linux-kernel - Re: [RFC][PATCH -next 2/2] printk: set may_schedule for some of console

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Date:	Thu, 21 Jan 2016 14:51:46 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Jan Kara <jack@...e.com>,
	Kyle McMartin <kyle@...nel.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Calvin Owens <calvinowens@...com>,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCH -next 2/2] printk: set may_schedule for some of
 console_trylock callers

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