linux-kernel - Re: [PATCH] arm64/smp: Move rcu_cpu

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Date:   Thu, 29 Oct 2020 09:10:45 +0000
From:   Will Deacon <will@...nel.org>
To:     Qian Cai <cai@...hat.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/smp: Move rcu_cpu_starting() earlier

On Wed, Oct 28, 2020 at 02:26:14PM -0400, Qian Cai wrote:
> The call to rcu_cpu_starting() in secondary_start_kernel() is not early
> enough in the CPU-hotplug onlining process, which results in lockdep
> splats as follows:
> 
>  WARNING: suspicious RCU usage
>  -----------------------------
>  kernel/locking/lockdep.c:3497 RCU-list traversed in non-reader section!!
> 
>  other info that might help us debug this:
> 
>  RCU used illegally from offline CPU!
>  rcu_scheduler_active = 1, debug_locks = 1
>  no locks held by swapper/1/0.
> 
>  Call trace:
>   dump_backtrace+0x0/0x3c8
>   show_stack+0x14/0x60
>   dump_stack+0x14c/0x1c4
>   lockdep_rcu_suspicious+0x134/0x14c
>   __lock_acquire+0x1c30/0x2600
>   lock_acquire+0x274/0xc48
>   _raw_spin_lock+0xc8/0x140
>   vprintk_emit+0x90/0x3d0
>   vprintk_default+0x34/0x40
>   vprintk_func+0x378/0x590
>   printk+0xa8/0xd4
>   __cpuinfo_store_cpu+0x71c/0x868
>   cpuinfo_store_cpu+0x2c/0xc8
>   secondary_start_kernel+0x244/0x318
> 
> This is avoided by moving the call to rcu_cpu_starting up near the
> beginning of the secondary_start_kernel() function.

Hmm, it's not really a move though -- we'll end up calling this thing twice
afaict. It would be better to make sure we've called notify_cpu_starting()
early enough. Can we do that instead?

Will