On Mon, Aug 21, 2017 at 09:25:50PM +0200, Peter Zijlstra wrote: > On Mon, Aug 21, 2017 at 07:00:02PM +0100, Will Deacon wrote: > > > No, I meant _from_ the LL load, not _to_ a later load. > > > > Sorry, I'm still not following enough to give you a definitive answer on > > that. Could you give an example, please? These sequences usually run in > > a loop, so the conditional branch back (based on the status flag) is where > > the read-after-read comes in. > > > > Any control dependencies from the loaded data exist regardless of the status > > flag. > > Basically what Waiman ended up doing, something like: > > if (cmpxchg_relaxed(&pn->state, vcpu_halted, vcpu_hashed) != > vcpu_halted) > return; > > WRITE_ONCE(l->locked, _Q_SLOW_VAL); > > Where the STORE depends on the LL value being 'complete'.
Yup, that's ordered as you would expect. Thanks for the example! Will
