Messages in this thread | | | Date | Tue, 2 Feb 2016 04:02:52 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [RFC][PATCH] mips: Fix arch_spin_unlock() |
| |
On Tue, Feb 02, 2016 at 12:19:04AM -0800, Linus Torvalds wrote: > On Tue, Feb 2, 2016 at 12:07 AM, Linus Torvalds > <torvalds@linux-foundation.org> wrote: > > > > So we *absolutely* should say that *OF COURSE* these things work: > > > > - CPU A: > > > > .. initialize data structure -> smp_wmb() -> WRITE_ONCE(ptr); > > > > - CPU B: > > > > smp_load_acquire(ptr) - we can rely on things behind "ptr" being initialized > > That's a bad example, btw. I shouldn't have made it be a "pointer", > because then we get the whole address dependency chain ordering > anyway. > > So instead of "ptr", read "state flag". It might just be an "int" that > says "data has been initialized". > > So > > .. initialize memory .. > smp_wmb(); > WRITE_ONCE(&is_initialized, 1); > > should pair with > > if (smp_load_acquire(&is_initialized)) > ... we can read and write the data, knowing it has been initialized .. > > exactly because "smp_wmb()" (cheap write barrier) might be cheaper > than "smp_store_release()" (expensive full barrier) and thus > preferred. > > So mixing ordering metaphors actually does make sense, and should be > entirely well-defined.
I don't believe that anyone is arguing that this particular example should not work the way that you want it to.
> There's likely less reason to do it the other way (ie > "smp_store_release()" on one side pairing with "LOAD_ONCE() + > smp_rmb()" on the other) since there likely isn't the same kind of > performance reason for that pairing. But even if we would never > necessarily want to do it, I think our memory ordering rules would be > *much* better for strongly stating that it has to work, than being > timid and trying to make the rules weak. > > Memory ordering is confusing enough as it is. We should not make > people worry more than they already have to. Strong rules are good.
The sorts of things I am really worried about are abominations like this (and far worse):
void thread0(void) { r1 = smp_load_acquire(&a); smp_store_release(&b, 1); }
void thread1(void) { r2 = smp_load_acquire(&b); smp_store_release(&c, 1); }
void thread2(void) { WRITE_ONCE(c, 2); smp_mb(); r3 = READ_ONCE(d); }
void thread3(void) { WRITE_ONCE(d, 1); smp_store_release(&a, 1); }
r1 == 1 && r2 == 1 && c == 2 && r3 == 0 ???
I advise discouraging this sort of thing. But it is your kernel, so what is your preference?
Thanx, Paul
| |