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From: Sergey Senozhatsky <senozhatsky@chromium.org>
To: John Ogness <john.ogness@linutronix.de>
Cc: Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	Vitor Massaru Iha <vitor@massaru.org>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	Changbin Du <changbin.du@intel.com>
Subject: Re: [PATCH printk v1 07/10] console: add write_atomic interface
Date: Tue, 31 Aug 2021 11:55:01 +0900	[thread overview]
Message-ID: <YS2aBeymxuiW3CIw@google.com> (raw)
In-Reply-To: <20210803131301.5588-8-john.ogness@linutronix.de>

On (21/08/03 15:18), John Ogness wrote:
[..]
> @@ -1993,6 +1993,16 @@ static int console_trylock_spinning(void)
>  	bool spin = false;
>  	unsigned long flags;
>  
> +#ifdef CONFIG_SMP
> +	/*
> +	 * CPUs holding the printk cpulock must not spin on any lock. Even
> +	 * console_trylock() must not be called because its implementation
> +	 * uses spinlocks.
> +	 */
> +	if (atomic_read(&printk_cpulock_owner) == smp_processor_id())
> +		return 0;
> +#endif
> +
>  	if (console_trylock())
>  		return 1;
>  
> @@ -2719,7 +2729,17 @@ static int have_callable_console(void)
>   */
>  static inline int can_use_console(void)
>  {
> -	return cpu_online(raw_smp_processor_id()) || have_callable_console();
> +	int cpu = raw_smp_processor_id();
> +#ifdef CONFIG_SMP
> +	/*
> +	 * CPUs holding the printk cpulock must not spin on any lock.
> +	 * Allowing console usage could call into the spinlocks of the
> +	 * various console drivers.
> +	 */
> +	if (atomic_read(&printk_cpulock_owner) == cpu)
> +		return 0;

I guess the only reason this is done in can_use_console() is
console_flush_on_panic()?

Because otherwise, I think, we can move this check to vprintk_emit().

can_use_console() can be called from preemptible() context. But
if it's called from preemptible() then we know that this is not
printk()/NMI path (but console_device() and friends instead) and
that this CPU is definitely not holding printk CPU lock.

console_trylock_spinning() and console_unlock()->can_use_console()
follow each other

	if (console_trylock_spinning())
		console_unlock();

so a single `atomic_read(&printk_cpulock_owner) == cpu` can suffice.

Now we get to the console_flush_on_panic(), which still calls console_unlock(),
iterates over messages, but when called by the CPU that owns printk_lock,
just skips all the messages. But there is no point in calling console_unlock()
in such a case, we can check if we're printk_cpulock_owner and bail out if so.

Or am I missing something?

  parent reply	other threads:[~2021-08-31  2:55 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
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 [this message]
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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