x86: fix global_flush_tlb() bug

While we were reviewing pageattr_32/64.c for unification,
Thomas Gleixner noticed the following serious SMP bug in
global_flush_tlb():

	down_read(&init_mm.mmap_sem);
	list_replace_init(&deferred_pages, &l);
	up_read(&init_mm.mmap_sem);

this is SMP-unsafe because list_replace_init() done on two CPUs in
parallel can corrupt the list.

This bug has been introduced about a year ago in the 64-bit tree:

       commit ea7322de
       Author: Andi Kleen <ak@suse.de>
       Date:   Thu Dec 7 02:14:05 2006 +0100

       [PATCH] x86-64: Speed and clean up cache flushing in change_page_attr

                down_read(&init_mm.mmap_sem);
        -       dpage = xchg(&deferred_pages, NULL);
        +       list_replace_init(&deferred_pages, &l);
                up_read(&init_mm.mmap_sem);

the xchg() based version was SMP-safe, but list_replace_init() is not.
So this "cleanup" introduced a nasty bug.

why this bug never become prominent is a mystery - it can probably be
explained with the (still) relative obscurity of the x86_64 architecture.

the safe fix for now is to write-lock init_mm.mmap_sem.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/mm/pageattr_64.c

@@ -230,9 +230,14 @@ void global_flush_tlb(void)
 	struct page *pg, *next;
 	struct list_head l;
 
-	down_read(&init_mm.mmap_sem);
+	/*
+	 * Write-protect the semaphore, to exclude two contexts
+	 * doing a list_replace_init() call in parallel and to
+	 * exclude new additions to the deferred_pages list:
+	 */
+	down_write(&init_mm.mmap_sem);
 	list_replace_init(&deferred_pages, &l);
-	up_read(&init_mm.mmap_sem);
+	up_write(&init_mm.mmap_sem);
 
 	flush_map(&l);