Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles

[Peter Zijlstra]

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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