either bug or unnecessary code in exit_mm()

["Christopher Friesen" <cfriesen@nortel.com>]

In the last few kernels (at least as far back as 2.6.10), exit_mm() 
looks like this:


static void exit_mm(struct task_struct * tsk)
{
	struct mm_struct *mm = tsk->mm;

	mm_release(tsk, mm);
	if (!mm)
		return;




while mm_release() looks like this:




void mm_release(struct task_struct *tsk, struct mm_struct *mm)
{
	<snip>
	if (tsk->clear_child_tid && atomic_read(&mm->mm_users) > 1) {





If it's valid to call exit_mm() with tsk->mm being NULL, then it seems 
like we'll get problems when we dereference it unconditionally in 
mm_release().

If mm_release() is valid, then the check for !mm in exit_mm() is 
unnecessary but there should probably be a comment.

Can someone tell me which one it is?

Thanks,

Chris
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