fix for account_group_exec_runtime(), make sure ->signal can't be freed under rq...

fix for account_group_exec_runtime(), make sure ->signal can't be freed under rq->lock

Impact: fix hang/crash on ia64 under high load

This is ugly, but the simplest patch by far.

Unlike other similar routines, account_group_exec_runtime() could be
called "implicitly" from within scheduler after exit_notify(). This
means we can race with the parent doing release_task(), we can't just
check ->signal != NULL.

Change __exit_signal() to do spin_unlock_wait(&task_rq(tsk)->lock)
before __cleanup_signal() to make sure ->signal can't be freed under
task_rq(tsk)->lock. Note that task_rq_unlock_wait() doesn't care
about the case when tsk changes cpu/rq under us, this should be OK.

Thanks to Ingo who nacked my previous buggy patch.

Signed-off-by: Oleg Nesterov <[email protected]>
Acked-by: Peter Zijlstra <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Reported-by: Doug Chapman <[email protected]>