Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 9 Dec 2020 12:11:13 +0100
From:   Petr Mladek <pmladek@suse.com>
To:     John Ogness <john.ogness@linutronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH next v4 2/2] printk: remove logbuf_lock writer-protection
 of ringbuffer
Message-ID: <X9Cw0WK4kweSB1yi@alley>
References: <20201209004453.17720-1-john.ogness@linutronix.de>
 <20201209004453.17720-3-john.ogness@linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20201209004453.17720-3-john.ogness@linutronix.de>
Precedence: bulk

On Wed 2020-12-09 01:50:53, John Ogness wrote:
> Since the ringbuffer is lockless, there is no need for it to be
> protected by @logbuf_lock. Remove @logbuf_lock writer-protection of
> the ringbuffer. The reader-protection is not removed because some
> some variables used by readers are using @logbuf_lock for
> synchronization: @syslog_seq, @syslog_time, @syslog_partial,
> @console_seq, struct kmsg_dumper.
> 
> For PRINTK_NMI_DIRECT_CONTEXT_MASK, @logbuf_lock usage is not removed
> because it may be used for dumper synchronization.
> 
> Without @logbuf_lock synchronization of vprintk_store() it is no
> longer possible to use the single static buffer for temporarily
> sprint'ing the message. Instead, use vsnprintf() to determine the
> length and perform the real vscnprintf() using the area reserved from
> the ringbuffer. This leads to suboptimal packing of the message data,
> but will result in less wasted storage than multiple per-cpu buffers
> to support lockless temporary sprint'ing.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr