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* [PATCH] printk: CON_PRINTBUFFER console registration is a bit racy
@ 2018-09-14  2:34 Sergey Senozhatsky
  2018-09-14  8:59 ` Petr Mladek
  0 siblings, 1 reply; 3+ messages in thread
From: Sergey Senozhatsky @ 2018-09-14  2:34 UTC (permalink / raw)
  To: Petr Mladek, Steven Rostedt
  Cc: Sergey Senozhatsky, linux-kernel, Sergey Senozhatsky

CON_PRINTBUFFER console registration requires us to do several
preparation steps:
- Rollback console_seq to replay logbuf messages which were already
  seen on other consoles;
- Set exclusive_console flag so console_unlock() will ->write() logbuf
  messages only to the exclusive_console driver.

The way we do it, however, is a bit racy

	logbuf_lock_irqsave(flags);
	console_seq = syslog_seq;
	console_idx = syslog_idx;
	logbuf_unlock_irqrestore(flags);
						<< preemption enabled
						<< irqs enabled
	exclusive_console = newcon;
	console_unlock();

We rollback console_seq under logbuf_lock with IRQs disabled, but
we set exclusive_console with local IRQs enabled and logbuf unlocked.
If the system oops-es or panic-s before we set exclusive_console - and
given that we have IRQs and preemption enabled there is such a
possibility - we will re-play all logbuf messages to every registered
console, which may be a bit annoying and time consuming.

Move exclusive_console assignment under the same IRQs-disabled and
logbuf_lock protected section where we rollback console_seq.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 kernel/printk/printk.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 992bb6bb7ac2..c52a986a72b3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2705,7 +2705,6 @@ void register_console(struct console *newcon)
 		logbuf_lock_irqsave(flags);
 		console_seq = syslog_seq;
 		console_idx = syslog_idx;
-		logbuf_unlock_irqrestore(flags);
 		/*
 		 * We're about to replay the log buffer.  Only do this to the
 		 * just-registered console to avoid excessive message spam to
@@ -2713,6 +2712,7 @@ void register_console(struct console *newcon)
 		 */
 		exclusive_console = newcon;
 		exclusive_console_stop_seq = console_seq;
+		logbuf_unlock_irqrestore(flags);
 	}
 	console_unlock();
 	console_sysfs_notify();
-- 
2.19.0


^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: [PATCH] printk: CON_PRINTBUFFER console registration is a bit racy
  2018-09-14  2:34 [PATCH] printk: CON_PRINTBUFFER console registration is a bit racy Sergey Senozhatsky
@ 2018-09-14  8:59 ` Petr Mladek
  2018-09-14 11:19   ` Sergey Senozhatsky
  0 siblings, 1 reply; 3+ messages in thread
From: Petr Mladek @ 2018-09-14  8:59 UTC (permalink / raw)
  To: Sergey Senozhatsky; +Cc: Steven Rostedt, linux-kernel, Sergey Senozhatsky

On Fri 2018-09-14 11:34:28, Sergey Senozhatsky wrote:
> CON_PRINTBUFFER console registration requires us to do several
> preparation steps:
> - Rollback console_seq to replay logbuf messages which were already
>   seen on other consoles;
> - Set exclusive_console flag so console_unlock() will ->write() logbuf
>   messages only to the exclusive_console driver.
> 
> The way we do it, however, is a bit racy
> 
> 	logbuf_lock_irqsave(flags);
> 	console_seq = syslog_seq;
> 	console_idx = syslog_idx;
> 	logbuf_unlock_irqrestore(flags);
> 						<< preemption enabled
> 						<< irqs enabled
> 	exclusive_console = newcon;
> 	console_unlock();
> 
> We rollback console_seq under logbuf_lock with IRQs disabled, but
> we set exclusive_console with local IRQs enabled and logbuf unlocked.
> If the system oops-es or panic-s before we set exclusive_console - and
> given that we have IRQs and preemption enabled there is such a
> possibility - we will re-play all logbuf messages to every registered
> console, which may be a bit annoying and time consuming.
>
> Move exclusive_console assignment under the same IRQs-disabled and
> logbuf_lock protected section where we rollback console_seq.
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
>  kernel/printk/printk.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 992bb6bb7ac2..c52a986a72b3 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2705,7 +2705,6 @@ void register_console(struct console *newcon)
>  		logbuf_lock_irqsave(flags);
>  		console_seq = syslog_seq;
>  		console_idx = syslog_idx;
> -		logbuf_unlock_irqrestore(flags);
>  		/*
>  		 * We're about to replay the log buffer.  Only do this to the
>  		 * just-registered console to avoid excessive message spam to
> @@ -2713,6 +2712,7 @@ void register_console(struct console *newcon)
>  		 */
>  		exclusive_console = newcon;
>  		exclusive_console_stop_seq = console_seq;
> +		logbuf_unlock_irqrestore(flags);

This would make a false feeling that these variables need to
be synchronized using logbug_lock. It might either confuse people
or it could get removed during a code clean up.

A better solution would be to explicitly disable preemption
around the entire section and add a comment.

Well, I am not sure if it is worth the code complexity.

Best Regards,
Petr

^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: [PATCH] printk: CON_PRINTBUFFER console registration is a bit racy
  2018-09-14  8:59 ` Petr Mladek
@ 2018-09-14 11:19   ` Sergey Senozhatsky
  0 siblings, 0 replies; 3+ messages in thread
From: Sergey Senozhatsky @ 2018-09-14 11:19 UTC (permalink / raw)
  To: Petr Mladek
  Cc: Sergey Senozhatsky, Steven Rostedt, linux-kernel, Sergey Senozhatsky

On (09/14/18 10:59), Petr Mladek wrote:
> 
> Well, I am not sure if it is worth the code complexity.
>

Well, I don't think we need to bother that much here. Besides,
exclusive_console is cleared under logbuf_lock with preemption
disabled now. So we set it under logbuf_lock and !irq and we
clear it under logbuf_lock and !irq. Looks quite OK to me.

	-ss

^ permalink raw reply	[flat|nested] 3+ messages in thread

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Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
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2018-09-14  2:34 [PATCH] printk: CON_PRINTBUFFER console registration is a bit racy Sergey Senozhatsky
2018-09-14  8:59 ` Petr Mladek
2018-09-14 11:19   ` Sergey Senozhatsky

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