[PATCH RT 18/32] futex: Make the futex_hash_bucket spinlock_t again and bring back its old state

Steven Rostedt
2020-01-17T17:42:03Z

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

[ Upstream commit 954ad80c23edfe71f4e8ce70b961eac884320c3a ]

This is an all-in-one patch that reverts the patches:
futex: Make the futex_hash_bucket lock raw
futex: Delay deallocation of pi_state

and adds back the old patches we had:
futex: workaround migrate_disable/enable in different context
rtmutex: Handle the various new futex race conditions
futex: Fix bug on when a requeued RT task times out
futex: Ensure lock/unlock symetry versus pi_lock and hash bucket lock

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
kernel/futex.c | 230 ++++++++++++++++++--------------
kernel/locking/rtmutex.c | 65 ++++++++-
kernel/locking/rtmutex_common.h | 3 +
3 files changed, 194 insertions(+), 104 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 0b8cff8d9162..e815cf542b82 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -243,7 +243,7 @@ struct futex_q {
struct plist_node list;

struct task_struct *task;
- raw_spinlock_t *lock_ptr;
+ spinlock_t *lock_ptr;
union futex_key key;
struct futex_pi_state *pi_state;
struct rt_mutex_waiter *rt_waiter;
@@ -264,7 +264,7 @@ static const struct futex_q futex_q_init = {
*/
struct futex_hash_bucket {
atomic_t waiters;
- raw_spinlock_t lock;
+ spinlock_t lock;
struct plist_head chain;
} ____cacheline_aligned_in_smp;

@@ -825,13 +825,13 @@ static void get_pi_state(struct futex_pi_state *pi_state)
* Drops a reference to the pi_state object and frees or caches it
* when the last reference is gone.
*/
-static struct futex_pi_state *__put_pi_state(struct futex_pi_state *pi_state)
+static void put_pi_state(struct futex_pi_state *pi_state)
{
if (!pi_state)
- return NULL;
+ return;

if (!atomic_dec_and_test(&pi_state->refcount))
- return NULL;
+ return;

/*
* If pi_state->owner is NULL, the owner is most probably dying
@@ -851,7 +851,9 @@ static struct futex_pi_state *__put_pi_state(struct futex_pi_state *pi_state)
raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
}

- if (!current->pi_state_cache) {
+ if (current->pi_state_cache) {
+ kfree(pi_state);
+ } else {
/*
* pi_state->list is already empty.
* clear pi_state->owner.
@@ -860,30 +862,6 @@ static struct futex_pi_state *__put_pi_state(struct futex_pi_state *pi_state)
pi_state->owner = NULL;
atomic_set(&pi_state->refcount, 1);
current->pi_state_cache = pi_state;
- pi_state = NULL;
- }
- return pi_state;
-}
-
-static void put_pi_state(struct futex_pi_state *pi_state)
-{
- kfree(__put_pi_state(pi_state));
-}
-
-static void put_pi_state_atomic(struct futex_pi_state *pi_state,
- struct list_head *to_free)
-{
- if (__put_pi_state(pi_state))
- list_add(&pi_state->list, to_free);
-}
-
-static void free_pi_state_list(struct list_head *to_free)
-{
- struct futex_pi_state *p, *next;
-
- list_for_each_entry_safe(p, next, to_free, list) {
- list_del(&p->list);
- kfree(p);
}
}

@@ -900,7 +878,6 @@ void exit_pi_state_list(struct task_struct *curr)
struct futex_pi_state *pi_state;
struct futex_hash_bucket *hb;
union futex_key key = FUTEX_KEY_INIT;
- LIST_HEAD(to_free);

if (!futex_cmpxchg_enabled)
return;
@@ -934,7 +911,7 @@ void exit_pi_state_list(struct task_struct *curr)
}
raw_spin_unlock_irq(&curr->pi_lock);

- raw_spin_lock(&hb->lock);
+ spin_lock(&hb->lock);
raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
raw_spin_lock(&curr->pi_lock);
/*
@@ -944,8 +921,10 @@ void exit_pi_state_list(struct task_struct *curr)
if (head->next != next) {
/* retain curr->pi_lock for the loop invariant */
raw_spin_unlock(&pi_state->pi_mutex.wait_lock);
- raw_spin_unlock(&hb->lock);
- put_pi_state_atomic(pi_state, &to_free);
+ raw_spin_unlock_irq(&curr->pi_lock);
+ spin_unlock(&hb->lock);
+ raw_spin_lock_irq(&curr->pi_lock);
+ put_pi_state(pi_state);
continue;
}

@@ -956,7 +935,7 @@ void exit_pi_state_list(struct task_struct *curr)

raw_spin_unlock(&curr->pi_lock);
raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
- raw_spin_unlock(&hb->lock);
+ spin_unlock(&hb->lock);

rt_mutex_futex_unlock(&pi_state->pi_mutex);
put_pi_state(pi_state);
@@ -964,8 +943,6 @@ void exit_pi_state_list(struct task_struct *curr)
raw_spin_lock_irq(&curr->pi_lock);
}
raw_spin_unlock_irq(&curr->pi_lock);
-
- free_pi_state_list(&to_free);
}

#endif
@@ -1452,7 +1429,7 @@ static void __unqueue_futex(struct futex_q *q)
{
struct futex_hash_bucket *hb;

- if (WARN_ON_SMP(!q->lock_ptr || !raw_spin_is_locked(q->lock_ptr))
+ if (WARN_ON_SMP(!q->lock_ptr || !spin_is_locked(q->lock_ptr))
|| WARN_ON(plist_node_empty(&q->list)))
return;

@@ -1580,21 +1557,21 @@ static inline void
double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
{
if (hb1 <= hb2) {
- raw_spin_lock(&hb1->lock);
+ spin_lock(&hb1->lock);
if (hb1 < hb2)
- raw_spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING);
+ spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING);
} else { /* hb1 > hb2 */
- raw_spin_lock(&hb2->lock);
- raw_spin_lock_nested(&hb1->lock, SINGLE_DEPTH_NESTING);
+ spin_lock(&hb2->lock);
+ spin_lock_nested(&hb1->lock, SINGLE_DEPTH_NESTING);
}
}

static inline void
double_unlock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
{
- raw_spin_unlock(&hb1->lock);
+ spin_unlock(&hb1->lock);
if (hb1 != hb2)
- raw_spin_unlock(&hb2->lock);
+ spin_unlock(&hb2->lock);
}

/*
@@ -1622,7 +1599,7 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
if (!hb_waiters_pending(hb))
goto out_put_key;

- raw_spin_lock(&hb->lock);
+ spin_lock(&hb->lock);

plist_for_each_entry_safe(this, next, &hb->chain, list) {
if (match_futex (&this->key, &key)) {
@@ -1641,7 +1618,7 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
}
}

- raw_spin_unlock(&hb->lock);
+ spin_unlock(&hb->lock);
wake_up_q(&wake_q);
out_put_key:
put_futex_key(&key);
@@ -1948,7 +1925,6 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
struct futex_hash_bucket *hb1, *hb2;
struct futex_q *this, *next;
DEFINE_WAKE_Q(wake_q);
- LIST_HEAD(to_free);

if (nr_wake < 0 || nr_requeue < 0)
return -EINVAL;
@@ -2176,6 +2152,16 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
requeue_pi_wake_futex(this, &key2, hb2);
drop_count++;
continue;
+ } else if (ret == -EAGAIN) {
+ /*
+ * Waiter was woken by timeout or
+ * signal and has set pi_blocked_on to
+ * PI_WAKEUP_INPROGRESS before we
+ * tried to enqueue it on the rtmutex.
+ */
+ this->pi_state = NULL;
+ put_pi_state(pi_state);
+ continue;
} else if (ret) {
/*
* rt_mutex_start_proxy_lock() detected a
@@ -2186,7 +2172,7 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
* object.
*/
this->pi_state = NULL;
- put_pi_state_atomic(pi_state, &to_free);
+ put_pi_state(pi_state);
/*
* We stop queueing more waiters and let user
* space deal with the mess.
@@ -2203,7 +2189,7 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
* in futex_proxy_trylock_atomic() or in lookup_pi_state(). We
* need to drop it here again.
*/
- put_pi_state_atomic(pi_state, &to_free);
+ put_pi_state(pi_state);

out_unlock:
double_unlock_hb(hb1, hb2);
@@ -2224,7 +2210,6 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
out_put_key1:
put_futex_key(&key1);
out:
- free_pi_state_list(&to_free);
return ret ? ret : task_count;
}

@@ -2248,7 +2233,7 @@ static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)

q->lock_ptr = &hb->lock;

- raw_spin_lock(&hb->lock); /* implies smp_mb(); (A) */
+ spin_lock(&hb->lock); /* implies smp_mb(); (A) */
return hb;
}

@@ -2256,7 +2241,7 @@ static inline void
queue_unlock(struct futex_hash_bucket *hb)
__releases(&hb->lock)
{
- raw_spin_unlock(&hb->lock);
+ spin_unlock(&hb->lock);
hb_waiters_dec(hb);
}

@@ -2295,7 +2280,7 @@ static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
__releases(&hb->lock)
{
__queue_me(q, hb);
- raw_spin_unlock(&hb->lock);
+ spin_unlock(&hb->lock);
}

/**
@@ -2311,41 +2296,41 @@ static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
*/
static int unqueue_me(struct futex_q *q)
{
- raw_spinlock_t *lock_ptr;
+ spinlock_t *lock_ptr;
int ret = 0;

/* In the common case we don't take the spinlock, which is nice. */
retry:
/*
- * q->lock_ptr can change between this read and the following
- * raw_spin_lock. Use READ_ONCE to forbid the compiler from reloading
- * q->lock_ptr and optimizing lock_ptr out of the logic below.
+ * q->lock_ptr can change between this read and the following spin_lock.
+ * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
+ * optimizing lock_ptr out of the logic below.
*/
lock_ptr = READ_ONCE(q->lock_ptr);
if (lock_ptr != NULL) {
- raw_spin_lock(lock_ptr);
+ spin_lock(lock_ptr);
/*
* q->lock_ptr can change between reading it and
- * raw_spin_lock(), causing us to take the wrong lock. This
+ * spin_lock(), causing us to take the wrong lock. This
* corrects the race condition.
*
* Reasoning goes like this: if we have the wrong lock,
* q->lock_ptr must have changed (maybe several times)
- * between reading it and the raw_spin_lock(). It can
- * change again after the raw_spin_lock() but only if it was
- * already changed before the raw_spin_lock(). It cannot,
+ * between reading it and the spin_lock(). It can
+ * change again after the spin_lock() but only if it was
+ * already changed before the spin_lock(). It cannot,
* however, change back to the original value. Therefore
* we can detect whether we acquired the correct lock.
*/
if (unlikely(lock_ptr != q->lock_ptr)) {
- raw_spin_unlock(lock_ptr);
+ spin_unlock(lock_ptr);
goto retry;
}
__unqueue_futex(q);

BUG_ON(q->pi_state);

- raw_spin_unlock(lock_ptr);
+ spin_unlock(lock_ptr);
ret = 1;
}

@@ -2361,16 +2346,13 @@ static int unqueue_me(struct futex_q *q)
static void unqueue_me_pi(struct futex_q *q)
__releases(q->lock_ptr)
{
- struct futex_pi_state *ps;
-
__unqueue_futex(q);

BUG_ON(!q->pi_state);
- ps = __put_pi_state(q->pi_state);
+ put_pi_state(q->pi_state);
q->pi_state = NULL;

- raw_spin_unlock(q->lock_ptr);
- kfree(ps);
+ spin_unlock(q->lock_ptr);
}

static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
@@ -2503,7 +2485,7 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
*/
handle_err:
raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
- raw_spin_unlock(q->lock_ptr);
+ spin_unlock(q->lock_ptr);

switch (err) {
case -EFAULT:
@@ -2521,7 +2503,7 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
break;
}

- raw_spin_lock(q->lock_ptr);
+ spin_lock(q->lock_ptr);
raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);

/*
@@ -2617,7 +2599,7 @@ static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q,
/*
* The task state is guaranteed to be set before another task can
* wake it. set_current_state() is implemented using smp_store_mb() and
- * queue_me() calls raw_spin_unlock() upon completion, both serializing
+ * queue_me() calls spin_unlock() upon completion, both serializing
* access to the hash list and forcing another memory barrier.
*/
set_current_state(TASK_INTERRUPTIBLE);
@@ -2908,7 +2890,15 @@ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags,
* before __rt_mutex_start_proxy_lock() is done.
*/
raw_spin_lock_irq(&q.pi_state->pi_mutex.wait_lock);
- raw_spin_unlock(q.lock_ptr);
+ /*
+ * the migrate_disable() here disables migration in the in_atomic() fast
+ * path which is enabled again in the following spin_unlock(). We have
+ * one migrate_disable() pending in the slow-path which is reversed
+ * after the raw_spin_unlock_irq() where we leave the atomic context.
+ */
+ migrate_disable();
+
+ spin_unlock(q.lock_ptr);
/*
* __rt_mutex_start_proxy_lock() unconditionally enqueues the @rt_waiter
* such that futex_unlock_pi() is guaranteed to observe the waiter when
@@ -2916,6 +2906,7 @@ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags,
*/
ret = __rt_mutex_start_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter, current);
raw_spin_unlock_irq(&q.pi_state->pi_mutex.wait_lock);
+ migrate_enable();

if (ret) {
if (ret == 1)
@@ -2929,7 +2920,7 @@ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags,
ret = rt_mutex_wait_proxy_lock(&q.pi_state->pi_mutex, to, &rt_waiter);

cleanup:
- raw_spin_lock(q.lock_ptr);
+ spin_lock(q.lock_ptr);
/*
* If we failed to acquire the lock (deadlock/signal/timeout), we must
* first acquire the hb->lock before removing the lock from the
@@ -3030,7 +3021,7 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
return ret;

hb = hash_futex(&key);
- raw_spin_lock(&hb->lock);
+ spin_lock(&hb->lock);

/*
* Check waiters first. We do not trust user space values at
@@ -3064,10 +3055,19 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
* rt_waiter. Also see the WARN in wake_futex_pi().
*/
raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
- raw_spin_unlock(&hb->lock);
+ /*
+ * Magic trickery for now to make the RT migrate disable
+ * logic happy. The following spin_unlock() happens with
+ * interrupts disabled so the internal migrate_enable()
+ * won't undo the migrate_disable() which was issued when
+ * locking hb->lock.
+ */
+ migrate_disable();
+ spin_unlock(&hb->lock);

/* drops pi_state->pi_mutex.wait_lock */
ret = wake_futex_pi(uaddr, uval, pi_state);
+ migrate_enable();

put_pi_state(pi_state);

@@ -3103,7 +3103,7 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
* owner.
*/
if ((ret = cmpxchg_futex_value_locked(&curval, uaddr, uval, 0))) {
- raw_spin_unlock(&hb->lock);
+ spin_unlock(&hb->lock);
switch (ret) {
case -EFAULT:
goto pi_faulted;
@@ -3123,7 +3123,7 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
ret = (curval == uval) ? 0 : -EAGAIN;

out_unlock:
- raw_spin_unlock(&hb->lock);
+ spin_unlock(&hb->lock);
out_putkey:
put_futex_key(&key);
return ret;
@@ -3239,7 +3239,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
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_hash_bucket *hb, *hb2;
union futex_key key2 = FUTEX_KEY_INIT;
struct futex_q q = futex_q_init;
int res, ret;
@@ -3297,20 +3297,55 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
/* Queue the futex_q, drop the hb lock, wait for wakeup. */
futex_wait_queue_me(hb, &q, to);

- raw_spin_lock(&hb->lock);
- ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
- raw_spin_unlock(&hb->lock);
- if (ret)
- goto out_put_keys;
+ /*
+ * On RT we must avoid races with requeue and trying to block
+ * on two mutexes (hb->lock and uaddr2's rtmutex) by
+ * serializing access to pi_blocked_on with pi_lock.
+ */
+ raw_spin_lock_irq(&current->pi_lock);
+ if (current->pi_blocked_on) {
+ /*
+ * We have been requeued or are in the process of
+ * being requeued.
+ */
+ raw_spin_unlock_irq(&current->pi_lock);
+ } else {
+ /*
+ * Setting pi_blocked_on to PI_WAKEUP_INPROGRESS
+ * prevents a concurrent requeue from moving us to the
+ * uaddr2 rtmutex. After that we can safely acquire
+ * (and possibly block on) hb->lock.
+ */
+ current->pi_blocked_on = PI_WAKEUP_INPROGRESS;
+ raw_spin_unlock_irq(&current->pi_lock);
+
+ spin_lock(&hb->lock);
+
+ /*
+ * Clean up pi_blocked_on. We might leak it otherwise
+ * when we succeeded with the hb->lock in the fast
+ * path.
+ */
+ raw_spin_lock_irq(&current->pi_lock);
+ current->pi_blocked_on = NULL;
+ raw_spin_unlock_irq(&current->pi_lock);
+
+ ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
+ spin_unlock(&hb->lock);
+ if (ret)
+ goto out_put_keys;
+ }

/*
- * In order for us to be here, we know our q.key == key2, and since
- * we took the hb->lock above, we also know that futex_requeue() has
- * completed and we no longer have to concern ourselves with a wakeup
- * race with the atomic proxy lock acquisition by the requeue code. The
- * futex_requeue dropped our key1 reference and incremented our key2
- * reference count.
+ * In order to be here, we have either been requeued, are in
+ * the process of being requeued, or requeue successfully
+ * acquired uaddr2 on our behalf. If pi_blocked_on was
+ * non-null above, we may be racing with a requeue. Do not
+ * rely on q->lock_ptr to be hb2->lock until after blocking on
+ * hb->lock or hb2->lock. The futex_requeue dropped our key1
+ * reference and incremented our key2 reference count.
*/
+ hb2 = hash_futex(&key2);

/* Check if the requeue code acquired the second futex for us. */
if (!q.rt_waiter) {
@@ -3319,9 +3354,8 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
* did a lock-steal - fix up the PI-state in that case.
*/
if (q.pi_state && (q.pi_state->owner != current)) {
- struct futex_pi_state *ps_free;
-
- raw_spin_lock(q.lock_ptr);
+ spin_lock(&hb2->lock);
+ BUG_ON(&hb2->lock != q.lock_ptr);
ret = fixup_pi_state_owner(uaddr2, &q, current);
if (ret && rt_mutex_owner(&q.pi_state->pi_mutex) == current) {
pi_state = q.pi_state;
@@ -3331,9 +3365,8 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
* Drop the reference to the pi state which
* the requeue_pi() code acquired for us.
*/
- ps_free = __put_pi_state(q.pi_state);
- raw_spin_unlock(q.lock_ptr);
- kfree(ps_free);
+ put_pi_state(q.pi_state);
+ spin_unlock(&hb2->lock);
}
} else {
struct rt_mutex *pi_mutex;
@@ -3347,7 +3380,8 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
pi_mutex = &q.pi_state->pi_mutex;
ret = rt_mutex_wait_proxy_lock(pi_mutex, to, &rt_waiter);

- raw_spin_lock(q.lock_ptr);
+ spin_lock(&hb2->lock);
+ BUG_ON(&hb2->lock != q.lock_ptr);
if (ret && !rt_mutex_cleanup_proxy_lock(pi_mutex, &rt_waiter))
ret = 0;

@@ -4014,7 +4048,7 @@ static int __init futex_init(void)
for (i = 0; i < futex_hashsize; i++) {
atomic_set(&futex_queues[i].waiters, 0);
plist_head_init(&futex_queues[i].chain);
- raw_spin_lock_init(&futex_queues[i].lock);
+ spin_lock_init(&futex_queues[i].lock);
}

return 0;
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 44a33057a83a..2a9bf2443acc 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -142,6 +142,12 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
WRITE_ONCE(*p, owner & ~RT_MUTEX_HAS_WAITERS);
}

+static int rt_mutex_real_waiter(struct rt_mutex_waiter *waiter)
+{
+ return waiter && waiter != PI_WAKEUP_INPROGRESS &&
+ waiter != PI_REQUEUE_INPROGRESS;
+}
+
/*
* We can speed up the acquire/release, if there's no debugging state to be
* set up.
@@ -415,7 +421,8 @@ int max_lock_depth = 1024;

static inline struct rt_mutex *task_blocked_on_lock(struct task_struct *p)
{
- return p->pi_blocked_on ? p->pi_blocked_on->lock : NULL;
+ return rt_mutex_real_waiter(p->pi_blocked_on) ?
+ p->pi_blocked_on->lock : NULL;
}

/*
@@ -551,7 +558,7 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
* reached or the state of the chain has changed while we
* dropped the locks.
*/
- if (!waiter)
+ if (!rt_mutex_real_waiter(waiter))
goto out_unlock_pi;

/*
@@ -1321,6 +1328,22 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
return -EDEADLK;

raw_spin_lock(&task->pi_lock);
+ /*
+ * In the case of futex requeue PI, this will be a proxy
+ * lock. The task will wake unaware that it is enqueueed on
+ * this lock. Avoid blocking on two locks and corrupting
+ * pi_blocked_on via the PI_WAKEUP_INPROGRESS
+ * flag. futex_wait_requeue_pi() sets this when it wakes up
+ * before requeue (due to a signal or timeout). Do not enqueue
+ * the task if PI_WAKEUP_INPROGRESS is set.
+ */
+ if (task != current && task->pi_blocked_on == PI_WAKEUP_INPROGRESS) {
+ raw_spin_unlock(&task->pi_lock);
+ return -EAGAIN;
+ }
+
+ BUG_ON(rt_mutex_real_waiter(task->pi_blocked_on));
+
waiter->task = task;
waiter->lock = lock;
waiter->prio = task->prio;
@@ -1344,7 +1367,7 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
rt_mutex_enqueue_pi(owner, waiter);

rt_mutex_adjust_prio(owner);
- if (owner->pi_blocked_on)
+ if (rt_mutex_real_waiter(owner->pi_blocked_on))
chain_walk = 1;
} else if (rt_mutex_cond_detect_deadlock(waiter, chwalk)) {
chain_walk = 1;
@@ -1444,7 +1467,7 @@ static void remove_waiter(struct rt_mutex *lock,
{
bool is_top_waiter = (waiter == rt_mutex_top_waiter(lock));
struct task_struct *owner = rt_mutex_owner(lock);
- struct rt_mutex *next_lock;
+ struct rt_mutex *next_lock = NULL;

lockdep_assert_held(&lock->wait_lock);

@@ -1470,7 +1493,8 @@ static void remove_waiter(struct rt_mutex *lock,
rt_mutex_adjust_prio(owner);

/* Store the lock on which owner is blocked or NULL */
- next_lock = task_blocked_on_lock(owner);
+ if (rt_mutex_real_waiter(owner->pi_blocked_on))
+ next_lock = task_blocked_on_lock(owner);

raw_spin_unlock(&owner->pi_lock);

@@ -1506,7 +1530,8 @@ void rt_mutex_adjust_pi(struct task_struct *task)
raw_spin_lock_irqsave(&task->pi_lock, flags);

waiter = task->pi_blocked_on;
- if (!waiter || rt_mutex_waiter_equal(waiter, task_to_waiter(task))) {
+ if (!rt_mutex_real_waiter(waiter) ||
+ rt_mutex_waiter_equal(waiter, task_to_waiter(task))) {
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
return;
}
@@ -2325,6 +2350,34 @@ int __rt_mutex_start_proxy_lock(struct rt_mutex *lock,
if (try_to_take_rt_mutex(lock, task, NULL))
return 1;

+#ifdef CONFIG_PREEMPT_RT_FULL
+ /*
+ * In PREEMPT_RT there's an added race.
+ * If the task, that we are about to requeue, times out,
+ * it can set the PI_WAKEUP_INPROGRESS. This tells the requeue
+ * to skip this task. But right after the task sets
+ * its pi_blocked_on to PI_WAKEUP_INPROGRESS it can then
+ * block on the spin_lock(&hb->lock), which in RT is an rtmutex.
+ * This will replace the PI_WAKEUP_INPROGRESS with the actual
+ * lock that it blocks on. We *must not* place this task
+ * on this proxy lock in that case.
+ *
+ * To prevent this race, we first take the task's pi_lock
+ * and check if it has updated its pi_blocked_on. If it has,
+ * we assume that it woke up and we return -EAGAIN.
+ * Otherwise, we set the task's pi_blocked_on to
+ * PI_REQUEUE_INPROGRESS, so that if the task is waking up
+ * it will know that we are in the process of requeuing it.
+ */
+ raw_spin_lock(&task->pi_lock);
+ if (task->pi_blocked_on) {
+ raw_spin_unlock(&task->pi_lock);
+ return -EAGAIN;
+ }
+ task->pi_blocked_on = PI_REQUEUE_INPROGRESS;
+ raw_spin_unlock(&task->pi_lock);
+#endif
+
/* We enforce deadlock detection for futexes */
ret = task_blocks_on_rt_mutex(lock, waiter, task,
RT_MUTEX_FULL_CHAINWALK);
diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
index 758dc43872e5..546aaf058b9e 100644
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -132,6 +132,9 @@ enum rtmutex_chainwalk {
/*
* PI-futex support (proxy locking functions, etc.):
*/
+#define PI_WAKEUP_INPROGRESS ((struct rt_mutex_waiter *) 1)
+#define PI_REQUEUE_INPROGRESS ((struct rt_mutex_waiter *) 2)
+
extern struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock);
extern void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
struct task_struct *proxy_owner);
--
2.24.1