On Mon, Sep 14, 2015 at 01:11:56PM +0100, Peter Zijlstra wrote: > On Mon, Sep 14, 2015 at 02:01:53PM +0200, Peter Zijlstra wrote: > > 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. > > So with a bit more through this seems fundamentally impossible, you > always needs some stores in a lock() implementation, the above for > instance needs to queue itself, otherwise CPU0 will not be able to find > it etc..
Which brings us back round to separating LOCK/UNLOCK from ACQUIRE/RELEASE. If we say that UNLOCK(foo) -> LOCK(bar) is ordered but RELEASE(baz) -> ACQUIRE(boz) is only ordered by smp_mb__release_acquire(), then I think we're in a position where we can at least build arbitrary locks portably out of ACQUIRE/RELEASE operations, even though I don't see any users of that macro in the imminent future. I'll have a crack at some documentation. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
