On Sun, Sep 24, 2017 at 05:12:13PM -0700, Linus Torvalds wrote: > On Sun, Sep 24, 2017 at 5:03 PM, Paul E. McKenney > <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > Mostly just paranoia on my part. I would be happy to remove it if > > you prefer. Or you or Steve can do so if that is more convenient. > > I really don't think it's warranted. The values are *stable*. There's > no subtle lack of locking, or some optimistic access to a value that > can change. > > The compiler can generate code to read the value fifteen billion > times, and it will always get the same value. > > Yes, maybe in between the different accesses, an NMI will happen, and > the value will be incremented, but then as the NMI exits, it will > decrement again, so the code that got interrupted will not actually > see the change. > > So the READ_ONCE() isn't "paranoia". It's just confusing. > > > And yes, consistency would dictate that the uses in rcu_nmi_enter() > > and rcu_nmi_exit() should be _ONCE(), particularly the stores to > > ->dynticks_nmi_nesting. > > NO. > > That would be just more of that confusion. > > That value is STABLE. It's stable even within an NMI handler. The NMI > code can read it, modify it, write it back, do a little dance, all > without having to care. There's no "_ONCE()" about it - not for the > readers, not for the writers, not for _anybody_. > > So adding even more READ/WRITE_ONCE() accesses wouldn't be > "consistent", it would just be insanity. > > Now, if an NMI happens and the value would be different on entry than > it is on exit, that would be something else. Then it really wouldn't > be stable wrt random users. But that would also be a major bug in the > NMI handler, as far as I can tell. > > So the reason I'm objecting to that READ_ONCE() is that it isn't > "paranoia", it's "voodoo programming". And we don't do voodoo > programming. I already agreed that the READ_ONCE() can be removed. But without the WRITE_ONCE(), the compiler could theoretically tear the store. Now we might be asserting that our compilers don't do that, and that if they ever do, we will file a bug or whatever. So are we asserting that our compilers won't ever do store tearing? Thanx, Paul