On Mon, Sep 14, 2015 at 01:35:20PM +0200, Peter Zijlstra wrote:
>
> Sorry for being tardy, I had a wee spell of feeling horrible and then I
> procrastinated longer than I should have.
>
> On Fri, Sep 11, 2015 at 01:45:07PM +0100, Will Deacon wrote:
>
> > Peter, any thoughts? I'm not au fait with the x86 memory model, but what
> > Paul's saying is worrying.
>
> Right, so Paul is right -- and I completely forgot (I used to know about
> that).
>
> So all the TSO archs (SPARC-TSO, x86 (!OOSTORE) and s390) can do
> smp_load_acquire()/smp_store_release() with just barrier(), and while:
>
> smp_store_release(&x);
> smp_load_acquire(&x);
>
> will provide full order by means of the address dependency,
>
> smp_store_release(&x);
> smp_load_acquire(&y);
>
> will not. Because the one reorder TSO allows is exactly that one.
>
> > Peter -- if the above reordering can happen on x86, then moving away
> > from RCpc is going to be less popular than I hoped...
>
> Sadly yes.. We could of course try and split LOCK from ACQUIRE again,
> but I'm not sure that's going to help anything except confusion.
This of course also means we need something like:
smp_mb__release_acquire()
which cannot be a no-op for TSO archs. And it might even mean it needs
to be the same as smp_mb__unlock_lock(), but I need to think more on
this.
The scenario is:
CPU0 CPU1
unlock(x)
smp_store_release(&x->lock, 0);
unlock(y)
smp_store_release(&next->lock, 1); /* next == &y */
lock(y)
while (!(smp_load_acquire(&y->lock))
cpu_relax();
Where the lock does _NOT_ issue a store to acquire the lock at all. Now
I don't think any of our current primitives manage this, so we should be
good, but it might just be possible.
And at the same time; having both:
smp_mb__release_acquire()
smp_mb__unlock_lock()
is quite horrible, for it clearly shows a LOCK isn't quite the same as
ACQUIRE :/
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