mm, writeback: prevent race when calculating dirty limits

Setting vm_dirty_bytes and dirty_background_bytes is not protected by
any serialization.

Therefore, it's possible for either variable to change value after the
test in global_dirty_limits() to determine whether available_memory
needs to be initialized or not.

Always ensure that available_memory is properly initialized.
Signed-off-by: David Rientjes <rientjes@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page-writeback.c

@@ -261,14 +261,11 @@ static unsigned long global_dirtyable_memory(void)
  */
 void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
 {
+	const unsigned long available_memory = global_dirtyable_memory();
 	unsigned long background;
 	unsigned long dirty;
-	unsigned long uninitialized_var(available_memory);
 	struct task_struct *tsk;
 
-	if (!vm_dirty_bytes || !dirty_background_bytes)
-		available_memory = global_dirtyable_memory();
-
 	if (vm_dirty_bytes)
 		dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
 	else