diff options
| -rw-r--r-- | kernel/futex.c | 308 |
1 files changed, 259 insertions, 49 deletions
diff --git a/kernel/futex.c b/kernel/futex.c index 82660b9b837a..e7b4c6121da4 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -197,6 +197,8 @@ struct futex_pi_state { * @rt_waiter: rt_waiter storage for use with requeue_pi * @requeue_pi_key: the requeue_pi target futex key * @bitset: bitset for the optional bitmasked wakeup + * @requeue_state: State field for futex_requeue_pi() + * @requeue_wait: RCU wait for futex_requeue_pi() (RT only) * * We use this hashed waitqueue, instead of a normal wait_queue_entry_t, so * we can wake only the relevant ones (hashed queues may be shared). @@ -219,12 +221,68 @@ struct futex_q { struct rt_mutex_waiter *rt_waiter; union futex_key *requeue_pi_key; u32 bitset; + atomic_t requeue_state; +#ifdef CONFIG_PREEMPT_RT + struct rcuwait requeue_wait; +#endif } __randomize_layout; +/* + * On PREEMPT_RT, the hash bucket lock is a 'sleeping' spinlock with an + * underlying rtmutex. The task which is about to be requeued could have + * just woken up (timeout, signal). After the wake up the task has to + * acquire hash bucket lock, which is held by the requeue code. As a task + * can only be blocked on _ONE_ rtmutex at a time, the proxy lock blocking + * and the hash bucket lock blocking would collide and corrupt state. + * + * On !PREEMPT_RT this is not a problem and everything could be serialized + * on hash bucket lock, but aside of having the benefit of common code, + * this allows to avoid doing the requeue when the task is already on the + * way out and taking the hash bucket lock of the original uaddr1 when the + * requeue has been completed. + * + * The following state transitions are valid: + * + * On the waiter side: + * Q_REQUEUE_PI_NONE -> Q_REQUEUE_PI_IGNORE + * Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_WAIT + * + * On the requeue side: + * Q_REQUEUE_PI_NONE -> Q_REQUEUE_PI_INPROGRESS + * Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_DONE/LOCKED + * Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_NONE (requeue failed) + * Q_REQUEUE_PI_WAIT -> Q_REQUEUE_PI_DONE/LOCKED + * Q_REQUEUE_PI_WAIT -> Q_REQUEUE_PI_IGNORE (requeue failed) + * + * The requeue side ignores a waiter with state Q_REQUEUE_PI_IGNORE as this + * signals that the waiter is already on the way out. It also means that + * the waiter is still on the 'wait' futex, i.e. uaddr1. + * + * The waiter side signals early wakeup to the requeue side either through + * setting state to Q_REQUEUE_PI_IGNORE or to Q_REQUEUE_PI_WAIT depending + * on the current state. In case of Q_REQUEUE_PI_IGNORE it can immediately + * proceed to take the hash bucket lock of uaddr1. If it set state to WAIT, + * which means the wakeup is interleaving with a requeue in progress it has + * to wait for the requeue side to change the state. Either to DONE/LOCKED + * or to IGNORE. DONE/LOCKED means the waiter q is now on the uaddr2 futex + * and either blocked (DONE) or has acquired it (LOCKED). IGNORE is set by + * the requeue side when the requeue attempt failed via deadlock detection + * and therefore the waiter q is still on the uaddr1 futex. + */ +enum { + Q_REQUEUE_PI_NONE = 0, + Q_REQUEUE_PI_IGNORE, + Q_REQUEUE_PI_IN_PROGRESS, + Q_REQUEUE_PI_WAIT, + Q_REQUEUE_PI_DONE, + Q_REQUEUE_PI_LOCKED, +}; + static const struct futex_q futex_q_init = { /* list gets initialized in queue_me()*/ - .key = FUTEX_KEY_INIT, - .bitset = FUTEX_BITSET_MATCH_ANY + .key = FUTEX_KEY_INIT, + .bitset = FUTEX_BITSET_MATCH_ANY, + .requeue_state = ATOMIC_INIT(Q_REQUEUE_PI_NONE), }; /* @@ -1772,6 +1830,108 @@ void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1, q->key = *key2; } +static inline bool futex_requeue_pi_prepare(struct futex_q *q, + struct futex_pi_state *pi_state) +{ + int old, new; + + /* + * Set state to Q_REQUEUE_PI_IN_PROGRESS unless an early wakeup has + * already set Q_REQUEUE_PI_IGNORE to signal that requeue should + * ignore the waiter. + */ + old = atomic_read_acquire(&q->requeue_state); + do { + if (old == Q_REQUEUE_PI_IGNORE) + return false; + + /* + * futex_proxy_trylock_atomic() might have set it to + * IN_PROGRESS and a interleaved early wake to WAIT. + * + * It was considered to have an extra state for that + * trylock, but that would just add more conditionals + * all over the place for a dubious value. + */ + if (old != Q_REQUEUE_PI_NONE) + break; + + new = Q_REQUEUE_PI_IN_PROGRESS; + } while (!atomic_try_cmpxchg(&q->requeue_state, &old, new)); + + q->pi_state = pi_state; + return true; +} + +static inline void futex_requeue_pi_complete(struct futex_q *q, int locked) +{ + int old, new; + + old = atomic_read_acquire(&q->requeue_state); + do { + if (old == Q_REQUEUE_PI_IGNORE) + return; + + if (locked >= 0) { + /* Requeue succeeded. Set DONE or LOCKED */ + WARN_ON_ONCE(old != Q_REQUEUE_PI_IN_PROGRESS && + old != Q_REQUEUE_PI_WAIT); + new = Q_REQUEUE_PI_DONE + locked; + } else if (old == Q_REQUEUE_PI_IN_PROGRESS) { + /* Deadlock, no early wakeup interleave */ + new = Q_REQUEUE_PI_NONE; + } else { + /* Deadlock, early wakeup interleave. */ + WARN_ON_ONCE(old != Q_REQUEUE_PI_WAIT); + new = Q_REQUEUE_PI_IGNORE; + } + } while (!atomic_try_cmpxchg(&q->requeue_state, &old, new)); + +#ifdef CONFIG_PREEMPT_RT + /* If the waiter interleaved with the requeue let it know */ + if (unlikely(old == Q_REQUEUE_PI_WAIT)) + rcuwait_wake_up(&q->requeue_wait); +#endif +} + +static inline int futex_requeue_pi_wakeup_sync(struct futex_q *q) +{ + int old, new; + + old = atomic_read_acquire(&q->requeue_state); + do { + /* Is requeue done already? */ + if (old >= Q_REQUEUE_PI_DONE) + return old; + + /* + * If not done, then tell the requeue code to either ignore + * the waiter or to wake it up once the requeue is done. + */ + new = Q_REQUEUE_PI_WAIT; + if (old == Q_REQUEUE_PI_NONE) + new = Q_REQUEUE_PI_IGNORE; + } while (!atomic_try_cmpxchg(&q->requeue_state, &old, new)); + + /* If the requeue was in progress, wait for it to complete */ + if (old == Q_REQUEUE_PI_IN_PROGRESS) { +#ifdef CONFIG_PREEMPT_RT + rcuwait_wait_event(&q->requeue_wait, + atomic_read(&q->requeue_state) != Q_REQUEUE_PI_WAIT, + TASK_UNINTERRUPTIBLE); +#else + (void)atomic_cond_read_relaxed(&q->requeue_state, VAL != Q_REQUEUE_PI_WAIT); +#endif + } + + /* + * Requeue is now either prohibited or complete. Reread state + * because during the wait above it might have changed. Nothing + * will modify q->requeue_state after this point. + */ + return atomic_read(&q->requeue_state); +} + /** * requeue_pi_wake_futex() - Wake a task that acquired the lock during requeue * @q: the futex_q @@ -1799,6 +1959,8 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key, q->lock_ptr = &hb->lock; + /* Signal locked state to the waiter */ + futex_requeue_pi_complete(q, 1); wake_up_state(q->task, TASK_NORMAL); } @@ -1869,6 +2031,10 @@ futex_proxy_trylock_atomic(u32 __user *pifutex, struct futex_hash_bucket *hb1, if (!match_futex(top_waiter->requeue_pi_key, key2)) return -EINVAL; + /* Ensure that this does not race against an early wakeup */ + if (!futex_requeue_pi_prepare(top_waiter, NULL)) + return -EAGAIN; + /* * Try to take the lock for top_waiter. Set the FUTEX_WAITERS bit in * the contended case or if set_waiters is 1. The pi_state is returned @@ -1878,8 +2044,22 @@ futex_proxy_trylock_atomic(u32 __user *pifutex, struct futex_hash_bucket *hb1, ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task, exiting, set_waiters); if (ret == 1) { + /* Dequeue, wake up and update top_waiter::requeue_state */ requeue_pi_wake_futex(top_waiter, key2, hb2); return vpid; + } else if (ret < 0) { + /* Rewind top_waiter::requeue_state */ + futex_requeue_pi_complete(top_waiter, ret); + } else { + /* + * futex_lock_pi_atomic() did not acquire the user space + * futex, but managed to establish the proxy lock and pi + * state. top_waiter::requeue_state cannot be fixed up here + * because the waiter is not enqueued on the rtmutex + * yet. This is handled at the callsite depending on the + * result of rt_mutex_start_proxy_lock() which is + * guaranteed to be reached with this function returning 0. + */ } return ret; } @@ -2020,6 +2200,8 @@ retry_private: * intend to requeue waiters, force setting the FUTEX_WAITERS * bit. We force this here where we are able to easily handle * faults rather in the requeue loop below. + * + * Updates topwaiter::requeue_state if a top waiter exists. */ ret = futex_proxy_trylock_atomic(uaddr2, hb1, hb2, &key1, &key2, &pi_state, @@ -2033,6 +2215,24 @@ retry_private: * VPID of the top waiter task. * If the lock was not taken, we have pi_state and an initial * refcount on it. In case of an error we have nothing. + * + * The top waiter's requeue_state is up to date: + * + * - If the lock was acquired atomically (ret > 0), then + * the state is Q_REQUEUE_PI_LOCKED. + * + * - If the trylock failed with an error (ret < 0) then + * the state is either Q_REQUEUE_PI_NONE, i.e. "nothing + * happened", or Q_REQUEUE_PI_IGNORE when there was an + * interleaved early wakeup. + * + * - If the trylock did not succeed (ret == 0) then the + * state is either Q_REQUEUE_PI_IN_PROGRESS or + * Q_REQUEUE_PI_WAIT if an early wakeup interleaved. + * This will be cleaned up in the loop below, which + * cannot fail because futex_proxy_trylock_atomic() did + * the same sanity checks for requeue_pi as the loop + * below does. */ if (ret > 0) { WARN_ON(pi_state); @@ -2064,7 +2264,10 @@ retry_private: /* We hold a reference on the pi state. */ break; - /* If the above failed, then pi_state is NULL */ + /* + * If the above failed, then pi_state is NULL and + * waiter::requeue_state is correct. + */ case -EFAULT: double_unlock_hb(hb1, hb2); hb_waiters_dec(hb2); @@ -2140,21 +2343,39 @@ retry_private: * object of the waiter. */ get_pi_state(pi_state); - this->pi_state = pi_state; + + /* Don't requeue when the waiter is already on the way out. */ + if (!futex_requeue_pi_prepare(this, pi_state)) { + /* + * Early woken waiter signaled that it is on the + * way out. Drop the pi_state reference and try the + * next waiter. @this->pi_state is still NULL. + */ + put_pi_state(pi_state); + continue; + } + ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex, - this->rt_waiter, this->task); + this->rt_waiter, + this->task); + if (ret == 1) { /* * We got the lock. We do neither drop the refcount * on pi_state nor clear this->pi_state because the * waiter needs the pi_state for cleaning up the * user space value. It will drop the refcount - * after doing so. + * after doing so. this::requeue_state is updated + * in the wakeup as well. */ requeue_pi_wake_futex(this, &key2, hb2); task_count++; - continue; - } else if (ret) { + } else if (!ret) { + /* Waiter is queued, move it to hb2 */ + requeue_futex(this, hb1, hb2, &key2); + futex_requeue_pi_complete(this, 0); + task_count++; + } else { /* * rt_mutex_start_proxy_lock() detected a potential * deadlock when we tried to queue that waiter. @@ -2164,15 +2385,13 @@ retry_private: */ this->pi_state = NULL; put_pi_state(pi_state); + futex_requeue_pi_complete(this, ret); /* * We stop queueing more waiters and let user space * deal with the mess. */ break; } - /* Waiter is queued, move it to hb2 */ - requeue_futex(this, hb1, hb2, &key2); - task_count++; } /* @@ -3161,6 +3380,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags, struct futex_hash_bucket *hb; union futex_key key2 = FUTEX_KEY_INIT; struct futex_q q = futex_q_init; + struct rt_mutex_base *pi_mutex; int res, ret; if (!IS_ENABLED(CONFIG_FUTEX_PI)) @@ -3210,32 +3430,22 @@ 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); - spin_lock(&hb->lock); - /* Is @q still queued on uaddr1? */ - if (!match_futex(&q->key, key2)) + switch (futex_requeue_pi_wakeup_sync(&q)) { + case Q_REQUEUE_PI_IGNORE: + /* The waiter is still on uaddr1 */ + spin_lock(&hb->lock); ret = handle_early_requeue_pi_wakeup(hb, &q, to); - spin_unlock(&hb->lock); - if (ret) - goto out; - - /* - * 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. - */ + spin_unlock(&hb->lock); + break; - /* - * Check if the requeue code acquired the second futex for us and do - * any pertinent fixup. - */ - if (!q.rt_waiter) { + case Q_REQUEUE_PI_LOCKED: + /* The requeue acquired the lock */ if (q.pi_state && (q.pi_state->owner != current)) { spin_lock(q.lock_ptr); ret = fixup_owner(uaddr2, &q, true); /* - * Drop the reference to the pi state which - * the requeue_pi() code acquired for us. + * Drop the reference to the pi state which the + * requeue_pi() code acquired for us. */ put_pi_state(q.pi_state); spin_unlock(q.lock_ptr); @@ -3245,18 +3455,14 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags, */ ret = ret < 0 ? ret : 0; } - } else { - struct rt_mutex_base *pi_mutex; + break; - /* - * We have been woken up by futex_unlock_pi(), a timeout, or a - * signal. futex_unlock_pi() will not destroy the lock_ptr nor - * the pi_state. - */ - WARN_ON(!q.pi_state); + case Q_REQUEUE_PI_DONE: + /* Requeue completed. Current is 'pi_blocked_on' the rtmutex */ pi_mutex = &q.pi_state->pi_mutex; ret = rt_mutex_wait_proxy_lock(pi_mutex, to, &rt_waiter); + /* Current is not longer pi_blocked_on */ spin_lock(q.lock_ptr); if (ret && !rt_mutex_cleanup_proxy_lock(pi_mutex, &rt_waiter)) ret = 0; @@ -3276,17 +3482,21 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags, unqueue_me_pi(&q); spin_unlock(q.lock_ptr); - } - if (ret == -EINTR) { - /* - * We've already been requeued, but cannot restart by calling - * futex_lock_pi() directly. We could restart this syscall, but - * it would detect that the user space "val" changed and return - * -EWOULDBLOCK. Save the overhead of the restart and return - * -EWOULDBLOCK directly. - */ - ret = -EWOULDBLOCK; + if (ret == -EINTR) { + /* + * We've already been requeued, but cannot restart + * by calling futex_lock_pi() directly. We could + * restart this syscall, but it would detect that + * the user space "val" changed and return + * -EWOULDBLOCK. Save the overhead of the restart + * and return -EWOULDBLOCK directly. + */ + ret = -EWOULDBLOCK; + } + break; + default: + BUG(); } out: |