linux-kernel - Re: [PATCH v2] rcu: mollify sparse with RCU guard

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Message-ID: <df1ecb31-fed2-4069-93bf-8773f30842b2@paulmck-laptop>
Date: Tue, 26 Mar 2024 10:21:40 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Johannes Berg <johannes@...solutions.net>, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <frederic@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Berg <johannes.berg@...el.com>
Subject: Re: [PATCH v2] rcu: mollify sparse with RCU guard

On Mon, Mar 25, 2024 at 10:41:36AM -0700, Boqun Feng wrote:
> On Mon, Mar 25, 2024 at 06:39:08PM +0100, Johannes Berg wrote:
> > From: Johannes Berg <johannes.berg@...el.com>
> > 
> > When using "guard(rcu)();" sparse will complain, because even
> > though it now understands the cleanup attribute, it doesn't
> > evaluate the calls from it at function exit, and thus doesn't
> > count the context correctly.
> > 
> > Given that there's a conditional in the resulting code:
> > 
> >   static inline void class_rcu_destructor(class_rcu_t *_T)
> >   {
> >       if (_T->lock) {
> >           rcu_read_unlock();
> >       }
> >   }
> > 
> > it seems that even trying to teach sparse to evalulate the
> > cleanup attribute function it'd still be difficult to really
> > make it understand the full context here.
> > 
> > Suppress the sparse warning by just releasing the context in
> > the acquisition part of the function, after all we know it's
> > safe with the guard, that's the whole point of it.
> > 
> > Signed-off-by: Johannes Berg <johannes.berg@...el.com>
> 
> Reviewed-by: Boqun Feng <boqun.feng@...il.com>

Queued, thank you both!

							Thanx, Paul

> Regards,
> Boqun
> 
> > ---
> > v2: add a comment after discussion with Boqun
> > 
> > ---
> >  include/linux/rcupdate.h | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 0746b1b0b663..6a3c52b3c180 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -1059,6 +1059,18 @@ rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
> >  extern int rcu_expedited;
> >  extern int rcu_normal;
> >  
> > -DEFINE_LOCK_GUARD_0(rcu, rcu_read_lock(), rcu_read_unlock())
> > +DEFINE_LOCK_GUARD_0(rcu,
> > +		    do {
> > +			rcu_read_lock();
> > +			/*
> > +			 * sparse doesn't call the cleanup function,
> > +			 * so just release immediately and don't track
> > +			 * the context. We don't need to anyway, since
> > +			 * the whole point of the guard is to not need
> > +			 * the explicit unlock.
> > +			 */
> > +			__release(RCU);
> > +		    } while(0),
> > +		    rcu_read_unlock())
> >  
> >  #endif /* __LINUX_RCUPDATE_H */
> > -- 
> > 2.44.0
> >