Ulrich has been working on the glibc code using posix-timers and
stressing it more now than it has before.  He ran into an SMP deadlock
on process exit in the case there are pending queued signals from a
timer.

The deadlock arises because in the path through exit_itimers, the
tasklist_lock is already held (for writing).  When a timer is being
deleted, sigqueue_free will try to take it (for reading) in the case
where that timer has a pending signal queued on somebody's queue.  This
patch avoids the problem by making sure the queues are flushed before
calling exit_itimers, thus ensuring its code path won't try to take
tasklist_lock.