Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

[Alan Stern]

On Fri, Jan 20, 2023 at 11:20:32AM -0800, Paul E. McKenney wrote:
> On Fri, Jan 20, 2023 at 01:37:51PM -0500, Alan Stern wrote:
> > srcu_read_unlock() does not need a full smp_mb().
> 
> That is quite possible, and that is what we are looking into.  And testing
> thus far agrees with you.  But the grace-period ordering constraints
> are quite severe, so this requires careful checking and severe testing.

If you're interested, I can provide a simple argument to show that the 
Fundamental Law of RCU would continue to hold with only a release fence.  
There is an added requirement: merely that synchronize_srcu() must have 
an smp_mb() somewhere after its final read of the unlock counters -- 
which your version of the algorithm already has.

Alan