Prev: [PATCH 3/4] futex: free_pi_state outside of hb->lock sections
Next: [PATCH 0/4][RT] futex: fix tasks blocking on two rt_mutex locks
From: Darren Hart on 9 Jul 2010 18:40
The requeue_pi mechanism introduced proxy locking of the rtmutex. This creates
a scenario where a task can wake-up, not knowing it has been enqueued on an
rtmutex. In order to detect this, the task would have to be able to take either
task->pi_blocked_on->lock->wait_lock and/or the hb->lock. Unfortunately,
without already holding one of these, the pi_blocked_on variable can change
from NULL to valid or from valid to NULL. Therefor, the task cannot be allowed
to take a sleeping lock after wakeup or it could end up trying to block on two
locks, the second overwriting a valid pi_blocked_on value. This obviously
breaks the pi mechanism.

This patch increases latency, while running the ltp pthread_cond_many test
which Michal reported the bug with, I see double digit hrtimer latencies
(typically only on the first run after boo):

kernel: hrtimer: interrupt took 75911 ns

This might be addressed by changing the various loops in the futex code to be
incremental, probably at an additional throughput hit. The private hash_bucket
lists discussed in the past could reduce hb->lock contention in some scenarios.
It should be noted that pthread_cond_many is a rather pathological case.

This also introduces problems for plists which want a spinlock_t rather
than a raw_spinlock_t. Any thoughts on how to address this?

Signed-off-by: Darren Hart <dvhltc(a)us.ibm.com>
Cc: Thomas Gleixner <tglx(a)linutronix.de>
Cc: Peter Zijlstra <peterz(a)infradead.org>
Cc: Ingo Molnar <mingo(a)elte.hu>
Cc: Eric Dumazet <eric.dumazet(a)gmail.com>
Cc: John Kacur <jkacur(a)redhat.com>
Cc: Steven Rostedt <rostedt(a)goodmis.org>
Cc: Mike Galbraith <efault(a)gmx.de>
---
kernel/futex.c | 67 ++++++++++++++++++++++++++++++--------------------------
1 files changed, 36 insertions(+), 31 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index b217972..0ad5a85 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -128,7 +128,7 @@ struct futex_q {
* waiting on a futex.
*/
struct futex_hash_bucket {
- spinlock_t lock;
+ raw_spinlock_t lock;
struct plist_head chain;
};

@@ -479,7 +479,7 @@ void exit_pi_state_list(struct task_struct *curr)
hb = hash_futex(&key);
raw_spin_unlock_irq(&curr->pi_lock);

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

raw_spin_lock_irq(&curr->pi_lock);
/*
@@ -499,7 +499,7 @@ void exit_pi_state_list(struct task_struct *curr)

rt_mutex_unlock(&pi_state->pi_mutex);

- spin_unlock(&hb->lock);
+ raw_spin_unlock(&hb->lock);

raw_spin_lock_irq(&curr->pi_lock);
}
@@ -860,21 +860,21 @@ static inline void
double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
{
if (hb1 <= hb2) {
- spin_lock(&hb1->lock);
+ raw_spin_lock(&hb1->lock);
if (hb1 < hb2)
- spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING);
+ raw_spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING);
} else { /* hb1 > hb2 */
- spin_lock(&hb2->lock);
- spin_lock_nested(&hb1->lock, SINGLE_DEPTH_NESTING);
+ raw_spin_lock(&hb2->lock);
+ raw_spin_lock_nested(&hb1->lock, SINGLE_DEPTH_NESTING);
}
}

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

/*
@@ -896,7 +896,7 @@ static int futex_wake(u32 __user *uaddr, int fshared, int nr_wake, u32 bitset)
goto out;

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

plist_for_each_entry_safe(this, next, head, list) {
@@ -916,7 +916,7 @@ static int futex_wake(u32 __user *uaddr, int fshared, int nr_wake, u32 bitset)
}
}

- spin_unlock(&hb->lock);
+ raw_spin_unlock(&hb->lock);
put_futex_key(fshared, &key);
out:
return ret;
@@ -1070,6 +1070,7 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,

q->lock_ptr = &hb->lock;
#ifdef CONFIG_DEBUG_PI_LIST
+ /* FIXME: we're converting this to a raw lock... */
q->list.plist.spinlock = &hb->lock;
#endif

@@ -1377,14 +1378,14 @@ static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
hb = hash_futex(&q->key);
q->lock_ptr = &hb->lock;

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

static inline void
queue_unlock(struct futex_q *q, struct futex_hash_bucket *hb)
{
- spin_unlock(&hb->lock);
+ raw_spin_unlock(&hb->lock);
drop_futex_key_refs(&q->key);
}

@@ -1416,11 +1417,12 @@ static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb)

plist_node_init(&q->list, prio);
#ifdef CONFIG_DEBUG_PI_LIST
+ /* FIXME: we're converting this to a raw_spinlock */
q->list.plist.spinlock = &hb->lock;
#endif
plist_add(&q->list, &hb->chain);
q->task = current;
- spin_unlock(&hb->lock);
+ raw_spin_unlock(&hb->lock);
}

/**
@@ -1444,7 +1446,7 @@ retry:
lock_ptr = q->lock_ptr;
barrier();
if (lock_ptr != NULL) {
- spin_lock(lock_ptr);
+ raw_spin_lock(lock_ptr);
/*
* q->lock_ptr can change between reading it and
* spin_lock(), causing us to take the wrong lock. This
@@ -1459,7 +1461,7 @@ retry:
* we can detect whether we acquired the correct lock.
*/
if (unlikely(lock_ptr != q->lock_ptr)) {
- spin_unlock(lock_ptr);
+ raw_spin_unlock(lock_ptr);
goto retry;
}
WARN_ON(plist_node_empty(&q->list));
@@ -1467,7 +1469,7 @@ retry:

BUG_ON(q->pi_state);

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

@@ -1491,7 +1493,7 @@ static void unqueue_me_pi(struct futex_q *q)
pi_state = q->pi_state;
q->pi_state = NULL;

- spin_unlock(q->lock_ptr);
+ raw_spin_unlock(q->lock_ptr);
drop_futex_key_refs(&q->key);

free_pi_state(pi_state);
@@ -1579,11 +1581,11 @@ retry:
* simply return.
*/
handle_fault:
- spin_unlock(q->lock_ptr);
+ raw_spin_unlock(q->lock_ptr);

ret = fault_in_user_writeable(uaddr);

- spin_lock(q->lock_ptr);
+ raw_spin_lock(q->lock_ptr);

/*
* Check if someone else fixed it for us:
@@ -1976,7 +1978,7 @@ retry_private:
ret = ret ? 0 : -EWOULDBLOCK;
}

- spin_lock(q.lock_ptr);
+ raw_spin_lock(q.lock_ptr);
/*
* Fixup the pi_state owner and possibly acquire the lock if we
* haven't already.
@@ -2053,7 +2055,7 @@ retry:
goto out;

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

/*
* To avoid races, try to do the TID -> 0 atomic transition
@@ -2102,14 +2104,14 @@ retry:
}

out_unlock:
- spin_unlock(&hb->lock);
+ raw_spin_unlock(&hb->lock);
put_futex_key(fshared, &key);

out:
return ret;

pi_faulted:
- spin_unlock(&hb->lock);
+ raw_spin_unlock(&hb->lock);
put_futex_key(fshared, &key);

ret = fault_in_user_writeable(uaddr);
@@ -2257,9 +2259,9 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
/* Queue the futex_q, drop the hb lock, wait for wakeup. */
futex_wait_queue_me(hb, &q, to);

- spin_lock(&hb->lock);
+ raw_spin_lock(&hb->lock);
ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
- spin_unlock(&hb->lock);
+ raw_spin_unlock(&hb->lock);
if (ret)
goto out_put_keys;

@@ -2277,10 +2279,10 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
* did a lock-steal - fix up the PI-state in that case.
*/
if (q.pi_state && (q.pi_state->owner != current)) {
- spin_lock(q.lock_ptr);
+ raw_spin_lock(q.lock_ptr);
ret = fixup_pi_state_owner(uaddr2, &q, current,
fshared);
- spin_unlock(q.lock_ptr);
+ raw_spin_unlock(q.lock_ptr);
}
} else {
/*
@@ -2293,7 +2295,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
ret = rt_mutex_finish_proxy_lock(pi_mutex, to, &rt_waiter, 1);
debug_rt_mutex_free_waiter(&rt_waiter);

- spin_lock(q.lock_ptr);
+ raw_spin_lock(q.lock_ptr);
/*
* Fixup the pi_state owner and possibly acquire the lock if we
* haven't already.
@@ -2668,8 +2670,11 @@ static int __init futex_init(void)
futex_cmpxchg_enabled = 1;

for (i = 0; i < ARRAY_SIZE(futex_queues); i++) {
- plist_head_init(&futex_queues[i].chain, &futex_queues[i].lock);
- spin_lock_init(&futex_queues[i].lock);
+ /*
+ * FIXME: plist wants a spinlock, but the hb->lock is a raw_spinlock_t
+ */
+ plist_head_init(&futex_queues[i].chain, NULL /*&futex_queues[i].lock*/);
+ raw_spin_lock_init(&futex_queues[i].lock);
}

return 0;
--
1.7.0.4

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