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SubjectRe: [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;
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