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From: Petr Mladek <pmladek@suse.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: "Daniel Thompson" <daniel.thompson@linaro.org>,
	"Sergey Senozhatsky" <senozhatsky@chromium.org>,
	"Steven Rostedt" <rostedt@goodmis.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org,
	"Michael Ellerman" <mpe@ellerman.id.au>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Paul Mackerras" <paulus@samba.org>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	"Jason Wessel" <jason.wessel@windriver.com>,
	"Douglas Anderson" <dianders@chromium.org>,
	"Srikar Dronamraju" <srikar@linux.vnet.ibm.com>,
	"Gautham R. Shenoy" <ego@linux.vnet.ibm.com>,
	"Chengyang Fan" <cy.fan@huawei.com>,
	"Christophe Leroy" <christophe.leroy@csgroup.eu>,
	"Bhaskar Chowdhury" <unixbhaskar@gmail.com>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"Cédric Le Goater" <clg@kaod.org>,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	linuxppc-dev@lists.ozlabs.org,
	kgdb-bugreport@lists.sourceforge.net
Subject: Re: [PATCH printk v1 03/10] kgdb: delay roundup if holding printk cpulock
Date: Wed, 4 Aug 2021 14:31:36 +0200	[thread overview]
Message-ID: <YQqIqKjRcNhZSaAZ@alley> (raw)
In-Reply-To: <87tuk635rb.fsf@jogness.linutronix.de>

On Tue 2021-08-03 17:36:32, John Ogness wrote:
> On 2021-08-03, Daniel Thompson <daniel.thompson@linaro.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);
> +	}
>  }

  parent reply	other threads:[~2021-08-04 12:31 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-03 13:12 [PATCH printk v1 00/10] printk: introduce atomic consoles and sync mode John Ogness
2021-08-03 13:12 ` [PATCH printk v1 01/10] printk: relocate printk cpulock functions John Ogness
2021-08-04  9:24   ` Petr Mladek
2021-08-03 13:12 ` [PATCH printk v1 02/10] printk: rename printk cpulock API and always disable interrupts John Ogness
2021-08-04  9:52   ` Petr Mladek
2021-08-03 13:12 ` [PATCH printk v1 03/10] kgdb: delay roundup if holding printk cpulock John Ogness
2021-08-03 14:25   ` Daniel Thompson
2021-08-03 15:30     ` John Ogness
2021-08-04 11:31       ` Daniel Thompson
2021-08-04 12:12         ` Petr Mladek
2021-08-04 15:04           ` Daniel Thompson
2021-08-05  3:46             ` John Ogness
2021-08-06 12:06               ` Daniel Thompson
2021-08-04 12:31       ` Petr Mladek [this message]
2021-08-03 13:12 ` [PATCH printk v1 04/10] printk: relocate printk_delay() John Ogness
2021-08-04 13:07   ` Petr Mladek
2021-08-03 13:12 ` [PATCH printk v1 05/10] printk: call boot_delay_msec() in printk_delay() John Ogness
2021-08-04 13:09   ` Petr Mladek
2021-08-31  1:04   ` Sergey Senozhatsky
2021-08-03 13:12 ` [PATCH printk v1 06/10] printk: use seqcount_latch for console_seq John Ogness
2021-08-05 12:16   ` Petr Mladek
2021-08-05 15:26     ` John Ogness
2021-08-06 15:56       ` Petr Mladek
2021-08-31  3:05         ` Sergey Senozhatsky
2021-08-03 13:12 ` [PATCH printk v1 07/10] console: add write_atomic interface John Ogness
2021-08-03 14:02   ` Andy Shevchenko
2021-08-06 10:56     ` John Ogness
2021-08-06 11:18       ` Andy Shevchenko
2021-08-31  2:55   ` Sergey Senozhatsky
2021-08-03 13:12 ` [PATCH printk v1 08/10] printk: introduce kernel sync mode John Ogness
2021-08-05 17:11   ` Petr Mladek
2021-08-05 21:25     ` John Ogness
2021-08-03 13:13 ` [PATCH printk v1 09/10] kdb: if available, only use atomic consoles for output mirroring John Ogness
2021-08-03 13:13 ` [PATCH printk v1 10/10] serial: 8250: implement write_atomic John Ogness
2021-08-03 14:07   ` Andy Shevchenko
2021-08-05  7:47     ` Jiri Slaby
2021-08-05  8:26       ` John Ogness
2021-08-03 13:52 ` [PATCH printk v1 00/10] printk: introduce atomic consoles and sync mode Andy Shevchenko
2021-08-05 15:47 ` Petr Mladek
2021-08-31  0:33   ` Sergey Senozhatsky

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