linux-kernel - Re: [PATCH printk v1 03/10] kgdb: delay roundup if holding printk cpulock

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Date:   Wed, 4 Aug 2021 14:31:36 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Jason Wessel <jason.wessel@...driver.com>,
        Douglas Anderson <dianders@...omium.org>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        Chengyang Fan <cy.fan@...wei.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Bhaskar Chowdhury <unixbhaskar@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Cédric Le Goater <clg@...d.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linuxppc-dev@...ts.ozlabs.org, kgdb-bugreport@...ts.sourceforge.net
Subject: Re: [PATCH printk v1 03/10] kgdb: delay roundup if holding printk
 cpulock

On Tue 2021-08-03 17:36:32, John Ogness wrote:
> On 2021-08-03, Daniel Thompson <daniel.thompson@...aro.org> wrote:
> > On Tue, Aug 03, 2021 at 03:18:54PM +0206, John Ogness wrote:
> >> kgdb makes use of its own cpulock (@dbg_master_lock, @kgdb_active)
> >> during cpu roundup. This will conflict with the printk cpulock.
> >
> >> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> >> index 3d0c933937b4..1b546e117f10 100644
> >> --- a/kernel/printk/printk.c
> >> +++ b/kernel/printk/printk.c
> >> @@ -214,6 +215,7 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
> >>  #ifdef CONFIG_SMP
> >>  static atomic_t printk_cpulock_owner = ATOMIC_INIT(-1);
> >>  static atomic_t printk_cpulock_nested = ATOMIC_INIT(0);
> >> +static unsigned int kgdb_cpu = -1;
> >
> > Is this the flag to provoke retriggering? It appears to be a write-only
> > variable (at least in this patch). How is it consumed?
> 
> Critical catch! Thank you. I am quite unhappy to see these hunks were
> accidentally dropped when generating this series.
> 
> @@ -3673,6 +3675,9 @@ EXPORT_SYMBOL(__printk_cpu_trylock);
>   */
>  void __printk_cpu_unlock(void)
>  {
> +	bool trigger_kgdb = false;
> +	unsigned int cpu;
> +
>  	if (atomic_read(&printk_cpulock_nested)) {
>  		atomic_dec(&printk_cpulock_nested);
>  		return;
> @@ -3683,6 +3688,12 @@ void __printk_cpu_unlock(void)
>  	 * LMM(__printk_cpu_unlock:A)
>  	 */
>  
> +	cpu = smp_processor_id();
> +	if (kgdb_cpu == cpu) {
> +		trigger_kgdb = true;
> +		kgdb_cpu = -1;
> +	}

Just in case that this approach is used in the end.

This code looks racy. kgdb_roundup_delay() seems to be called in NMI
context. NMI might happen at this point and set kgdb_cpu after
it was checked.

I am afraid that it won't be easy to make this safe using a single
global variable. A solution might be a per-CPU variable set
by kgdb_roundup_delay() when it owns printk_cpu_lock.
__printk_cpu_unlock() would call kgdb_roundup_cpu(cpu) when
the variable is set.

Nit: The name "kgdb_cpu" is too generic. It is not clear what is
     so special about this CPU. I would call the per-CPU variable
     "kgdb_delayed_roundup" or so.


Best Regards,
Petr

>  	/*
>  	 * Guarantee loads and stores from this CPU when it was the
>  	 * lock owner are visible to the next lock owner. This pairs
> @@ -3703,6 +3714,21 @@ void __printk_cpu_unlock(void)
>  	 */
>  	atomic_set_release(&printk_cpulock_owner,
>  			   -1); /* LMM(__printk_cpu_unlock:B) */
> +
> +	if (trigger_kgdb) {
> +		pr_warn("re-triggering kgdb roundup for CPU#%d\n", cpu);
> +		kgdb_roundup_cpu(cpu);
> +	}
>  }