[PATCH v2 sched_ext/for-6.16] sched_ext: Add RCU protection to scx_root in DSQ iterator
From: Andrea Righi
Date: Wed Apr 30 2025 - 16:22:14 EST
Using a DSQ iterators from a timer callback can trigger the following
lockdep splat when accessing scx_root:
=============================
WARNING: suspicious RCU usage
6.14.0-virtme #1 Not tainted
-----------------------------
kernel/sched/ext.c:6907 suspicious rcu_dereference_check() usage!
other info that might help us debug this:
rcu_scheduler_active = 2, debug_locks = 1
no locks held by swapper/0/0.
stack backtrace:
CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.14.0-virtme #1 PREEMPT(full)
Sched_ext: tickless (enabled+all)
Call Trace:
<IRQ>
dump_stack_lvl+0x6f/0xb0
lockdep_rcu_suspicious.cold+0x4e/0xa3
bpf_iter_scx_dsq_new+0xb1/0xd0
bpf_prog_63f4fd1bccc101e7_dispatch_cpu+0x3e/0x156
bpf_prog_8320d4217989178c_dispatch_all_cpus+0x153/0x1b6
bpf_prog_97f847d871513f95_sched_timerfn+0x4c/0x264
? hrtimer_run_softirq+0x4f/0xd0
bpf_timer_cb+0x7a/0x140
__hrtimer_run_queues+0x1f9/0x3a0
hrtimer_run_softirq+0x8c/0xd0
handle_softirqs+0xd3/0x3d0
__irq_exit_rcu+0x9a/0xc0
irq_exit_rcu+0xe/0x20
sysvec_apic_timer_interrupt+0x73/0x80
Add a proper dereference check to explicitly validate RCU-safe access to
scx_root from rcu_read_lock() contexts and also from contexts that hold
rcu_read_lock_bh(), such as timer callbacks.
Fixes: cdf5a6faa8cf0 ("sched_ext: Move dsq_hash into scx_sched")
Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
---
kernel/sched/ext.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
Changes in v2:
- check rcu_read_lock_bh_held() instead of adding rcu_read_lock/unlock()
- link to v1: https://lore.kernel.org/all/20250430090930.355732-1-arighi@xxxxxxxxxx/
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 1ee5a0088b1a1..00e18eb072bf8 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -6913,7 +6913,7 @@ __bpf_kfunc int bpf_iter_scx_dsq_new(struct bpf_iter_scx_dsq *it, u64 dsq_id,
BUILD_BUG_ON(__alignof__(struct bpf_iter_scx_dsq_kern) !=
__alignof__(struct bpf_iter_scx_dsq));
- sch = rcu_dereference(scx_root);
+ sch = rcu_dereference_check(scx_root, rcu_read_lock_bh_held());
if (!sch)
return -ENODEV;
--
2.49.0