On Fri, Feb 20, 2015 at 07:28:16PM +0100, Manfred Spraul wrote: > Hi Oleg, > > my example was bad, let's continue with your example. > > And: If sem_lock() needs another smp_xmb(), then we must add it: > Some apps do not have a user space hot path, i.e. it seems that on > some setups, we have millions of calls per second. > If there is a race, then it will happen. > > I've tried to merge your example: > > > > int X = 0, Y = 0; > > > > void func(void) > > { > > bool ll = rand(); > > > > if (ll) { > > spin_lock(&local); > > if (!spin_is_locked(&global)) > > goto done; > > spin_unlock(&local); > > } > > ll = false; > > spin_lock(&global); > > spin_unlock_wait(&local); > > done: > > smp_rmb(); <<<<<<<<<<<<<<< > > BUG_ON(X != Y); > > > > ++X; ++Y; > > > > if (ll) > > spin_unlock(&local); > > else > > spin_unlock(&global); > > } > I agree, we need the smp_rmb(). > I'll write a patch. > > >We need the full barrier to serialize STORE's as well, but probably we can > >rely on control dependancy and thus we only need rmb(). > Do we need a full barrier or not? > > I don't manage to create a proper line of reasoning.
This has to be one of the more bizarre forms of Dekker's algorithm that I have seen. ;-) I am going to have to put this through one of the tools... Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/