Subject: Re: [tip: locking/core] lockdep: Fix lockdep recursion

From: Qian Cai <[email-masked]>
Date: Fri, 09 Oct 2020 13:36:47 -0400

On Fri, 2020-10-09 at 18:23 +0200, Peter Zijlstra wrote:
> On Fri, Oct 09, 2020 at 06:58:37AM -0700, Paul E. McKenney wrote:
> > On Fri, Oct 09, 2020 at 09:41:24AM -0400, Qian Cai wrote:
> > > On Fri, 2020-10-09 at 07:58 +0000, tip-bot2 for Peter Zijlstra wrote:
> > > > The following commit has been merged into the locking/core branch of
> > > > tip:
> > > > 
> > > > Commit-ID:     4d004099a668c41522242aa146a38cc4eb59cb1e
> > > > Gitweb:        
> > > > https://git.kernel.org/tip/4d004099a668c41522242aa146a38cc4eb59cb1e
> > > > Author:        Peter Zijlstra <[email-masked]>
> > > > AuthorDate:    Fri, 02 Oct 2020 11:04:21 +02:00
> > > > Committer:     Ingo Molnar <[email-masked]>
> > > > CommitterDate: Fri, 09 Oct 2020 08:53:30 +02:00
> > > > 
> > > > lockdep: Fix lockdep recursion
> > > > 
> > > > Steve reported that lockdep_assert*irq*(), when nested inside lockdep
> > > > itself, will trigger a false-positive.
> > > > 
> > > > One example is the stack-trace code, as called from inside lockdep,
> > > > triggering tracing, which in turn calls RCU, which then uses
> > > > lockdep_assert_irqs_disabled().
> > > > 
> > > > Fixes: a21ee6055c30 ("lockdep: Change hardirq{s_enabled,_context} to
> > > > per-cpu
> > > > variables")
> > > > Reported-by: Steven Rostedt <[email-masked]>
> > > > Signed-off-by: Peter Zijlstra (Intel) <[email-masked]>
> > > > Signed-off-by: Ingo Molnar <[email-masked]>
> > > 
> > > Reverting this linux-next commit fixed booting RCU-list warnings
> > > everywhere.
> > 
> > Is it possible that the RCU-list warnings were being wrongly suppressed
> > without a21ee6055c30?  As in are you certain that these RCU-list warnings
> > are in fact false positives?
> > > [    4.002695][    T0]  init_timer_key+0x29/0x220
> > > [    4.002695][    T0]  identify_cpu+0xfcb/0x1980
> > > [    4.002695][    T0]  identify_secondary_cpu+0x1d/0x190
> > > [    4.002695][    T0]  smp_store_cpu_info+0x167/0x1f0
> > > [    4.002695][    T0]  start_secondary+0x5b/0x290
> > > [    4.002695][    T0]  secondary_startup_64_no_verify+0xb8/0xbb
> 
> They're actually correct warnings, this is trying to use RCU before that
> CPU is reported to RCU.
> 
> Possibly something like the below works, but I've not tested it, nor
> have I really thought hard about it, bring up tricky and this is just
> moving code.

I don't think this will always work. Basically, anything like printk() would
trigger the warning because it tries to acquire a lock. For example, on arm64:

[    0.418627]  lockdep_rcu_suspicious+0x134/0x14c
[    0.418629]  __lock_acquire+0x1c30/0x2600
[    0.418631]  lock_acquire+0x274/0xc48
[    0.418632]  _raw_spin_lock+0xc8/0x140
[    0.418634]  vprintk_emit+0x90/0x3d0
[    0.418636]  vprintk_default+0x34/0x40
[    0.418638]  vprintk_func+0x378/0x590
[    0.418640]  printk+0xa8/0xd4
[    0.418642]  __cpuinfo_store_cpu+0x71c/0x868
[    0.418644]  cpuinfo_store_cpu+0x2c/0xc8
[    0.418645]  secondary_start_kernel+0x244/0x318

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?

> 
> ---
> 
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 35ad8480c464..9173d64ee69d 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1670,6 +1670,9 @@ void __init identify_boot_cpu(void)
>  void identify_secondary_cpu(struct cpuinfo_x86 *c)
>  {
>  	BUG_ON(c == &boot_cpu_data);
> +
> +	rcu_cpu_starting(smp_processor_id());
> +
>  	identify_cpu(c);
>  #ifdef CONFIG_X86_32
>  	enable_sep_cpu();
> diff --git a/arch/x86/kernel/cpu/mtrr/mtrr.c b/arch/x86/kernel/cpu/mtrr/mtrr.c
> index 6a80f36b5d59..5f436cb4f7c4 100644
> --- a/arch/x86/kernel/cpu/mtrr/mtrr.c
> +++ b/arch/x86/kernel/cpu/mtrr/mtrr.c
> @@ -794,8 +794,6 @@ void mtrr_ap_init(void)
>  	if (!use_intel() || mtrr_aps_delayed_init)
>  		return;
>  
> -	rcu_cpu_starting(smp_processor_id());
> -
>  	/*
>  	 * Ideally we should hold mtrr_mutex here to avoid mtrr entries
>  	 * changed, but this routine will be called in cpu boot time,
>