Re: [RFC][PATCH -next 2/2] printk: set may_schedule for some of console_trylock callers

[Sergey Senozhatsky]

Thanks for the review.

On (01/18/16 17:17), Petr Mladek wrote:
> On Sun 2016-01-17 23:23:32, Sergey Senozhatsky wrote:
> >  console_unlock() allows to cond_resched() if its caller has
> > set `console_may_schedule' to 1 (this functionality present
> > since commit 'printk: do cond_resched() between lines while
> > outputting to consoles').
> > 
> > The rules are:
> > -- console_lock() always sets `console_may_schedule' to 1
> > -- console_trylock() always sets `console_may_schedule' to 0
> > 
> > However, console_trylock() callers (among them is printk()) are
> > not necessarily executing in atomic contexts, and some of them
> > can cond_resched() in console_unlock(). So console_trylock()
> > can set `console_may_schedule' to 0 only if cond_resched() is
> > invalid in the current context, and set it to 1 otherwise.
> > 
> > The patch also drops explicit preempt_disable()/preempt_enable()
> > calls in vprintk_emit().
> 
> I do not see any explanation why it is safe to remove these
> calls in this patch. If they were required only because of the
> can_use_console() call

The comment in the code states

	/*
	 * Disable preemption to avoid being preempted while holding
	 * console_sem which would prevent anyone from printing to
	 * console
	 */

which is not really a problem -- we schedule from console_unlock() with
the console_sem being held, it's fine.

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/kernel/printk/printk.c?id=8d91f8b15361dfb438ab6eb3b319e2ded43458ff

> it would make sense to move this change to the previous patch.
> The previous patch moved the can_use_console() to locations
> protected by lockbuf_lock that have disabled preemption because
> of the lock.

old:
vprintk_emit
  preempt_disable
   can_use_console
    console_unlock
      spin_lock, save IRQ
      spin_unlock
      call_console_drivers
      restore IRQ
  preempt_enable

new:
vprintk_emit
  console_unlock
    spin_lock, save IRQ
    can_use_console
    spin_unlock
    call_console_drivers
    restore IRQ

so preemption is disabled during the transition from can_use_console()
to call_console_drivers().


[..]
> >  	console_locked = 1;
> > -	console_may_schedule = 0;
> > +	console_may_schedule = !(oops_in_progress ||
> > +			irqs_disabled() ||
> > +			in_atomic() ||
> > +			rcu_preempt_depth());
> 
> Is it safe to call cond_resched() when the CPU is not online
> and preemption is enabled, please? Your previous patch
> suggests that this situation might happen. I guess that
> it might break the CPU initialization code.

CPU notifies are preemptible. CPU_UP_PREPARE/etc. can schedule,
there are GFP_KERNEL kmalloc-s from CPU_UP_PREPARE (where cpu
is not yet online), mutex_lock() calls in cpu_notify handlers,
and so on.

	-ss