linux-kernel - Re: [PATCH v3 17/22] ipmi: Convert kipmi kthread into kthread worker API

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Date:	Tue, 24 Nov 2015 13:12:33 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Corey Minyard <minyard@....org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>, Borislav Petkov <bp@...e.de>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	Vlastimil Babka <vbabka@...e.cz>, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	openipmi-developer@...ts.sourceforge.net
Subject: Re: [PATCH v3 17/22] ipmi: Convert kipmi kthread into kthread worker
 API

On Mon 2015-11-23 13:36:06, Corey Minyard wrote:
> 
> 
> On 11/18/2015 07:25 AM, Petr Mladek wrote:
> > Kthreads are currently implemented as an infinite loop. Each
> > has its own variant of checks for terminating, freezing,
> > awakening. In many cases it is unclear to say in which state
> > it is and sometimes it is done a wrong way.
> >
> > The plan is to convert kthreads into kthread_worker or workqueues
> > API. It allows to split the functionality into separate operations.
> > It helps to make a better structure. Also it defines a clean state
> > where no locks are taken, IRQs blocked, the kthread might sleep
> > or even be safely migrated.
> >
> > The kthread worker API is useful when we want to have a dedicated
> > single thread for the work. It helps to make sure that it is
> > available when needed. Also it allows a better control, e.g.
> > define a scheduling priority.
> >
> > This patch converts kipmi kthread into the kthread worker API because
> > it modifies the scheduling priority. The change is quite straightforward.
> 
> I think this is correct.  That code was hard to get right, but I don't
> see where any
> logic is actually changed.

I believe that it was hard to make it working.


> This also doesn't really look any simpler (you end up with more LOC than
> you did before :) ),
> though it will make things more consistent and reduce errors and that's
> a good thing.

I have just realized that the original code actually looks racy. For
example, it does:

	__set_current_state(TASK_INTERRUPTIBLE);
	schedule();

without rechecking the state in between. There might already be a new
message and it might miss the wake_up_process(). Similar problem is
with the schedule_timeout_interruptible(100); I mean:


CPU 0					CPU 1


ipmi_thread()
  spin_lock_irqsave();
  smi_result = smi_event_handler();
  spin_unlock_irqrestore();

  [...]
  else if (smi_result == SI_SM_IDLE)
    /* true */
    if (atomic_read(need_watch)) {
      /* true */

					sender()
					  spin_lock_irqsave()
					  check_start_timer_thread()
					    wake_up_process()

					    /*
					     * NOPE because kthread
					     * is not sleeping
					     */

     schedule_timeout_interruptible(100);

     /*
      * We sleep 100 jiffies but
      * there is a pending message.
      */


This is not a problem with the kthread worker API because

	mod_delayed_kthread_work(smi_info->worker,
				 &smi_info->work, 0);

would queue the work to be done immediately and

	queue_delayed_kthread_work(smi_info->worker,
				   &smi_info->work, 100);

would do nothing in this case.


> My only comment is I would like the worker function named ipmi_worker,
> not ipmi_func.

You probably want it because the original name was ipmi_thread. But
it might cause confusion with new_smi->worker. The function gets
assigned to work->func, see struct kthread_work. Therefore I think that
_func suffix makes more sense.

> Reviewed-by: Corey Minyard <cminyard@...sta.com>


Thanks a lot for review,
Petr
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