[PATCH] percpu: fix race on alloc failed warning limit

[Vlad Dumitrescu]

The 'allocation failed, ...' warning messages can cause unlimited log
spam, contrary to the implementation's intent.

The warn_limit variable is accessed without synchronization. If more
than <warn_limit> threads enter the warning path at the same time, the
variable will get decremented past 0. Once it becomes negative, the
non-zero check will always return true leading to unlimited log spam.

Use atomic operations to access warn_limit and change the check to test
for positive (> 0) as it can still become negative.

While the change cited in Fixes is only adjacent, the warning limit
implementation was correct before it. Only non-atomic allocations were
considered for warnings, and those happened to hold pcpu_alloc_mutex
while accessing warn_limit.

Fixes: f7d77dfc91f7 ("mm/percpu.c: print error message too if atomic alloc failed")
Signed-off-by: Vlad Dumitrescu <vdumitrescu@xxxxxxxxxx>
---
 mm/percpu.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index a56f35dcc417..c1a4089eb4c3 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1734,7 +1734,7 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved,
 	bool is_atomic;
 	bool do_warn;
 	struct obj_cgroup *objcg = NULL;
-	static int warn_limit = 10;
+	static atomic_t warn_limit = ATOMIC_INIT(10);
 	struct pcpu_chunk *chunk, *next;
 	const char *err;
 	int slot, off, cpu, ret;
@@ -1904,13 +1904,17 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved,
 fail:
 	trace_percpu_alloc_percpu_fail(reserved, is_atomic, size, align);
 
-	if (do_warn && warn_limit) {
-		pr_warn("allocation failed, size=%zu align=%zu atomic=%d, %s\n",
-			size, align, is_atomic, err);
-		if (!is_atomic)
-			dump_stack();
-		if (!--warn_limit)
-			pr_info("limit reached, disable warning\n");
+	if (do_warn && atomic_read(&warn_limit) > 0) {
+		int remaining = atomic_dec_return(&warn_limit);
+
+		if (remaining >= 0) {
+			pr_warn("allocation failed, size=%zu align=%zu atomic=%d, %s\n",
+				size, align, is_atomic, err);
+			if (!is_atomic)
+				dump_stack();
+			if (remaining == 0)
+				pr_info("limit reached, disable warning\n");
+		}
 	}
 
 	if (is_atomic) {
-- 
2.49.0