[PATCH 6.15.y] sched/ext: Prevent update_locked_rq() calls with NULL rq

[Sasha Levin <sashal@kernel.org>]

From: Breno Leitao <leitao@debian.org>

[ Upstream commit e14fd98c6d66cb76694b12c05768e4f9e8c95664 ]

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.

rq is NULL when an ops invoked from an unlocked context. In such cases, we
don't need to store any rq, since the value should already be NULL
(unlocked). 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>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org # v6.15
Signed-off-by: Sasha Levin <sashal@kernel.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 86ce43fa36693..bb5148c87750e 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -1149,7 +1149,8 @@ static inline struct rq *scx_locked_rq(void)
 
 #define SCX_CALL_OP(mask, op, rq, args...)					\
 do {										\
-	update_locked_rq(rq);							\
+	if (rq)									\
+		update_locked_rq(rq);						\
 	if (mask) {								\
 		scx_kf_allow(mask);						\
 		scx_ops.op(args);						\
@@ -1157,14 +1158,16 @@ do {										\
 	} else {								\
 		scx_ops.op(args);						\
 	}									\
-	update_locked_rq(NULL);							\
+	if (rq)									\
+		update_locked_rq(NULL);						\
 } while (0)
 
 #define SCX_CALL_OP_RET(mask, op, rq, args...)					\
 ({										\
 	__typeof__(scx_ops.op(args)) __ret;					\
 										\
-	update_locked_rq(rq);							\
+	if (rq)									\
+		update_locked_rq(rq);						\
 	if (mask) {								\
 		scx_kf_allow(mask);						\
 		__ret = scx_ops.op(args);					\
@@ -1172,7 +1175,8 @@ do {										\
 	} else {								\
 		__ret = scx_ops.op(args);					\
 	}									\
-	update_locked_rq(NULL);							\
+	if (rq)									\
+		update_locked_rq(NULL);						\
 	__ret;									\
 })
 
-- 
2.39.5