Robust futexes utilize the robust_list mechanism to allow the kernel to
release futexes which are held when a task exits. The exit can be voluntary
or caused by a signal or fault. This prevents that waiters block forever.

The futex operations in user space store a pointer to the futex they are
either locking or unlocking in the op_pending member of the per task robust
list.

After a lock operation has succeeded the futex is queued in the robust list
linked list and the op_pending pointer is cleared.

After an unlock operation has succeeded the futex is removed from the
robust list linked list and the op_pending pointer is cleared.

The robust list exit code checks for the pending operation and any futex
which is queued in the linked list. It carefully checks whether the futex
value is the TID of the exiting task. If so, it sets the OWNER_DIED bit and
tries to wake up a potential waiter.

This is race free for the lock operation but unlock has two race scenarios
where waiters might not be woken up. These issues can be observed with
regular robust pthread mutexes. PI aware pthread mutexes are not affected.

(1) Unlocking task is killed after unlocking the futex value in user space
    before being able to wake a waiter.

        pthread_mutex_unlock()
                |
                V
        atomic_exchange_rel (&mutex->__data.__lock, 0)
                        <------------------------killed
            lll_futex_wake ()                   |
                                                |
                                                |(__lock = 0)
                                                |(enter kernel)
                                                |
                                                V
                                            do_exit()
                                            exit_mm()
                                          mm_release()
                                        exit_robust_list()
                                        handle_futex_death()
                                                |
                                                |(__lock = 0)
                                                |(uval = 0)
                                                |
                                                V
        if ((uval & FUTEX_TID_MASK) != task_pid_vnr(curr))
                return 0;

    The sanity check which ensures that the user space futex is owned by
    the exiting task prevents the wakeup of waiters which in consequence
    block infinitely.

(2) Waiting task is killed after a wakeup and before it can acquire the
    futex in user space.

        OWNER                         WAITER
				futex_wait()      		
   pthread_mutex_unlock()               |
                |                       |
                |(__lock = 0)           |
                |                       |
                V                       |
         futex_wake() ------------>  wakeup()
                                        |
                                        |(return to userspace)
                                        |(__lock = 0)
                                        |
                                        V
                        oldval = mutex->__data.__lock
                                          <-----------------killed
    atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,  |
                        id | assume_other_futex_waiters, 0)      |
                                                                 |
                                                                 |
                                                   (enter kernel)|
                                                                 |
                                                                 V
                                                         do_exit()
                                                        |
                                                        |
                                                        V
                                        handle_futex_death()
                                        |
                                        |(__lock = 0)
                                        |(uval = 0)
                                        |
                                        V
        if ((uval & FUTEX_TID_MASK) != task_pid_vnr(curr))
                return 0;

    The sanity check which ensures that the user space futex is owned
    by the exiting task prevents the wakeup of waiters, which seems to
    be correct as the exiting task does not own the futex value, but
    the consequence is that other waiters wont be woken up and block
    infinitely.

In both scenarios the following conditions are true:

   - task->robust_list->list_op_pending != NULL
   - user space futex value == 0
   - Regular futex (not PI)

If these conditions are met then it is reasonably safe to wake up a
potential waiter in order to prevent the above problems.

As this might be a false positive it can cause spurious wakeups, but the
waiter side has to handle other types of unrelated wakeups, e.g. signals
gracefully anyway. So such a spurious wakeup will not affect the
correctness of these operations.

This workaround must not touch the user space futex value and cannot set
the OWNER_DIED bit because the lock value is 0, i.e. uncontended. Setting
OWNER_DIED in this case would result in inconsistent state and subsequently
in malfunction of the owner died handling in user space.

The rest of the user space state is still consistent as no other task can
observe the list_op_pending entry in the exiting tasks robust list.

The eventually woken up waiter will observe the uncontended lock value and
take it over.

[ tglx: Massaged changelog and comment. Made the return explicit and not
  	depend on the subsequent check and added constants to hand into
  	handle_futex_death() instead of plain numbers. Fixed a few coding
	style issues. ]

Fixes: 0771dfef ("[PATCH] lightweight robust futexes: core")
Signed-off-by: Yang Tao <yang.tao172@zte.com.cn>
Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/1573010582-35297-1-git-send-email-wang.yi59@zte.com.cn
Link: https://lkml.kernel.org/r/20191106224555.943191378@linutronix.de