linux-kernel - Re: [PATCH next v4 2/2] printk: remove logbuf

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Date:   Wed, 9 Dec 2020 14:31:46 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        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: [PATCH next v4 2/2] printk: remove logbuf_lock writer-protection
 of ringbuffer

On (20/12/09 01:50), 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@...utronix.de>

Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>

	-ss