[PATCH -v4 10/10] futex: Pull rt_mutex_futex_unlock() out from under hb->lock
From: Peter Zijlstra
Date: Tue Dec 13 2016 - 04:03:00 EST
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 DL crash because of pointer instability.
Because of all the previous patches that:
- allow us to do rt_mutex_futex_unlock() without dropping wait_lock;
which in turn allows us to rely on wait_lock atomicy.
- changed locking rules to cover {uval,pi_state} with wait_lock.
- removed the hb queue vs rt_mutex waiters inconsistency.
We can now quite simply pull rt_mutex_futex_unlock() out from under
hb->lock, a pi_state reference and wait_lock are sufficient.
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
kernel/futex.c | 139 +++++++++++++++++++++++++++++++++++++--------------------
1 file changed, 92 insertions(+), 47 deletions(-)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -800,7 +800,7 @@ static int refill_pi_state_cache(void)
return 0;
}
-static struct futex_pi_state * alloc_pi_state(void)
+static struct futex_pi_state *alloc_pi_state(void)
{
struct futex_pi_state *pi_state = current->pi_state_cache;
@@ -810,6 +810,11 @@ static struct futex_pi_state * alloc_pi_
return pi_state;
}
+static void get_pi_state(struct futex_pi_state *pi_state)
+{
+ WARN_ON_ONCE(!atomic_inc_not_zero(&pi_state->refcount));
+}
+
/*
* Drops a reference to the pi_state object and frees or caches it
* when the last reference is gone.
@@ -854,7 +859,7 @@ static void put_pi_state(struct futex_pi
* Look up the task based on what TID userspace gave us.
* We dont trust it.
*/
-static struct task_struct * futex_find_get_task(pid_t pid)
+static struct task_struct *futex_find_get_task(pid_t pid)
{
struct task_struct *p;
@@ -914,10 +919,12 @@ void exit_pi_state_list(struct task_stru
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);
@@ -1030,6 +1037,9 @@ 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.
+ *
+ * IOW, we cannot race against the unlocked put_pi_state() in
+ * futex_unlock_pi().
*/
WARN_ON(!atomic_read(&pi_state->refcount));
@@ -1101,7 +1111,7 @@ static int attach_to_pi_state(u32 __user
goto out_einval;
out_attach:
- atomic_inc(&pi_state->refcount);
+ get_pi_state(pi_state);
raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
*ps = pi_state;
return 0;
@@ -1373,35 +1383,22 @@ static void mark_wake_futex(struct wake_
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)
+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);
-
BUG_ON(!new_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.
+ * 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);
@@ -1424,10 +1421,8 @@ static int wake_futex_pi(u32 __user *uad
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));
@@ -1445,15 +1440,15 @@ 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;
}
/*
@@ -1982,7 +1977,7 @@ static int futex_requeue(u32 __user *uad
* refcount on the pi_state and store the pointer in
* the futex_q object of the waiter.
*/
- atomic_inc(&pi_state->refcount);
+ get_pi_state(pi_state);
this->pi_state = pi_state;
ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
this->rt_waiter,
@@ -2217,7 +2212,8 @@ static int fixup_pi_state_owner(u32 __us
/*
* 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.
*
@@ -2234,7 +2230,7 @@ static int fixup_pi_state_owner(u32 __us
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))
@@ -2324,6 +2320,10 @@ static int fixup_owner(u32 __user *uaddr
/*
* 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);
@@ -2334,11 +2334,12 @@ static int fixup_owner(u32 __user *uaddr
* Paranoia check. If we did not take the lock, then we should not be
* the owner of the rt_mutex.
*/
- if (rt_mutex_owner(&q->pi_state->pi_mutex) == current)
+ if (rt_mutex_owner(&q->pi_state->pi_mutex) == current) {
printk(KERN_ERR "fixup_owner: ret = %d pi-mutex: %p "
"pi-state %p\n", ret,
q->pi_state->pi_mutex.owner,
q->pi_state->owner);
+ }
out:
return ret ? ret : locked;
@@ -2562,6 +2563,7 @@ static int futex_lock_pi(u32 __user *uad
ktime_t *time, int trylock)
{
struct hrtimer_sleeper timeout, *to = NULL;
+ struct futex_pi_state *pi_state = NULL;
struct rt_mutex_waiter rt_waiter;
struct futex_hash_bucket *hb;
struct futex_q q = futex_q_init;
@@ -2673,12 +2675,19 @@ static int futex_lock_pi(u32 __user *uad
* 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:
@@ -2741,10 +2750,36 @@ static int futex_unlock_pi(u32 __user *u
*/
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;
+
+ /*
+ * 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);
+
/*
- * In case of success wake_futex_pi dropped the hash
- * bucket lock.
+ * Success, we're done! No tricky corner cases.
*/
if (!ret)
goto out_putkey;
@@ -2759,7 +2794,6 @@ static int futex_unlock_pi(u32 __user *u
* setting the FUTEX_WAITERS bit. Try again.
*/
if (ret == -EAGAIN) {
- spin_unlock(&hb->lock);
put_futex_key(&key);
goto retry;
}
@@ -2767,7 +2801,7 @@ static int futex_unlock_pi(u32 __user *u
* wake_futex_pi has detected invalid state. Tell user
* space.
*/
- goto out_unlock;
+ goto out_putkey;
}
/*
@@ -2777,8 +2811,10 @@ static int futex_unlock_pi(u32 __user *u
* 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.
@@ -2792,7 +2828,6 @@ static int futex_unlock_pi(u32 __user *u
return ret;
pi_faulted:
- spin_unlock(&hb->lock);
put_futex_key(&key);
ret = fault_in_user_writeable(uaddr);
@@ -2896,6 +2931,7 @@ static int futex_wait_requeue_pi(u32 __u
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;
@@ -2980,8 +3016,10 @@ static int futex_wait_requeue_pi(u32 __u
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.
@@ -3022,13 +3060,20 @@ static int futex_wait_requeue_pi(u32 __u
* 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