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

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Message-ID: <20181003122726.GB30267@tigerII.localdomain>
Date:   Wed, 3 Oct 2018 21:27:26 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        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 (10/03/18 11:23), Petr Mladek wrote:
> 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.

Thanks.

	-ss