Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter

[Andy Lutomirski]

On Thu, Nov 21, 2019 at 12:25 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Thu, Nov 21, 2019 at 10:53:03AM -0800, Fenghua Yu wrote:
>
> > 4. Otherwise, re-calculate addr to point the 32-bit address which contains
> >    the bit and operate on the bit. No split lock.
>
> That sounds confused, Even BT{,CRS} have a RmW size. There is no
> 'operate on the bit'.
>
> Specifically I hard rely on BTSL to be a 32bit RmW, see commit:
>
>   7aa54be29765 ("locking/qspinlock, x86: Provide liveness guarantee")
>

Okay, spent a bit of time trying to grok this.  Are you saying that
LOCK BTSL suffices in a case where LOCK BTSB or LOCK XCHG8 would not?
On x86, all the LOCK operations are full barriers, so they should
order with adjacent normal accesses even to unrelated addresses,
right?

I certainly understand that a *non-locked* RMW to a bit might need to
have a certain width to get the right ordering guarantees, but those
aren't affected by split-lock detection regardless.

--Andy