Re: [PATCH next v4 2/2] printk: remove logbuf_lock writer-protection of ringbuffer

[Petr Mladek]

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@xxxxxxxxxxxxx>

Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>

Best Regards,
Petr