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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>
This commit is contained in:
Peter Zijlstra 2017-03-22 11:35:55 +01:00 committed by Thomas Gleixner
parent 73d786bd04
commit 16ffa12d74
1 changed files with 100 additions and 54 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

154
kernel/futex.c
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@ -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;
/*
* 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;
@ -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