linux-kernel - Re: [GIT PULL] locking fixes

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Date:   Mon, 22 Jan 2018 11:39:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] locking fixes

On Mon, Jan 22, 2018 at 10:43:36AM +0100, Geert Uytterhoeven wrote:
> >  static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
> > +                               struct task_struct *argowner)
> >  {
> >         struct futex_pi_state *pi_state = q->pi_state;
> >         u32 uval, uninitialized_var(curval), newval;
> > +       struct task_struct *oldowner, *newowner;
> > +       u32 newtid;
> 
> new tid is no longer initialized...
> 
> >         int ret;
> >
> > +       lockdep_assert_held(q->lock_ptr);
> > +
> >         raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
> >
> >         oldowner = pi_state->owner;
> > @@ -2317,11 +2316,17 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
> >                 newtid |= FUTEX_OWNER_DIED;
> 
> ... leading to a compiler warning with gcc 4.1.2:
> 
>     warning: ‘newtid’ is used uninitialized in this function
> 
> I guess newer compilers don't give the warning, as the result of the
> assignment above is not used at all, and thus may be optimized away...
> 
> >
> >         /*
> > +        * We are here because either:
> > +        *
> > +        *  - we stole the lock and pi_state->owner needs updating to reflect
> > +        *    that (@argowner == current),
> > +        *
> > +        * or:
> > +        *
> > +        *  - someone stole our lock and we need to fix things to point to the
> > +        *    new owner (@argowner == NULL).
> >          *
> > +        * Either way, we have to replace the TID in the user space variable.
> >          * This must be atomic as we have to preserve the owner died bit here.
> >          *
> >          * Note: We write the user space value _before_ changing the pi_state
> > @@ -2334,6 +2339,42 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
> >          * in the PID check in lookup_pi_state.
> >          */
> >  retry:
> > +       if (!argowner) {
> > +               if (oldowner != current) {
> > +                       /*
> > +                        * We raced against a concurrent self; things are
> > +                        * already fixed up. Nothing to do.
> > +                        */
> > +                       ret = 0;
> > +                       goto out_unlock;
> > +               }
> > +
> > +               if (__rt_mutex_futex_trylock(&pi_state->pi_mutex)) {
> > +                       /* We got the lock after all, nothing to fix. */
> > +                       ret = 0;
> > +                       goto out_unlock;
> > +               }
> > +
> > +               /*
> > +                * Since we just failed the trylock; there must be an owner.
> > +                */
> > +               newowner = rt_mutex_owner(&pi_state->pi_mutex);
> > +               BUG_ON(!newowner);
> > +       } else {
> > +               WARN_ON_ONCE(argowner != current);
> > +               if (oldowner == current) {
> > +                       /*
> > +                        * We raced against a concurrent self; things are
> > +                        * already fixed up. Nothing to do.
> > +                        */
> > +                       ret = 0;
> > +                       goto out_unlock;
> > +               }
> > +               newowner = argowner;
> > +       }
> > +
> > +       newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
> 
> ... since it is always overwritten here.
> 
> Is that intentional?

No, I think you actually spotted a bug there. We now can't set
OWNER_DIED anymore, which is bad.

I think the below fixes things, but let me go trawl through the various
futex test things, because I think I've seen a unit test for this
_somewhere_.

---
 kernel/futex.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 8c5424dd5924..7f719d110908 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2311,9 +2311,6 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
 	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
 
 	oldowner = pi_state->owner;
-	/* Owner died? */
-	if (!pi_state->owner)
-		newtid |= FUTEX_OWNER_DIED;
 
 	/*
 	 * We are here because either:
@@ -2374,6 +2371,9 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
 	}
 
 	newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
+	/* Owner died? */
+	if (!pi_state->owner)
+		newtid |= FUTEX_OWNER_DIED;
 
 	if (get_futex_value_locked(&uval, uaddr))
 		goto handle_fault;