From: Ben Hutchings ben@decadent.org.uk
From: Peter Zijlstra peterz@infradead.org
commit 16ffa12d742534d4ff73e8b3a4e81c1de39196f0 upstream.
There's a number of 'interesting' problems, all caused by holding hb->lock while doing the rt_mutex_unlock() equivalient.
Notably:
- a PI inversion on hb->lock; and,
- a SCHED_DEADLINE crash because of pointer instability.
The previous changes:
- changed the locking rules to cover {uval,pi_state} with wait_lock.
- allow to do rt_mutex_futex_unlock() without dropping wait_lock; which in turn allows to rely on wait_lock atomicity completely.
- simplified the waiter conundrum.
It's now sufficient to hold rtmutex::wait_lock and a reference on the pi_state to protect the state consistency, so hb->lock can be dropped before calling rt_mutex_futex_unlock().
Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: juri.lelli@arm.com Cc: bigeasy@linutronix.de Cc: xlpang@redhat.com Cc: rostedt@goodmis.org Cc: mathieu.desnoyers@efficios.com Cc: jdesfossez@efficios.com Cc: dvhart@infradead.org Cc: bristot@redhat.com Link: http://lkml.kernel.org/r/20170322104151.900002056@infradead.org Signed-off-by: Thomas Gleixner tglx@linutronix.de [bwh: Backported to 4.9: adjust context] Signed-off-by: Ben Hutchings ben@decadent.org.uk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/futex.c | 111 ++++++++++++++++++++++++++++++++++----------------------- 1 file changed, 68 insertions(+), 43 deletions(-)
--- a/kernel/futex.c +++ b/kernel/futex.c @@ -966,10 +966,12 @@ static void exit_pi_state_list(struct ta pi_state->owner = NULL; raw_spin_unlock_irq(&curr->pi_lock);
- rt_mutex_futex_unlock(&pi_state->pi_mutex); - + get_pi_state(pi_state); spin_unlock(&hb->lock);
+ rt_mutex_futex_unlock(&pi_state->pi_mutex); + put_pi_state(pi_state); + raw_spin_lock_irq(&curr->pi_lock); } raw_spin_unlock_irq(&curr->pi_lock); @@ -1083,6 +1085,11 @@ static int attach_to_pi_state(u32 __user * has dropped the hb->lock in between queue_me() and unqueue_me_pi(), * which in turn means that futex_lock_pi() still has a reference on * our pi_state. + * + * The waiter holding a reference on @pi_state also protects against + * the unlocked put_pi_state() in futex_unlock_pi(), futex_lock_pi() + * and futex_wait_requeue_pi() as it cannot go to 0 and consequently + * free pi_state before we can take a reference ourselves. */ WARN_ON(!atomic_read(&pi_state->refcount));
@@ -1537,48 +1544,40 @@ static void mark_wake_futex(struct wake_ q->lock_ptr = NULL; }
-static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter, - struct futex_hash_bucket *hb) +/* + * Caller must hold a reference on @pi_state. + */ +static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_state) { - struct task_struct *new_owner; - struct futex_pi_state *pi_state = top_waiter->pi_state; u32 uninitialized_var(curval), newval; + struct task_struct *new_owner; + bool deboost = false; WAKE_Q(wake_q); - bool deboost; int ret = 0;
- if (!pi_state) - return -EINVAL; - - /* - * If current does not own the pi_state then the futex is - * inconsistent and user space fiddled with the futex value. - */ - if (pi_state->owner != current) - return -EINVAL; - raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); new_owner = rt_mutex_next_owner(&pi_state->pi_mutex); - - /* - * When we interleave with futex_lock_pi() where it does - * rt_mutex_timed_futex_lock(), we might observe @top_waiter futex_q waiter, - * but the rt_mutex's wait_list can be empty (either still, or again, - * depending on which side we land). - * - * When this happens, give up our locks and try again, giving the - * futex_lock_pi() instance time to complete, either by waiting on the - * rtmutex or removing itself from the futex queue. - */ if (!new_owner) { - raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); - return -EAGAIN; + /* + * Since we held neither hb->lock nor wait_lock when coming + * into this function, we could have raced with futex_lock_pi() + * such that we might observe @this futex_q waiter, but the + * rt_mutex's wait_list can be empty (either still, or again, + * depending on which side we land). + * + * When this happens, give up our locks and try again, giving + * the futex_lock_pi() instance time to complete, either by + * waiting on the rtmutex or removing itself from the futex + * queue. + */ + ret = -EAGAIN; + goto out_unlock; }
/* - * We pass it to the next owner. The WAITERS bit is always - * kept enabled while there is PI state around. We cleanup the - * owner died bit, because we are the owner. + * We pass it to the next owner. The WAITERS bit is always kept + * enabled while there is PI state around. We cleanup the owner + * died bit, because we are the owner. */ newval = FUTEX_WAITERS | task_pid_vnr(new_owner);
@@ -1611,15 +1610,15 @@ static int wake_futex_pi(u32 __user *uad deboost = __rt_mutex_futex_unlock(&pi_state->pi_mutex, &wake_q); }
+out_unlock: raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); - spin_unlock(&hb->lock);
if (deboost) { wake_up_q(&wake_q); rt_mutex_adjust_prio(current); }
- return 0; + return ret; }
/* @@ -2493,7 +2492,7 @@ retry: if (get_futex_value_locked(&uval, uaddr)) goto handle_fault;
- while (1) { + for (;;) { newval = (uval & FUTEX_OWNER_DIED) | newtid;
if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) @@ -3006,10 +3005,36 @@ retry: */ top_waiter = futex_top_waiter(hb, &key); if (top_waiter) { - ret = wake_futex_pi(uaddr, uval, top_waiter, hb); + struct futex_pi_state *pi_state = top_waiter->pi_state; + + ret = -EINVAL; + if (!pi_state) + goto out_unlock; + /* - * In case of success wake_futex_pi dropped the hash - * bucket lock. + * If current does not own the pi_state then the futex is + * inconsistent and user space fiddled with the futex value. + */ + if (pi_state->owner != current) + goto out_unlock; + + /* + * Grab a reference on the pi_state and drop hb->lock. + * + * The reference ensures pi_state lives, dropping the hb->lock + * is tricky.. wake_futex_pi() will take rt_mutex::wait_lock to + * close the races against futex_lock_pi(), but in case of + * _any_ fail we'll abort and retry the whole deal. + */ + get_pi_state(pi_state); + spin_unlock(&hb->lock); + + ret = wake_futex_pi(uaddr, uval, pi_state); + + put_pi_state(pi_state); + + /* + * Success, we're done! No tricky corner cases. */ if (!ret) goto out_putkey; @@ -3024,7 +3049,6 @@ retry: * setting the FUTEX_WAITERS bit. Try again. */ if (ret == -EAGAIN) { - spin_unlock(&hb->lock); put_futex_key(&key); goto retry; } @@ -3032,7 +3056,7 @@ retry: * wake_futex_pi has detected invalid state. Tell user * space. */ - goto out_unlock; + goto out_putkey; }
/* @@ -3042,8 +3066,10 @@ retry: * preserve the WAITERS bit not the OWNER_DIED one. We are the * owner. */ - if (cmpxchg_futex_value_locked(&curval, uaddr, uval, 0)) + if (cmpxchg_futex_value_locked(&curval, uaddr, uval, 0)) { + spin_unlock(&hb->lock); goto pi_faulted; + }
/* * If uval has changed, let user space handle it. @@ -3057,7 +3083,6 @@ out_putkey: return ret;
pi_faulted: - spin_unlock(&hb->lock); put_futex_key(&key);
ret = fault_in_user_writeable(uaddr);