Re: [RFC] locking: rwbase: Take care of ordering guarantee for fastpath reader

[Peter Zijlstra]

On Fri, Sep 03, 2021 at 10:50:58PM +0800, Boqun Feng wrote:
> On Thu, Sep 02, 2021 at 01:55:29PM +0200, Peter Zijlstra wrote:
> [...]
> > >  	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
> > >  	rwbase_rtmutex_unlock(rtm);
> > >  }
> > > @@ -216,8 +229,14 @@ static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
> > >  	 */
> > >  	rwbase_set_and_save_current_state(state);
> > >  
> > > -	/* Block until all readers have left the critical section. */
> > > -	for (; atomic_read(&rwb->readers);) {
> > > +	/*
> > > +	 * Block until all readers have left the critical section.
> > > +	 *
> > > +	 * _acqurie() is needed in case that the reader side runs in the fast
> > > +	 * path, pairing with the atomic_dec_and_test() in rwbase_read_unlock(),
> > > +	 * provides ACQUIRE.
> > > +	 */
> > > +	for (; atomic_read_acquire(&rwb->readers);) {
> > >  		/* Optimized out for rwlocks */
> > >  		if (rwbase_signal_pending_state(state, current)) {
> > >  			__set_current_state(TASK_RUNNING);
> > 
> > I think we can restructure things to avoid this one, but yes. Suppose we
> > do:
> > 
> > 	readers = atomic_sub_return_relaxed(READER_BIAS, &rwb->readers);
> > 
> > 	/*
> > 	 * These two provide either an smp_mb() or an UNLOCK+LOCK
> 
> By "UNLOCK+LOCK", you mean unlock(->pi_lock) + lock(->wait_lock), right?
> This may be unrelated, but in our memory model only unlock+lock pairs on
> the same lock provide TSO-like ordering ;-) IOW, unlock(->pi_lock) +
> lock(->wait_lock) on the same CPU doesn't order reads before and after.

Hurpm.. what actual hardware does that? PPC uses LWSYNC for
ACQUIRE/RELEASE, and ARM64 has RCsc RELEASE+ACQUIRE ordering.

Both should provide RC-TSO (or stronger) for UNLOCK-A + LOCK-B.