Messages in this thread | | | Date | Wed, 18 Feb 2015 23:43:17 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles |
| |
On Wed, Feb 18, 2015 at 08:14:01PM +0100, Manfred Spraul wrote:
> >spinlock_t local, global; > >bool force_global; > >bool my_lock(bool try_local) > >{ > > if (try_local) { > > spin_lock(&local); > > if (!spin_is_locked(&global)) { > > if (!force_global) { > > return true; > > } > > } > > spin_unlock(&local); > > > > > > spin_lock(&global); > > spin_unlock_wait(&local); > > return false; > > } > > > > void my_unlock(bool drop_local) > > { > > if (drop_local) > > spin_unlock(&local); > > else > > spin_unlock(&global); > > } > >}
> >Another question is do we need a barrier AFTER spin_unlock_wait(). I do not > >know what ipc/sem.c actually needs, but in general (I think) this does need > >mb(). Otherwise my_lock / my_unlock itself does not have the proper acq/rel > >semantics. For example, my_lock(false) can miss the changes which were done > >under my_lock(true).
> How could that happen? > I thought that > thread A: > protected_var = 1234; > spin_unlock(&lock_a) > > thread B: > spin_lock(&lock_b) > if (protected_var)
> is safe. i.e, there is no need that acquire and releases is done on the same pointer.
Well, just those four statements can of course be executed like:
CPU0 CPU1
spin_lock(&b) if (prot_var)
prot_var = 1; spin_unlock(&a);
And you would see the old var. Lock a and b are completely independent here.
Now of course the local/global thing in sysvsem is more complex.
As to what Oleg meant:
X := 0
CPU0 CPU1
spin_lock(&global); spin_lock(&local); X = 1; spin_unlock(&local); spin_unlock_wait(&local);
assert(X == 1); /* BOOM */
that assert can trigger, because spin_unlock_wait() are reads, the read of X can be lifted over and above, before the assignment of X on CPU1.
Again, the sysvsem code is slightly more complex, but I think Oleg is right, there is no guarantee you'll observe the full critical section of sem->lock if sem_lock() takes the slow path and does sem_wait_array(), because of the above.
| |