On (02/19/18 17:01), Petr Mladek wrote:
[..]
> - raw_spin_lock(&logbuf_lock);
> retry = console_seq != log_next_seq;
> + /*
> + * Check whether userland needs notification. Do this only when really
> + * leaving to avoid race with console_trylock_spinning().
> + */
> + if (seen_seq != log_next_seq && !retry) {
> + wake_klogd = true;
> + seen_seq = log_next_seq;
> + }
Let's add the "why" part. This "!retry" might be hard to understand. We
are looking at
- CPUa is about to leave console_unlock()
- printk on CPUb appends a new message
- CPUa detects that `console_seq != log_next_seq', updates `seen_seq'
- printk on CPUb is getting preempted
- CPUa re-takes the console_sem via retry path
- printk CPUb is becoming TASK_RUNNING again - it now spins for console_sem,
since we have an active console_sem owner
- CPUa detects that there is a console_sem waiter, so it offloads the
printing task, without ever waking up klogd
Either we can have that complex "seen_seq != log_next_seq && !retry"
check - or we simply can add
if (console_lock_spinning_disable_and_check()) {
printk_safe_exit_irqrestore(flags);
if (wake_klogd)
wake_up_klogd();
}
to the offloading return path.
The later is *may be* simpler to follow. The rule is: every
!console_suspend and !cant-use-consoles return path from console_unlock()
must wake_up_klogd() [if needed].
-ss
