[PATCH 03 of 12] get_task_mm should not succeed if mmput() is running and has reduced

[Andrea Arcangeli <andrea@qumranet.com>]

# HG changeset patch
# User Andrea Arcangeli <andrea@qumranet.com>
# Date 1208872186 -7200
# Node ID a6672bdeead0d41b2ebd6846f731d43a611645b7
# Parent  3c804dca25b15017b22008647783d6f5f3801fa9
get_task_mm should not succeed if mmput() is running and has reduced
the mm_users count to zero. This can occur if a processor follows
a tasks pointer to an mm struct because that pointer is only cleared
after the mmput().

If get_task_mm() succeeds after mmput() reduced the mm_users to zero then
we have the lovely situation that one portion of the kernel is doing
all the teardown work for an mm while another portion is happily using
it.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

diff --git a/kernel/fork.c b/kernel/fork.c
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -442,7 +442,8 @@
 		if (task->flags & PF_BORROWED_MM)
 			mm = NULL;
 		else
-			atomic_inc(&mm->mm_users);
+			if (!atomic_inc_not_zero(&mm->mm_users))
+				mm = NULL;
 	}
 	task_unlock(task);
 	return mm;

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