linux-kernel - Re: [PATCH] cgroup/debug: Fix lockdep splat with "%pK" format specifier

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Message-ID: <YVH0uKGGmRjrIlFy@slm.duckdns.org>
Date:   Mon, 27 Sep 2021 06:43:36 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cgroup/debug: Fix lockdep splat with "%pK" format
 specifier

Hello,

On Sun, Sep 26, 2021 at 10:58:07PM -0400, Waiman Long wrote:
> The lockdep splat is caused by the fact that the debug controller use the
> "%pK" format specifier to print out address of cset's with css_set_lock
> held. Under some circumstances, the use of "%pK" format specifier may
> acquire the selinux_ss.policy_rwlock.
> 
> To avoid this possible deadlock scenario, we have to abandon the use of
> the "%pK" format specifier and just use "%p" to always hash the cset
> addresses. The actual cset addresses aren't that important as long as
> they are unique for matching purpose.

Isn't the right thing to do here making the selinux rwlock an irqsafe one?
It's a bit crazy to have printf specifier to have restrictive locking
requirements.

Thanks.

-- 
tejun