Re: [PATCH] sched.h: Annotate sighand_struct with __rcu

[Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>]

On Thu, Jan 23, 2020 at 08:41:08AM -0800, Paul E. McKenney wrote:
> On Thu, Jan 23, 2020 at 08:23:05PM +0530, madhuparnabhowmik10@gmail.com wrote:
> > From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> > 
> > This patch fixes the following sparse errors by annotating the
> > sighand_struct with __rcu
> > 
> > kernel/fork.c:1511:9: error: incompatible types in comparison expression
> > kernel/exit.c:100:19: error: incompatible types in comparison expression
> > kernel/signal.c:1370:27: error: incompatible types in comparison expression
> > 
> > This fix introduces the following sparse error in signal.c due to
> > checking the sighand pointer without rcu primitives:
> > 
> > kernel/signal.c:1386:21: error: incompatible types in comparison expression
> > 
> > This new sparse error is also addressed in this patch.
> > 
> > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> > ---
> >  include/linux/sched.h | 2 +-
> >  kernel/signal.c       | 3 ++-
> >  2 files changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index b511e178a89f..7a351360ad54 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -918,7 +918,7 @@ struct task_struct {
> >  
> >  	/* Signal handlers: */
> >  	struct signal_struct		*signal;
> > -	struct sighand_struct		*sighand;
> > +	struct sighand_struct __rcu		*sighand;
> >  	sigset_t			blocked;
> >  	sigset_t			real_blocked;
> >  	/* Restored if set_restore_sigmask() was used: */
> > diff --git a/kernel/signal.c b/kernel/signal.c
> > index bcd46f547db3..1272def37462 100644
> > --- a/kernel/signal.c
> > +++ b/kernel/signal.c
> > @@ -1383,7 +1383,8 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
> >  		 * must see ->sighand == NULL.
> >  		 */
> >  		spin_lock_irqsave(&sighand->siglock, *flags);
> > -		if (likely(sighand == tsk->sighand))
> > +		if (likely(sighand == rcu_dereference_protected(tsk->sighand,
> > +						lockdep_is_held(&sighand->siglock))))
> 
> Given that the return value is never dereferenced, you can use
> rcu_access_pointer(), which may be used outside of an RCU read-side
> critical section, and thus does not need the lockdep_is_held().  So this
> change would save a line of code and would be a bit easier on the eyes.
>
Okay got it! I will send the updated patch soon.

Thank you,
Madhuparna
> 							Thanx, Paul
> 
> >  			break;
> >  		spin_unlock_irqrestore(&sighand->siglock, *flags);
> >  	}
> > -- 
> > 2.17.1
> >