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

[David Howells]

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.

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

David