppmem/ > > > > > > > > It requires somewhat special syntax, but the following, which should be > > > > equivalent to your example above, runs just fine: > > > > > > > > int main() { > > > > atomic_int foo = 0; > > > > atomic_int bar = 0; > > > > atomic_int baz = 0; > > > > {{{ { > > > > foo.store(42, memory_order_relaxed); > > > > bar.store(1, memory_order_seq_cst); > > > > baz.store(42, memory_order_relaxed); > > > > } > > > > ||| { > > > > r1=baz.load(memory_order_seq_cst).readsvalue(42); > > > > r2=foo.load(memory_order_seq_cst).readsvalue(0); > > > > } > > > > }}}; > > > > return 0; } > > > > > > > > That yields 3 consistent executions for me, and likewise if the last > > > > readsvalue() is using 42 as argument. > > > > > > > > If you add a "fence(memory_order_seq_cst);" after the store to foo, the > > > > program can't observe != 42 for foo anymore, because the seq-cst fence > > > > is adding a synchronizes-with edge via the baz reads-from. > > > > > > > > I think this is a really neat tool, and very helpful to answer such > > > > questions as in your example. > > > > > > Hmmm... The tool doesn't seem to like fetch_add(). But let's assume that > > > your substitution of store() for fetch_add() is correct. Then this shows > > > that we cannot substitute fetch_add() for atomic_add_return(). > > > > It should be in this example, I believe. > > You lost me on this one. I mean that in this example, substituting fetch_add() with store() should not change meaning, given that what the fetch_add reads-from seems irrelevant. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
