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.
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