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@xxxxxxxxx wrote:
> > From: Madhuparna Bhowmik <madhuparnabhowmik10@xxxxxxxxx>
> >
> > 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@xxxxxxxxx>
> > ---
> > 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
> >
