linux-kernel - Re: [PATCHv2] printk: CON_PRINTBUFFER console registration is a bit racy

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Date:   Wed, 3 Oct 2018 11:23:49 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCHv2] printk: CON_PRINTBUFFER console registration is a bit
 racy

On Fri 2018-09-28 18:53:04, 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 to the same IRQs-disabled and
> logbuf_lock-protected section where we rollback console_seq.
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>

JFYI, I have just pushed this into printk.git, for-4.20 branch.

Best Regards,
Petr