maple_tree: reset mas->index and mas->last on write retries

The following scenario can result in a race condition:

Consider a node with the following indices and values

	a<------->b<----------->c<--------->d
	    0xA        NULL          0xB

	CPU 1			  CPU 2
      ---------        		---------
	mas_set_range(a,b)
	mas_erase()
		-> range is expanded (a,c) because of null expansion

	mas_nomem()
	mas_unlock()
				mas_store_range(b,c,0xC)

The node now looks like:

	a<------->b<----------->c<--------->d
	    0xA        0xC          0xB

	mas_lock()
	mas_erase() <------ range of erase is still (a,c)

The node is now NULL from (a,c) but the write from CPU 2 should have been
retained and range (b,c) should still have 0xC as its value.  We can fix
this by re-intializing to the original index and last.  This does not need
a cc: Stable as there are no users of the maple tree which use internal
locking and this condition is only possible with internal locking.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Sidhartha Kumar <[email protected]>
Reviewed-by: Liam R. Howlett <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

lib/maple_tree.c

@@ -5451,14 +5451,19 @@ EXPORT_SYMBOL_GPL(mas_store);
  */
 int mas_store_gfp(struct ma_state *mas, void *entry, gfp_t gfp)
 {
+	unsigned long index = mas->index;
+	unsigned long last = mas->last;
 	MA_WR_STATE(wr_mas, mas, entry);
 
 	mas_wr_store_setup(&wr_mas);
 	trace_ma_write(__func__, mas, 0, entry);
 retry:
 	mas_wr_store_entry(&wr_mas);
-	if (unlikely(mas_nomem(mas, gfp)))
+	if (unlikely(mas_nomem(mas, gfp))) {
+		if (!entry)
+			__mas_set_range(mas, index, last);
 		goto retry;
+	}
 
 	if (unlikely(mas_is_err(mas)))
 		return xa_err(mas->node);
@@ -6245,23 +6250,26 @@ EXPORT_SYMBOL_GPL(mas_find_range_rev);
 void *mas_erase(struct ma_state *mas)
 {
 	void *entry;
+	unsigned long index = mas->index;
 	MA_WR_STATE(wr_mas, mas, NULL);
 
 	if (!mas_is_active(mas) || !mas_is_start(mas))
 		mas->status = ma_start;
 
-	/* Retry unnecessary when holding the write lock. */
+write_retry:
 	entry = mas_state_walk(mas);
 	if (!entry)
 		return NULL;
 
-write_retry:
 	/* Must reset to ensure spanning writes of last slot are detected */
 	mas_reset(mas);
 	mas_wr_store_setup(&wr_mas);
 	mas_wr_store_entry(&wr_mas);
-	if (mas_nomem(mas, GFP_KERNEL))
+	if (mas_nomem(mas, GFP_KERNEL)) {
+		/* in case the range of entry changed when unlocked */
+		mas->index = mas->last = index;
 		goto write_retry;
+	}
 
 	return entry;
 }