[PATCH v2] sched/ext: Prevent update_locked_rq() calls with NULL rq

[Breno Leitao <leitao@debian.org>]

Avoid invoking update_locked_rq() when the runqueue (rq) pointer is NULL
in the SCX_CALL_OP and SCX_CALL_OP_RET macros.

Previously, calling update_locked_rq(NULL) with preemption enabled could
trigger the following warning:

    BUG: using __this_cpu_write() in preemptible [00000000]

This happens because __this_cpu_write() is unsafe to use in preemptible
context.

Ensure that update_locked_rq() is only called when rq is non-NULL,
preventing calling __this_cpu_write() on preemptible context.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Fixes: 18853ba782bef ("sched_ext: Track currently locked rq")
Signed-off-by: Breno Leitao <leitao@debian.org>
---
Changes in v2:
- Avoid calling update_locked_rq() completely on preemptible mode (Peter Zijlstra)
- Link to v1: https://lore.kernel.org/r/20250716-scx_warning-v1-1-0e814f78eb8c@debian.org
---
 kernel/sched/ext.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index b498d867ba210..7dd5cbcb7a069 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -1272,7 +1272,8 @@ static inline struct rq *scx_locked_rq(void)
 
 #define SCX_CALL_OP(sch, mask, op, rq, args...)					\
 do {										\
-	update_locked_rq(rq);							\
+	if (rq)									\
+		update_locked_rq(rq);						\
 	if (mask) {								\
 		scx_kf_allow(mask);						\
 		(sch)->ops.op(args);						\
@@ -1280,14 +1281,16 @@ do {										\
 	} else {								\
 		(sch)->ops.op(args);						\
 	}									\
-	update_locked_rq(NULL);							\
+	if (rq)									\
+		update_locked_rq(NULL);						\
 } while (0)
 
 #define SCX_CALL_OP_RET(sch, mask, op, rq, args...)				\
 ({										\
 	__typeof__((sch)->ops.op(args)) __ret;					\
 										\
-	update_locked_rq(rq);							\
+	if (rq)									\
+		update_locked_rq(rq);						\
 	if (mask) {								\
 		scx_kf_allow(mask);						\
 		__ret = (sch)->ops.op(args);					\
@@ -1295,7 +1298,8 @@ do {										\
 	} else {								\
 		__ret = (sch)->ops.op(args);					\
 	}									\
-	update_locked_rq(NULL);							\
+	if (rq)									\
+		update_locked_rq(NULL);						\
 	__ret;									\
 })
 

---
base-commit: 155a3c003e555a7300d156a5252c004c392ec6b0
change-id: 20250716-scx_warning-5143cf17f806

Best regards,
--  
Breno Leitao <leitao@debian.org>