On Mon, Oct 05, 2020 at 05:35:19PM +0200, Peter Zijlstra wrote: > On Mon, Oct 05, 2020 at 11:16:39AM -0400, Alan Stern wrote: > > On Mon, Oct 05, 2020 at 04:13:13PM +0100, Will Deacon wrote: > > > > The failure to recognize the dependency in P0 should be considered a > > > > combined limitation of the memory model and herd7. It's not a simple > > > > mistake that can be fixed by a small rewrite of herd7; rather it's a > > > > deliberate choice we made based on herd7's inherent design. We > > > > explicitly said that control dependencies extend only to the code in the > > > > branches of an "if" statement; anything beyond the end of the statement > > > > is not considered to be dependent. > > > > > > Interesting. How does this interact with loops that are conditionally broken > > > out of, e.g. a relaxed cmpxchg() loop or an smp_cond_load_relaxed() call > > > prior to a WRITE_ONCE()? > > > > Heh -- We finesse this issue by not supporting loops at all! :-) > > Right, so something like: > > smp_cond_load_relaxed(x, !VAL); > WRITE_ONCE(*y, 1); > > Would be modeled like: > > r1 = READ_ONCE(*x); > if (!r1) > WRITE_ONCE(*y, 1); > > with an r1==0 constraint in the condition I suppose ? Yes, you got it! However, it is more efficient to use the "filter" clause to tell herd7 about executions that are to be discarded. Thanx, Paul
