netdev - Re: [PATCH net-next] net: add annotation for sock_{lock,unlock}

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Message-ID: <20201117165830.e44pu3nd5vx3jzmz@ltop.local>
Date:   Tue, 17 Nov 2020 17:58:30 +0100
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        linux-sparse@...r.kernel.org
Subject: Re: [PATCH net-next] net: add annotation for sock_{lock,unlock}_fast

On Tue, Nov 17, 2020 at 09:38:45AM +0100, Paolo Abeni wrote:
> Hello,
> 
> Thank you for the feedback!
> 
> On Mon, 2020-11-16 at 23:27 +0100, Luc Van Oostenryck wrote:
> > > @@ -1606,10 +1607,12 @@ bool lock_sock_fast(struct sock *sk);
> > >   */
> > >  static inline void unlock_sock_fast(struct sock *sk, bool slow)
> > >  {
> > > -	if (slow)
> > > +	if (slow) {
> > >  		release_sock(sk);
> > > -	else
> > > +		__release(&sk->sk_lock.slock);
> > 
> > The correct solution would be to annotate the declaration of
> > release_sock() with '__releases(&sk->sk_lock.slock)'.
> 
> If I add such annotation to release_sock(), I'll get several sparse
> warnings for context imbalance (on each lock_sock()/release_sock()
> pair), unless I also add an '__acquires()' annotation to lock_sock(). 
> 
> The above does not look correct to me ?!? When release_sock() completes
> the socket spin lock is not held.

Yes, that's fine, but I suppose it somehow releases the mutex that
is taken in lock_sock_fast() when returning true, right?

> The annotation added above is
> somewhat an artifact to let unlock_sock_fast() matches lock_sock_fast()
> from sparse perspective. I intentionally avoided changing
> the release_sock() annotation to avoid introducing more artifacts.
> 
> The proposed schema is not 100% accurate, as it will also allow e.g. a
> really-not-fitting bh_lock_sock()/unlock_sock_fast() pair, but I could
> not come-up with anything better.
> 
> Can we go with the schema I proposed?

Well, I suppose it's a first step.
But can you then add a '__releases(...)' to unlock_sock_fast()?
It's not needed by sparse because it's an inline function and sparse
can then deduce it but it will help to see the pairing with
lock_sock_fast() is OK.

-- Luc