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author | Peter Zijlstra <peterz@infradead.org> | 2017-03-22 11:35:55 +0100 |
---|---|---|
committer | Thomas Gleixner <tglx@linutronix.de> | 2017-03-23 19:10:08 +0100 |
commit | 16ffa12d742534d4ff73e8b3a4e81c1de39196f0 (patch) | |
tree | 2c19026cc476ea8f7bf12540788bfac1c0b41611 | |
parent | 73d786bd043ebc855f349c81ea805f6b11cbf2aa (diff) | |
download | op-kernel-dev-16ffa12d742534d4ff73e8b3a4e81c1de39196f0.zip op-kernel-dev-16ffa12d742534d4ff73e8b3a4e81c1de39196f0.tar.gz |
futex: Pull rt_mutex_futex_unlock() out from under hb->lock
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>
-rw-r--r-- | kernel/futex.c | 154 |
1 files changed, 100 insertions, 54 deletions
diff --git a/kernel/futex.c b/kernel/futex.c index 51a248a..3b0aace 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -921,10 +921,12 @@ void exit_pi_state_list(struct task_struct *curr) 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); @@ -1037,6 +1039,11 @@ static int attach_to_pi_state(u32 __user *uaddr, u32 uval, * 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)); @@ -1380,48 +1387,40 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q) smp_store_release(&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; DEFINE_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 @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. - */ 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); @@ -1444,10 +1443,8 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter ret = -EINVAL; } - if (ret) { - raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); - return ret; - } + if (ret) + goto out_unlock; raw_spin_lock(&pi_state->owner->pi_lock); WARN_ON(list_empty(&pi_state->list)); @@ -1465,15 +1462,15 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter */ 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; } /* @@ -2232,7 +2229,8 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q, /* * We are here either because we stole the rtmutex from the * previous highest priority waiter or we are the highest priority - * waiter but failed to get the rtmutex the first time. + * waiter but have failed to get the rtmutex the first time. + * * We have to replace the newowner TID in the user space variable. * This must be atomic as we have to preserve the owner died bit here. * @@ -2249,7 +2247,7 @@ 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)) @@ -2345,6 +2343,10 @@ static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked) /* * Got the lock. We might not be the anticipated owner if we * did a lock-steal - fix up the PI-state in that case: + * + * We can safely read pi_state->owner without holding wait_lock + * because we now own the rt_mutex, only the owner will attempt + * to change it. */ if (q->pi_state->owner != current) ret = fixup_pi_state_owner(uaddr, q, current); @@ -2584,6 +2586,7 @@ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int trylock) { struct hrtimer_sleeper timeout, *to = NULL; + struct futex_pi_state *pi_state = NULL; struct futex_hash_bucket *hb; struct futex_q q = futex_q_init; int res, ret; @@ -2670,12 +2673,19 @@ retry_private: * If fixup_owner() faulted and was unable to handle the fault, unlock * it and return the fault to userspace. */ - if (ret && (rt_mutex_owner(&q.pi_state->pi_mutex) == current)) - rt_mutex_futex_unlock(&q.pi_state->pi_mutex); + if (ret && (rt_mutex_owner(&q.pi_state->pi_mutex) == current)) { + pi_state = q.pi_state; + get_pi_state(pi_state); + } /* Unqueue and drop the lock */ unqueue_me_pi(&q); + if (pi_state) { + rt_mutex_futex_unlock(&pi_state->pi_mutex); + put_pi_state(pi_state); + } + goto out_put_key; out_unlock_put_key: @@ -2738,10 +2748,36 @@ 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; + + /* + * 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; + /* - * In case of success wake_futex_pi dropped the hash - * bucket lock. + * 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; @@ -2756,7 +2792,6 @@ retry: * setting the FUTEX_WAITERS bit. Try again. */ if (ret == -EAGAIN) { - spin_unlock(&hb->lock); put_futex_key(&key); goto retry; } @@ -2764,7 +2799,7 @@ retry: * wake_futex_pi has detected invalid state. Tell user * space. */ - goto out_unlock; + goto out_putkey; } /* @@ -2774,8 +2809,10 @@ 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. @@ -2789,7 +2826,6 @@ out_putkey: return ret; pi_faulted: - spin_unlock(&hb->lock); put_futex_key(&key); ret = fault_in_user_writeable(uaddr); @@ -2893,6 +2929,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags, u32 __user *uaddr2) { struct hrtimer_sleeper timeout, *to = NULL; + struct futex_pi_state *pi_state = NULL; struct rt_mutex_waiter rt_waiter; struct futex_hash_bucket *hb; union futex_key key2 = FUTEX_KEY_INIT; @@ -2977,8 +3014,10 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags, if (q.pi_state && (q.pi_state->owner != current)) { spin_lock(q.lock_ptr); ret = fixup_pi_state_owner(uaddr2, &q, current); - if (ret && rt_mutex_owner(&q.pi_state->pi_mutex) == current) - rt_mutex_futex_unlock(&q.pi_state->pi_mutex); + if (ret && rt_mutex_owner(&q.pi_state->pi_mutex) == current) { + pi_state = q.pi_state; + get_pi_state(pi_state); + } /* * Drop the reference to the pi state which * the requeue_pi() code acquired for us. @@ -3017,13 +3056,20 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags, * the fault, unlock the rt_mutex and return the fault to * userspace. */ - if (ret && rt_mutex_owner(pi_mutex) == current) - rt_mutex_futex_unlock(pi_mutex); + if (ret && rt_mutex_owner(&q.pi_state->pi_mutex) == current) { + pi_state = q.pi_state; + get_pi_state(pi_state); + } /* Unqueue and drop the lock. */ unqueue_me_pi(&q); } + if (pi_state) { + rt_mutex_futex_unlock(&pi_state->pi_mutex); + put_pi_state(pi_state); + } + if (ret == -EINTR) { /* * We've already been requeued, but cannot restart by calling |