From: Peter Zijlstra peterz@infradead.org
commit cfafcd117da0216520568c195cb2f6cd1980c4bb upstream.
By changing futex_lock_pi() to use rt_mutex_*_proxy_lock() all wait_list modifications are done under both hb->lock and wait_lock.
This closes the obvious interleave pattern between futex_lock_pi() and futex_unlock_pi(), but not entirely so. See below:
Before:
futex_lock_pi() futex_unlock_pi() unlock hb->lock
lock hb->lock unlock hb->lock
lock rt_mutex->wait_lock unlock rt_mutex_wait_lock -EAGAIN
lock rt_mutex->wait_lock list_add unlock rt_mutex->wait_lock
schedule()
lock rt_mutex->wait_lock list_del unlock rt_mutex->wait_lock
<idem> -EAGAIN
lock hb->lock
After:
futex_lock_pi() futex_unlock_pi()
lock hb->lock lock rt_mutex->wait_lock list_add unlock rt_mutex->wait_lock unlock hb->lock
schedule() lock hb->lock unlock hb->lock lock hb->lock lock rt_mutex->wait_lock list_del unlock rt_mutex->wait_lock
lock rt_mutex->wait_lock unlock rt_mutex_wait_lock -EAGAIN
unlock hb->lock
It does however solve the earlier starvation/live-lock scenario which got introduced with the -EAGAIN since unlike the before scenario; where the -EAGAIN happens while futex_unlock_pi() doesn't hold any locks; in the after scenario it happens while futex_unlock_pi() actually holds a lock, and then it is serialized on that lock.
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/20170322104152.062785528@infradead.org Signed-off-by: Thomas Gleixner tglx@linutronix.de [bwh: Backported to 4.9: adjust context] Signed-off-by: Ben Hutchings ben@decadent.org.uk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/futex.c | 77 ++++++++++++++++++++++++++++------------ kernel/locking/rtmutex.c | 26 +++---------- kernel/locking/rtmutex_common.h | 1 3 files changed, 62 insertions(+), 42 deletions(-)
--- a/kernel/futex.c +++ b/kernel/futex.c @@ -2333,20 +2333,7 @@ queue_unlock(struct futex_hash_bucket *h hb_waiters_dec(hb); }
-/** - * queue_me() - Enqueue the futex_q on the futex_hash_bucket - * @q: The futex_q to enqueue - * @hb: The destination hash bucket - * - * The hb->lock must be held by the caller, and is released here. A call to - * queue_me() is typically paired with exactly one call to unqueue_me(). The - * exceptions involve the PI related operations, which may use unqueue_me_pi() - * or nothing if the unqueue is done as part of the wake process and the unqueue - * state is implicit in the state of woken task (see futex_wait_requeue_pi() for - * an example). - */ -static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb) - __releases(&hb->lock) +static inline void __queue_me(struct futex_q *q, struct futex_hash_bucket *hb) { int prio;
@@ -2363,6 +2350,24 @@ static inline void queue_me(struct futex plist_node_init(&q->list, prio); plist_add(&q->list, &hb->chain); q->task = current; +} + +/** + * queue_me() - Enqueue the futex_q on the futex_hash_bucket + * @q: The futex_q to enqueue + * @hb: The destination hash bucket + * + * The hb->lock must be held by the caller, and is released here. A call to + * queue_me() is typically paired with exactly one call to unqueue_me(). The + * exceptions involve the PI related operations, which may use unqueue_me_pi() + * or nothing if the unqueue is done as part of the wake process and the unqueue + * state is implicit in the state of woken task (see futex_wait_requeue_pi() for + * an example). + */ +static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb) + __releases(&hb->lock) +{ + __queue_me(q, hb); spin_unlock(&hb->lock); }
@@ -2868,6 +2873,7 @@ static int futex_lock_pi(u32 __user *uad { struct hrtimer_sleeper timeout, *to = NULL; struct task_struct *exiting = NULL; + struct rt_mutex_waiter rt_waiter; struct futex_hash_bucket *hb; struct futex_q q = futex_q_init; int res, ret; @@ -2928,25 +2934,52 @@ retry_private: } }
+ WARN_ON(!q.pi_state); + /* * Only actually queue now that the atomic ops are done: */ - queue_me(&q, hb); + __queue_me(&q, hb);
- WARN_ON(!q.pi_state); - /* - * Block on the PI mutex: - */ - if (!trylock) { - ret = rt_mutex_timed_futex_lock(&q.pi_state->pi_mutex, to); - } else { + if (trylock) { ret = rt_mutex_futex_trylock(&q.pi_state->pi_mutex); /* Fixup the trylock return value: */ ret = ret ? 0 : -EWOULDBLOCK; + goto no_block; }
+ /* + * We must add ourselves to the rt_mutex waitlist while holding hb->lock + * such that the hb and rt_mutex wait lists match. + */ + rt_mutex_init_waiter(&rt_waiter); + ret = rt_mutex_start_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter, current); + if (ret) { + if (ret == 1) + ret = 0; + + goto no_block; + } + + spin_unlock(q.lock_ptr); + + if (unlikely(to)) + hrtimer_start_expires(&to->timer, HRTIMER_MODE_ABS); + + ret = rt_mutex_wait_proxy_lock(&q.pi_state->pi_mutex, to, &rt_waiter); + spin_lock(q.lock_ptr); /* + * If we failed to acquire the lock (signal/timeout), we must + * first acquire the hb->lock before removing the lock from the + * rt_mutex waitqueue, such that we can keep the hb and rt_mutex + * wait lists consistent. + */ + if (ret && !rt_mutex_cleanup_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter)) + ret = 0; + +no_block: + /* * Fixup the pi_state owner and possibly acquire the lock if we * haven't already. */ --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -1523,19 +1523,6 @@ int __sched rt_mutex_lock_interruptible( EXPORT_SYMBOL_GPL(rt_mutex_lock_interruptible);
/* - * Futex variant with full deadlock detection. - * Futex variants must not use the fast-path, see __rt_mutex_futex_unlock(). - */ -int __sched rt_mutex_timed_futex_lock(struct rt_mutex *lock, - struct hrtimer_sleeper *timeout) -{ - might_sleep(); - - return rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, - timeout, RT_MUTEX_FULL_CHAINWALK); -} - -/* * Futex variant, must not use fastpath. */ int __sched rt_mutex_futex_trylock(struct rt_mutex *lock) @@ -1808,12 +1795,6 @@ int rt_mutex_wait_proxy_lock(struct rt_m /* sleep on the mutex */ ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter);
- /* - * try_to_take_rt_mutex() sets the waiter bit unconditionally. We might - * have to fix that up. - */ - fixup_rt_mutex_waiters(lock); - raw_spin_unlock_irq(&lock->wait_lock);
return ret; @@ -1853,6 +1834,13 @@ bool rt_mutex_cleanup_proxy_lock(struct fixup_rt_mutex_waiters(lock); cleanup = true; } + + /* + * try_to_take_rt_mutex() sets the waiter bit unconditionally. We might + * have to fix that up. + */ + fixup_rt_mutex_waiters(lock); + raw_spin_unlock_irq(&lock->wait_lock);
return cleanup; --- a/kernel/locking/rtmutex_common.h +++ b/kernel/locking/rtmutex_common.h @@ -112,7 +112,6 @@ extern int rt_mutex_wait_proxy_lock(stru struct rt_mutex_waiter *waiter); extern bool rt_mutex_cleanup_proxy_lock(struct rt_mutex *lock, struct rt_mutex_waiter *waiter); -extern int rt_mutex_timed_futex_lock(struct rt_mutex *l, struct hrtimer_sleeper *to); extern int rt_mutex_futex_trylock(struct rt_mutex *l); extern int __rt_mutex_futex_trylock(struct rt_mutex *l);