[PATCH v2 12/19] maple_tree: Catch race in mas_alloc_cyclic()

[Liam R. Howlett (Oracle)]

If mas_alloc_cyclic() is called during a low memory situation, it is
possible the lock may be dropped so reclaim can occur.  There is a
window where some other task may allocate the same id and cause the
mas_insert() to fail with -EEXIST.  In this scenario the function will
return -EEXIST, which is not expected.

Modifying the retry on mas_nomem() to re-search for a slot means that
any race with other writes will not matter as the lock will be held
between finding the index and writing the index.

Moving the flag logic avoids cases where the flag is modified on drop
lock/reacquire or when the write fails after clearing the flag.

No existing users are exposed to this issue.

Fixes: 9b6713cc75229 ("maple_tree: Add mtree_alloc_cyclic()")
Reported-by: Chris Mason <clm@xxxxxxxx>
Cc: Chuck Lever <cel@xxxxxxxxxx>
Signed-off-by: Liam R. Howlett (Oracle) <liam@xxxxxxxxxxxxx>
---
 lib/maple_tree.c | 43 ++++++++++++++++++++++++-------------------
 1 file changed, 24 insertions(+), 19 deletions(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 768193406c3db..1bc177bf42f9f 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -3867,35 +3867,40 @@ int mas_alloc_cyclic(struct ma_state *mas, unsigned long *startp,
 		void *entry, unsigned long range_lo, unsigned long range_hi,
 		unsigned long *next, gfp_t gfp)
 {
-	unsigned long min = range_lo;
-	int ret = 0;
-
-	range_lo = max(min, *next);
-	ret = mas_empty_area(mas, range_lo, range_hi, 1);
-	if ((mas->tree->ma_flags & MT_FLAGS_ALLOC_WRAPPED) && ret == 0) {
-		mas->tree->ma_flags &= ~MT_FLAGS_ALLOC_WRAPPED;
-		ret = 1;
-	}
-	if (ret < 0 && range_lo > min) {
-		mas_reset(mas);
-		ret = mas_empty_area(mas, min, range_hi, 1);
-		if (ret == 0)
-			ret = 1;
-	}
-	if (ret < 0)
-		return ret;
+	int ret;
+	unsigned long min;
 
+	min = range_lo;
 	do {
+		range_lo = max(min, *next);
+		ret = mas_empty_area(mas, range_lo, range_hi, 1);
+		if (ret < 0 && range_lo > min) {
+			mas_reset(mas);
+			ret = mas_empty_area(mas, min, range_hi, 1);
+			if (ret == 0)
+				ret = 1;
+		}
+		if (ret < 0)
+			goto out;
+
 		mas_insert(mas, entry);
 	} while (mas_nomem(mas, gfp));
-	if (mas_is_err(mas))
-		return xa_err(mas->node);
 
+	if (mas_is_err(mas)) {
+		ret = xa_err(mas->node);
+		goto out;
+	}
+
+	if ((mas->tree->ma_flags & MT_FLAGS_ALLOC_WRAPPED) && ret == 0) {
+		mas->tree->ma_flags &= ~MT_FLAGS_ALLOC_WRAPPED;
+		ret = 1;
+	}
 	*startp = mas->index;
 	*next = *startp + 1;
 	if (*next == 0)
 		mas->tree->ma_flags |= MT_FLAGS_ALLOC_WRAPPED;
 
+out:
 	mas_destroy(mas);
 	return ret;
 }
-- 
2.47.3