linux-kernel - [PATCH v1] cgroup: drop preemption

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Message-ID: <20251119111402.153727-1-jiayuan.chen@linux.dev>
Date: Wed, 19 Nov 2025 19:14:01 +0800
From: Jiayuan Chen <jiayuan.chen@...ux.dev>
To: cgroups@...r.kernel.org
Cc: tj@...nel.org,
	hannes@...xchg.org,
	mkoutny@...e.com,
	linux-kernel@...r.kernel.org,
	Jiayuan Chen <jiayuan.chen@...ux.dev>
Subject: [PATCH v1] cgroup: drop preemption_disabled checking

BPF programs do not disable preemption, they only disable migration.
Therefore, when running the cgroup_hierarchical_stats selftest, a
warning [1] is generated.

The css_rstat_updated() function is lockless and reentrant, so checking
for disabled preemption is unnecessary (please correct me if I'm wrong).

[1]:
~/tools/testing/selftests/bpf$
test_progs -a cgroup_hierarchical_stats

...
------------[ cut here ]------------
WARNING: CPU: 0 PID: 382 at kernel/cgroup/rstat.c:84
Modules linked in:
RIP: 0010:css_rstat_updated+0x9d/0x160
...
PKRU: 55555554
Call Trace:
 <TASK>
 bpf_prog_16a1c2d081688506_counter+0x143/0x14e
 bpf_trampoline_6442524909+0x4b/0xb7
 cgroup_attach_task+0x5/0x330
 ? __cgroup_procs_write+0x1d7/0x2f0
 cgroup_procs_write+0x17/0x30
 cgroup_file_write+0xa6/0x2d0
 kernfs_fop_write_iter+0x188/0x240
 vfs_write+0x2da/0x5a0
 ksys_write+0x77/0x100
 __x64_sys_write+0x19/0x30
 x64_sys_call+0x79/0x26a0
 do_syscall_64+0x89/0x790
 ? irqentry_exit+0x77/0xb0
 ? __this_cpu_preempt_check+0x13/0x20
 ? lockdep_hardirqs_on+0xce/0x170
 ? irqentry_exit_to_user_mode+0xf2/0x290
 ? irqentry_exit+0x77/0xb0
 ? clear_bhb_loop+0x50/0xa0
 ? clear_bhb_loop+0x50/0xa0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
---[ end trace 0000000000000000 ]---

Signed-off-by: Jiayuan Chen <jiayuan.chen@...ux.dev>
---
 kernel/cgroup/rstat.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index a198e40c799b..fe0d22280cbd 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -81,8 +81,6 @@ __bpf_kfunc void css_rstat_updated(struct cgroup_subsys_state *css, int cpu)
 	if (!css_uses_rstat(css))
 		return;
 
-	lockdep_assert_preemption_disabled();
-
 	/*
 	 * For archs withnot nmi safe cmpxchg or percpu ops support, ignore
 	 * the requests from nmi context.
-- 
2.43.0