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

[Herbert Xu]

On Sat, May 03, 2025 at 05:39:16PM +0300, Jarkko Sakkinen wrote:
> On Wed, Apr 30, 2025 at 06:25:53PM +0300, Jarkko Sakkinen wrote:
> > Rely only on the memory ordering of spin_unlock() when setting
> > KEY_FLAG_FINAL_PUT under key->user->lock in key_put().
> > 
> > Signed-off-by: Jarkko Sakkinen <jarkko at kernel.org>
> > ---
> >  security/keys/key.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/security/keys/key.c b/security/keys/key.c
> > index 7198cd2ac3a3..aecbd624612d 100644
> > --- a/security/keys/key.c
> > +++ b/security/keys/key.c
> > @@ -656,10 +656,12 @@ void key_put(struct key *key)
> >  				spin_lock_irqsave(&key->user->lock, flags);
> >  				key->user->qnkeys--;
> >  				key->user->qnbytes -= key->quotalen;
> > +				set_bit(KEY_FLAG_FINAL_PUT, &key->flags);
> >  				spin_unlock_irqrestore(&key->user->lock, flags);
> > +			} else {
> > +				set_bit(KEY_FLAG_FINAL_PUT, &key->flags);
> > +				smp_mb(); /* key->user before FINAL_PUT set. */
> >  			}
> > -			smp_mb(); /* key->user before FINAL_PUT set. */
> > -			set_bit(KEY_FLAG_FINAL_PUT, &key->flags);
> 
> 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. */

You can use smp_mb__before_atomic here as it is equivalent to
smp_mb in this situation.

>  			}
> 			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.

I don't think spin_unlock alone is enough to replace an smp_mb.
A spin_lock + spin_unlock would be enough though.

However, looking at the bigger picture this smp_mb looks bogus.
What exactly is it protecting against?

The race condition that this is supposed to fix should have been
dealt with by the set_bit/test_bit of FINAL_PUT alone.  I don't
see any point in having this smb_mb at all.

Cheers,
-- 
Email: Herbert Xu <herbert at gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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