devkmsg: was [PATCH next v2 3/3] printk: remove logbuf_lock, add syslog_lock

[Petr Mladek <pmladek@suse.com>]

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.

I am going to split the feedback into few mails. It might make sense
to split also this patch into few more pieces that would remove the lock
from a particular interface.


> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index e9018c4e1b66..7385101210be 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -785,7 +749,6 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
>  	if (offset)
>  		return -ESPIPE;
>  
> -	logbuf_lock_irq();

user->seq manipulation is not longer safe from the atomicity point of view.

One solution would be to use atomic variable in struct devkmsg_user().
Another solution would be to synchronize it with user->lock like we do
in devkmsg_read().

user->lock looks like an overhead. But it actually would make sense to
prevent seek in the middle of a read.

>  	switch (whence) {
>  	case SEEK_SET:
>  		/* the first record */
> @@ -820,7 +782,6 @@ static __poll_t devkmsg_poll(struct file *file, poll_table *wait)
>  
>  	poll_wait(file, &log_wait, wait);
>  
> -	logbuf_lock_irq();
>  	if (prb_read_valid(prb, user->seq, NULL)) {

Same here. The atomicity of user->seq read/write is not guaranteed.


>  		/* return error when data has vanished underneath us */
>  		if (user->seq < prb_first_valid_seq(prb))

Best Regards,
Petr