linux-kernel - Re: syslog: was: [PATCH next v2 3/3] printk: remove logbuf_lock, add syslog

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Message-ID: <X839e1NXWPTX+X4J@alley>
Date:   Mon, 7 Dec 2020 11:01:31 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: syslog: was: [PATCH next v2 3/3] printk: remove logbuf_lock, add
 syslog_lock

On Sun 2020-12-06 22:12:21, John Ogness wrote:
> On 2020-12-04, Petr Mladek <pmladek@...e.com> wrote:
> > On Tue 2020-12-01 21:59:41, John Ogness wrote:
> >> Since the ringbuffer is lockless, there is no need for it to be
> >> protected by @logbuf_lock. Remove @logbuf_lock.
> >> 
> >> --- a/kernel/printk/printk.c
> >> +++ b/kernel/printk/printk.c
> >> @@ -2809,11 +2856,7 @@ void register_console(struct console *newcon)
> >>  		nr_ext_console_drivers++;
> >>  
> >>  	if (newcon->flags & CON_PRINTBUFFER) {
> >> -		/*
> >> -		 * console_unlock(); will print out the buffered messages
> >> -		 * for us.
> >> -		 */
> >> -		logbuf_lock_irqsave(flags);
> >> +		spin_lock_irqsave(&syslog_lock, flags);
> >
> > We should take the lock only around assigning syslog_seq. And add a
> > comment that it guarantees atomic update.
> 
> OK. So you just want "exclusive_console = newcon;" moved outside the
> critical section.

Exactly, I would like to make it clear that it synchronizes only the
single assignment. Otherwise, people might get wrong assumption.

I know that there is a comment describing the scope of each lock.
But people might miss it or do not search for it at all.

Best Regards,
Petr