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SubjectRe: replay log: Re: [PATCH printk v4 38/39] printk: relieve console_lock of list synchronization duties
On Tue 2022-11-15 18:21:34, John Ogness wrote:
> On 2022-11-15, Petr Mladek <pmladek@suse.com> wrote:
> >>> --- a/kernel/printk/printk.c
> >>> +++ b/kernel/printk/printk.c
> >>> @@ -3334,6 +3330,11 @@ void register_console(struct console *newcon)
> >>> * boot console that is the furthest behind.
> >>> */
> >>> if (bootcon_registered && !keep_bootcon) {
> >>> + /*
> >>> + * Hold the console_lock to guarantee safe access to
> >>> + * console->seq.
> >>> + */
> >>> + console_lock();
> >>> for_each_console(con) {
> >>> if ((con->flags & CON_BOOT) &&
> >>> (con->flags & CON_ENABLED) &&
> >>> @@ -3341,6 +3342,7 @@ void register_console(struct console *newcon)
> >>> newcon->seq = con->seq;
> >>> }
> >>> }
> >>> + console_unlock();
> >
> > So, without the above two hunks:
> >
> > Reviewed-by: Petr Mladek <pmladek@suse.com>
>
> Note that we actually need those hunks to guarantee a consistent @seq
> value. The console_lock is the only synchronization mechanism available
> to read console->seq.

Yes, we need a solution. But it does not need to be in this patch.

This patch removes console_lock() on some locations. But this
particular code was called without console_lock() even before
this patch.

Note that the regression was added in the 3rd patch that moved
this code outside console_lock().

Maybe, the easiest solution would be to do in the 3rd patch [*]:

} else {
/* Begin with next message. */
newcon->seq = prb_next_seq(prb);
/*
* Try hard to show the pending messages on boot consoles.
* so that the new console does not start too late.
*/
pr_flush();
}

It should behave as good and as bad as the original code.

[*] Or move the code and add this change before the 3rd patch
to keep this questionable solution separated and avoid
the regression.

Best Regards,
Petr

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