On Sun, 2015-08-02 at 00:29 +0200, Peter Zijlstra wrote:
> That's just gibberish, even in the same cacheline stuff can get
> reordered.
true dat
>
> So either we insert
> > + * memory barrier here and in the corresponding pv_wait_head()
> > + * function or we do an unconditional kick which is what is done here.
>
> why, why why ? You've added words, but you've not actually described
> what the problem is you're trying to fix.
>
> AFAICT the only thing we really care about here is that the load in
> question happens _after_ we observe SLOW, and that is still true.
>
> The order against the unlock is irrelevant.
>
> So we set ->state before we hash and before we set SLOW. Given that
> we've seen SLOW, we must therefore also see ->state.
>
> If ->state == halted, this means the CPU in question is blocked and the
> pv_node will not get re-used -- if it does get re-used, it wasn't
> blocked and we don't care either.
Right, if it does get re-used, we were burning SPIN_THRESHOLD and racing
only wastes a few spins, afaict. In fact this is explicitly stated:
/*
* The unlocker should have freed the lock before kicking the
* CPU. So if the lock is still not free, it is a spurious
* wakeup and so the vCPU should wait again after spinning for
* a while.
*/
The thing I like about this patch is that it simplifies the
pv_kick/pv_wait flow, not having to depend on minutia like ->state
checking. But the condition about spurious wakeups is already there, so
really nothing changes.
Thanks,
Davidlohr
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