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Message-ID: <942e0ffb37a4580982206d72404c521d72d38314.camel@redhat.com>
Subject: Re: [tip: locking/core] lockdep: Fix lockdep recursion
From:   Qian Cai <cai@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>, x86 <x86@kernel.org>,
        linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org,
        Linux Next Mailing List <linux-next@vger.kernel.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Boqun Feng <boqun.feng@gmail.com>
Date:   Fri, 09 Oct 2020 13:54:34 -0400
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         <e438b231c5e1478527af6c3e69bf0b37df650110.camel@redhat.com>
         <20201009135837.GD29330@paulmck-ThinkPad-P72>
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On Fri, 2020-10-09 at 13:36 -0400, Qian Cai wrote:
> Back to x86, we have:
> 
> start_secondary()
>   smp_callin()
>     apic_ap_setup()
>       setup_local_APIC()
>         printk() in certain conditions.
> 
> which is before smp_store_cpu_info().
> 
> Can't we add a rcu_cpu_starting() at the very top for each start_secondary(),
> secondary_start_kernel(), smp_start_secondary() etc, so we don't worry about
> any printk() later?

This is rather irony. rcu_cpu_starting() is taking a lock and then reports
itself.

[    8.826732][    T0]  __lock_acquire.cold.76+0x2ad/0x3e0
[    8.826732][    T0]  lock_acquire+0x1c8/0x820
[    8.826732][    T0]  _raw_spin_lock_irqsave+0x30/0x50
[    8.826732][    T0]  rcu_cpu_starting+0xd0/0x2c0
[    8.826732][    T0]  start_secondary+0x10/0x2a0
[    8.826732][    T0]  secondary_startup_64_no_verify+0xb8/0xbb