Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}_atomic()

[Paul E. McKenney]

On Tue, Apr 23, 2019 at 03:40:03PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 23, 2019 at 06:30:10AM -0700, Paul E. McKenney wrote:
> > There are a great many that look like this:
> > 
> > 	smp_mb__before_atomic();
> > 	clear_bit(NFSD4_CLIENT_UPCALL_LOCK, &clp->cl_flags);
> > 	smp_mb__after_atomic();
> 
> Ooh, marvel at the comment describing the ordering there. Oh wait :-(
> So much for checkpatch.pl I suppose.

Especially if the code was in the kernel before checkpatch.pl started
asking for comments.  Which might or might not be the case with this
code.  No idea either way.

> I think the first is a release order for the 'LOCK' bit and the second
> is because of wake_up_bit() being a shitty interface.
> 
> So maybe that should've been:
> 
> 	clear_bit_unlock(NFSD4_CLIENT_UPCALL_LOCK, &clp->cl_flags);
> 	smp_mb__after_atomic();
> 	wake_up_bit(&clp->cl_flags, NFSD4_CLIENT_UPCALL_LOCK);
> 
> instead?

Quite possibly, but my brain is too fried to be sure.

							Thanx, Paul