diff options
Diffstat (limited to 'kernel')
| -rw-r--r-- | kernel/futex.c | 106 |
1 files changed, 91 insertions, 15 deletions
diff --git a/kernel/futex.c b/kernel/futex.c index 660d6e259dca..f5aae14c247b 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1176,6 +1176,36 @@ out_error: return ret; } +/** + * wait_for_owner_exiting - Block until the owner has exited + * @exiting: Pointer to the exiting task + * + * Caller must hold a refcount on @exiting. + */ +static void wait_for_owner_exiting(int ret, struct task_struct *exiting) +{ + if (ret != -EBUSY) { + WARN_ON_ONCE(exiting); + return; + } + + if (WARN_ON_ONCE(ret == -EBUSY && !exiting)) + return; + + mutex_lock(&exiting->futex_exit_mutex); + /* + * No point in doing state checking here. If the waiter got here + * while the task was in exec()->exec_futex_release() then it can + * have any FUTEX_STATE_* value when the waiter has acquired the + * mutex. OK, if running, EXITING or DEAD if it reached exit() + * already. Highly unlikely and not a problem. Just one more round + * through the futex maze. + */ + mutex_unlock(&exiting->futex_exit_mutex); + + put_task_struct(exiting); +} + static int handle_exit_race(u32 __user *uaddr, u32 uval, struct task_struct *tsk) { @@ -1237,7 +1267,8 @@ static int handle_exit_race(u32 __user *uaddr, u32 uval, * it after doing proper sanity checks. */ static int attach_to_pi_owner(u32 __user *uaddr, u32 uval, union futex_key *key, - struct futex_pi_state **ps) + struct futex_pi_state **ps, + struct task_struct **exiting) { pid_t pid = uval & FUTEX_TID_MASK; struct futex_pi_state *pi_state; @@ -1276,7 +1307,19 @@ static int attach_to_pi_owner(u32 __user *uaddr, u32 uval, union futex_key *key, int ret = handle_exit_race(uaddr, uval, p); raw_spin_unlock_irq(&p->pi_lock); - put_task_struct(p); + /* + * If the owner task is between FUTEX_STATE_EXITING and + * FUTEX_STATE_DEAD then store the task pointer and keep + * the reference on the task struct. The calling code will + * drop all locks, wait for the task to reach + * FUTEX_STATE_DEAD and then drop the refcount. This is + * required to prevent a live lock when the current task + * preempted the exiting task between the two states. + */ + if (ret == -EBUSY) + *exiting = p; + else + put_task_struct(p); return ret; } @@ -1315,7 +1358,8 @@ static int attach_to_pi_owner(u32 __user *uaddr, u32 uval, union futex_key *key, static int lookup_pi_state(u32 __user *uaddr, u32 uval, struct futex_hash_bucket *hb, - union futex_key *key, struct futex_pi_state **ps) + union futex_key *key, struct futex_pi_state **ps, + struct task_struct **exiting) { struct futex_q *top_waiter = futex_top_waiter(hb, key); @@ -1330,7 +1374,7 @@ static int lookup_pi_state(u32 __user *uaddr, u32 uval, * We are the first waiter - try to look up the owner based on * @uval and attach to it. */ - return attach_to_pi_owner(uaddr, uval, key, ps); + return attach_to_pi_owner(uaddr, uval, key, ps, exiting); } static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval) @@ -1358,6 +1402,8 @@ static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval) * lookup * @task: the task to perform the atomic lock work for. This will * be "current" except in the case of requeue pi. + * @exiting: Pointer to store the task pointer of the owner task + * which is in the middle of exiting * @set_waiters: force setting the FUTEX_WAITERS bit (1) or not (0) * * Return: @@ -1366,11 +1412,17 @@ static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval) * - <0 - error * * The hb->lock and futex_key refs shall be held by the caller. + * + * @exiting is only set when the return value is -EBUSY. If so, this holds + * a refcount on the exiting task on return and the caller needs to drop it + * after waiting for the exit to complete. */ static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb, union futex_key *key, struct futex_pi_state **ps, - struct task_struct *task, int set_waiters) + struct task_struct *task, + struct task_struct **exiting, + int set_waiters) { u32 uval, newval, vpid = task_pid_vnr(task); struct futex_q *top_waiter; @@ -1440,7 +1492,7 @@ static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb, * attach to the owner. If that fails, no harm done, we only * set the FUTEX_WAITERS bit in the user space variable. */ - return attach_to_pi_owner(uaddr, newval, key, ps); + return attach_to_pi_owner(uaddr, newval, key, ps, exiting); } /** @@ -1859,6 +1911,8 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key, * @key1: the from futex key * @key2: the to futex key * @ps: address to store the pi_state pointer + * @exiting: Pointer to store the task pointer of the owner task + * which is in the middle of exiting * @set_waiters: force setting the FUTEX_WAITERS bit (1) or not (0) * * Try and get the lock on behalf of the top waiter if we can do it atomically. @@ -1866,16 +1920,20 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key, * then direct futex_lock_pi_atomic() to force setting the FUTEX_WAITERS bit. * hb1 and hb2 must be held by the caller. * + * @exiting is only set when the return value is -EBUSY. If so, this holds + * a refcount on the exiting task on return and the caller needs to drop it + * after waiting for the exit to complete. + * * Return: * - 0 - failed to acquire the lock atomically; * - >0 - acquired the lock, return value is vpid of the top_waiter * - <0 - error */ -static int futex_proxy_trylock_atomic(u32 __user *pifutex, - struct futex_hash_bucket *hb1, - struct futex_hash_bucket *hb2, - union futex_key *key1, union futex_key *key2, - struct futex_pi_state **ps, int set_waiters) +static int +futex_proxy_trylock_atomic(u32 __user *pifutex, struct futex_hash_bucket *hb1, + struct futex_hash_bucket *hb2, union futex_key *key1, + union futex_key *key2, struct futex_pi_state **ps, + struct task_struct **exiting, int set_waiters) { struct futex_q *top_waiter = NULL; u32 curval; @@ -1912,7 +1970,7 @@ static int futex_proxy_trylock_atomic(u32 __user *pifutex, */ vpid = task_pid_vnr(top_waiter->task); ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task, - set_waiters); + exiting, set_waiters); if (ret == 1) { requeue_pi_wake_futex(top_waiter, key2, hb2); return vpid; @@ -2041,6 +2099,8 @@ retry_private: } if (requeue_pi && (task_count - nr_wake < nr_requeue)) { + struct task_struct *exiting = NULL; + /* * Attempt to acquire uaddr2 and wake the top waiter. If we * intend to requeue waiters, force setting the FUTEX_WAITERS @@ -2048,7 +2108,8 @@ retry_private: * faults rather in the requeue loop below. */ ret = futex_proxy_trylock_atomic(uaddr2, hb1, hb2, &key1, - &key2, &pi_state, nr_requeue); + &key2, &pi_state, + &exiting, nr_requeue); /* * At this point the top_waiter has either taken uaddr2 or is @@ -2075,7 +2136,8 @@ retry_private: * If that call succeeds then we have pi_state and an * initial refcount on it. */ - ret = lookup_pi_state(uaddr2, ret, hb2, &key2, &pi_state); + ret = lookup_pi_state(uaddr2, ret, hb2, &key2, + &pi_state, &exiting); } switch (ret) { @@ -2105,6 +2167,12 @@ retry_private: hb_waiters_dec(hb2); put_futex_key(&key2); put_futex_key(&key1); + /* + * Handle the case where the owner is in the middle of + * exiting. Wait for the exit to complete otherwise + * this task might loop forever, aka. live lock. + */ + wait_for_owner_exiting(ret, exiting); cond_resched(); goto retry; default: @@ -2820,6 +2888,7 @@ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags, { struct hrtimer_sleeper timeout, *to = NULL; struct futex_pi_state *pi_state = NULL; + struct task_struct *exiting = NULL; struct rt_mutex_waiter rt_waiter; struct futex_hash_bucket *hb; struct futex_q q = futex_q_init; @@ -2847,7 +2916,8 @@ retry: retry_private: hb = queue_lock(&q); - ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current, 0); + ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current, + &exiting, 0); if (unlikely(ret)) { /* * Atomic work succeeded and we got the lock, @@ -2870,6 +2940,12 @@ retry_private: */ queue_unlock(hb); put_futex_key(&q.key); + /* + * Handle the case where the owner is in the middle of + * exiting. Wait for the exit to complete otherwise + * this task might loop forever, aka. live lock. + */ + wait_for_owner_exiting(ret, exiting); cond_resched(); goto retry; default: |