[PATCH] KEYS: Reduce smp_mb() calls in key_put()

[Jarkko Sakkinen]

On Sat, May 03, 2025 at 11:19:21PM +0100, David Howells wrote:
> Jarkko Sakkinen <jarkko at kernel.org> wrote:
> 
> > Oops, my bad (order swap), sorry. Should have been:
> > 	
> >  				spin_unlock_irqrestore(&key->user->lock, flags);
> > 			} else {
> > 				smp_mb(); /* key->user before FINAL_PUT set. */
> >  			}
> > 			set_bit(KEY_FLAG_FINAL_PUT, &key->flags);
> > 
> > Should spin_lock()/unlock() be good enough or what good does smp_mb() do
> > in that branch? Just checking if I'm missing something before sending
> > fixed version.
> 
> spin_unlock() is semi-permeable, so stuff after it can leak into the inside of
> it up as far as the spin_lock().  With your change, the garbage collector can
> no longer guarantee that key_put() will have done with accessing key->user
> when it sees KEY_FLAG_FINAL_PUT is set.
> 
> So, NAK on this patch, I think.  If you want a second opinion, I'd suggest
> waving it in front of Paul McKenney.

Fair enough.

If I revisit this in a way or another,  I'll cc to him for comments but
for this I'll buy what you said.

> 
> Possibly we only need smp_mb() in the IN_QUOTA branch in key_put().
> 
> David

Thank you for the comments.

BR, Jarkko