Re: [PATCH v2 cmpxchg 12/13] sh: Emulate one-byte cmpxchg

Paul E. McKenney
2024-05-02T23:45:30Z

On Fri, May 03, 2024 at 12:24:47AM +0100, Al Viro wrote:
> On Thu, May 02, 2024 at 04:12:44PM -0700, Paul E. McKenney wrote:
>
> > > I'm probably missing your point, though - what mix of cmpxchg and
> > > smp_store_release on 8bit values?
> >
> > One of RCU's state machines uses smp_store_release() to start the
> > state machine (only one task gets to do this) and cmpxchg() to update
> > state beyond that point. And the state is 8 bits so that it and other
> > state fits into 32 bits to allow a single check for multiple conditions
> > elsewhere.
>
> Humm... smp_store_release() of 8bit on old alpha is mb + fetch 64bit + replace
> 8 bits + store 64bit...

Agreed, which is why Arnd is moving his patches ahead. (He and I
discussed this some weeks back, so not a surprise for him.)

For my part, I dropped 16-bit cmpxchg emulation when moving from the
RFC series to v1.

Thanx, Paul