getxpid() parent lookup is broken

[Ben Hutchings <ben@decadent.org.uk>]

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I noticed that Tomoyo doesn't build on Alpha because Tomoyo tries to
call sys_getpid() and sys_getppid().  That's a bug in Tomoyo, but when I
looked at why Alpha is different I found that the implementation of
getxpid() hasn't been kept in sync getppid() for other architectures and
is presumably now incorrect.

sys_getppid():
	rcu_read_lock();
	pid = task_tgid_vnr(current->real_parent);
	rcu_read_unlock();

task_tgid_vnr(current->real_parent) expands through various inline
functions to:

pid_nr_ns(current->real_parent->group_leader->pids[PIDTYPE_PID].pid,
          current->nsproxy->pid_ns)

sys_getxpid():
	/* See linux/kernel/timer.c sys_getppid for discussion
	   about this loop.  */
	ldq	$3, TASK_GROUP_LEADER($2)
	ldq	$4, TASK_REAL_PARENT($3)
	ldl	$0, TASK_TGID($2)
1:	ldl	$1, TASK_TGID($4)
#ifdef CONFIG_SMP
	mov	$4, $5
	mb
	ldq	$3, TASK_GROUP_LEADER($2)
	ldq	$4, TASK_REAL_PARENT($3)
	cmpeq	$4, $5, $5
	beq	$5, 1b
#endif

The comment is obviously out-of-date.  This isn't following RCU protocol
and it isn't namespace-aware.  I think it needs to be turned into a
wrapper for the generic code.

Ben.

-- 
Ben Hutchings
Once a job is fouled up, anything done to improve it makes it worse.

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