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Date:   Wed, 16 Mar 2022 21:48:06 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "J. Bruce Fields" <bfields@fieldses.org>
Cc:     Chengming Zhou <zhouchengming@bytedance.com>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: failure to boot after dc6e0818bc9a "sched/cpuacct: Optimize away
 RCU read lock"
Message-ID: <20220316204806.GL8939@worktop.programming.kicks-ass.net>
References: <20220316174324.GA16511@fieldses.org>
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On Wed, Mar 16, 2022 at 01:43:24PM -0400, J. Bruce Fields wrote:
> One of my test VMs has been failing to boot linux-next recently.  I
> finally got around to a bisect this morning, and it landed on the below.
> 
> What other information would be useful to debug this?

A more recent -next should have this commit in it:


commit f2aa197e4794bf4c2c0c9570684f86e6fa103e8b
Author: Chengming Zhou <zhouchengming@bytedance.com>
Date:   Sat Mar 5 11:41:03 2022 +0800

    cgroup: Fix suspicious rcu_dereference_check() usage warning
    
    task_css_set_check() will use rcu_dereference_check() to check for
    rcu_read_lock_held() on the read-side, which is not true after commit
    dc6e0818bc9a ("sched/cpuacct: Optimize away RCU read lock"). This
    commit drop explicit rcu_read_lock(), change to RCU-sched read-side
    critical section. So fix the RCU warning by adding check for
    rcu_read_lock_sched_held().
    
    Fixes: dc6e0818bc9a ("sched/cpuacct: Optimize away RCU read lock")
    Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
    Reported-by: syzbot+16e3f2c77e7c5a0113f9@syzkaller.appspotmail.com
    Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Acked-by: Tejun Heo <tj@kernel.org>
    Tested-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
    Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
    Link: https://lore.kernel.org/r/20220305034103.57123-1-zhouchengming@bytedance.com

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 1e356c222756..0d1ada8968d7 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -450,6 +450,7 @@ extern struct mutex cgroup_mutex;
 extern spinlock_t css_set_lock;
 #define task_css_set_check(task, __c)					\
 	rcu_dereference_check((task)->cgroups,				\
+		rcu_read_lock_sched_held() ||				\
 		lockdep_is_held(&cgroup_mutex) ||			\
 		lockdep_is_held(&css_set_lock) ||			\
 		((task)->flags & PF_EXITING) || (__c))