On (01/21/16 10:25), Sergey Senozhatsky wrote:
[..]
> > First, the message "This stops the holder of console_sem just where we
> > want him" is suspitious.
>
> this comment is irrelevant, as of today. it was, a long time ago, because
> the entire thing was a bit different (linux-2.4.21 kernel/printk.c)
>
> /* This stops the holder of console_sem just where we want him */
> spin_lock_irqsave(&logbuf_lock, flags);
>
> logbuf_lock does stop the holder, local_irq_save() does not, you are right.
I meant 'irrelevant on its current place'.
[..]
> > As a result, I think that we do not need the extra checks
> > for the save context in printk(). IMHO, it is safe to remove
> > all the console_may_schedule stuff and also remove the extra
> > preempt_disable/preempt_enable() in vprintk_emit().
> >
> > Or did I miss anything?
>
> hm... I suspect the reason we have console_may_schedule is
> console_conditional_schedule() - console_sem owner may want
> to have an internal logic to re-schedule [fwiw], while still
> holding the console_sem. tty/vt/vt.c or video/console/fbcon.c
> for example. (in 2.4 kernel: video/fbcon.c and char/console.c).
>
> cond_resched() helps in console_unlock(); console_conditional_schedule()
> is called after console_lock() and _before_ console_unlock()....
for CONFIG_PREEMPT_COUNT kernel we can do something like
+void __sched console_conditional_schedule(void)
+{
+ if (!oops_in_progress && preemptible() && !rcu_preempt_depth())
+ cond_resched();
+}
and in console_unlock()
- if (do_cond_resched)
- cond_resched();
+ console_conditional_schedule();
but for !CONFIG_PREEMPT_COUNT we can't. because of currently held spin_locks/etc
that we don't know about.
`console_may_schedule' carries a bit of important information for
console_conditional_schedule() caller. if it has acquired console_sem
via console_lock() - then it can schedule, if via console_trylock() - it cannot.
the last `if via console_trylock() - it cannot' rule is not always true,
we clearly can have printk()->console_unlock() from non-atomic contexts
(if we know that its non-atomic, which is not the case with !PREEMPT_COUNT).
-ss
