[PATCH bpf-next v5 2/3] perf: Add atomic operation in get_recursion_context

[Tao Chen <chen.dylane@linux.dev>]

From BPF side, preemption usually is enabled. Yonghong said, it is
possible that both tasks (at process level) may reach right before
"recursion[rctx]++;". In such cases, both tasks will be able to get
buffer and this is not right.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Tao Chen <chen.dylane@linux.dev>
---
 kernel/events/internal.h | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index d9cc5708309..684bde972ba 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -214,12 +214,9 @@ static inline int get_recursion_context(u8 *recursion)
 {
 	unsigned char rctx = interrupt_context_level();
 
-	if (recursion[rctx])
+	if (cmpxchg(&recursion[rctx], 0, 1) != 0)
 		return -1;
 
-	recursion[rctx]++;
-	barrier();
-
 	return rctx;
 }
 
-- 
2.48.1