linux-kernel - Re: [PATCH] driver core: add wait event for deferred probe

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Message-Id: <20130214175456.729453E12FB@localhost>
Date:	Thu, 14 Feb 2013 17:54:56 +0000
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Haojian Zhuang <haojian.zhuang@...aro.org>,
	Arnd Bergmann <arnd@...db.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	viro@...iv.linux.org.uk, Rusty Russell <rusty@...tcorp.com.au>,
	hpa@...ux.intel.com, Jim Cromie <jim.cromie@...il.com>,
	linux-kernel@...r.kernel.org,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Linus Walleij <linus.walleij@...aro.org>,
	broonie@...nsource.wolfsonmicro.com,
	Patch Tracking <patches@...aro.org>
Subject: Re: [PATCH] driver core: add wait event for deferred probe

On Fri, 15 Feb 2013 00:58:23 +0800, Haojian Zhuang <haojian.zhuang@...aro.org> wrote:
> On 15 February 2013 00:50, Arnd Bergmann <arnd@...db.de> wrote:
> > On Thursday 14 February 2013, Haojian Zhuang wrote:
> >> On 14 February 2013 23:57, Arnd Bergmann <arnd@...db.de> wrote:
> >> > On Thursday 14 February 2013, Haojian Zhuang wrote:
> >> >> If you can change it into code in below, it could work. Otherwise, it
> >> >> always fails.
> >> >>         driver_deferred_probe_enable = true;
> >> >>         driver_deferred_probe_trigger();
> >> >> +      deferred_probe_work_func(NULL);
> >> >>         return 0;
> >> >>
> >> >> Because deferred_probe_work_func() depends on that deferred_probe is added
> >> >> into deferred_probe_active_list. If driver_deferred_probe_trigger() isn't called
> >> >> first, the deferred uart probe can't be added into active list. So even you call
> >> >> work_func at here, it doesn't help.
> >> >>
> >> >
> >> > Would that not cause two instances of the work function to run at the same time?
> >> > That sounds like a source for a lot of problems.
> >> >
> >> >         Arnd
> >>
> >> Two instances of the work function? I'm sorry that I don't
> >> understanding your meaning.
> >> Could you help explain your question?
> >
> > I mean you end up calling the work function directly, while it gets run as part
> > of the work queue on a different CPU at the same time. I just noticed that
> > there is actually locking in place in deferred_probe_work_func that prevents
> > any actual bugs, but you are still adding extra overhead here.
> >
> > Maybe just add
> >
> >         flush_workqueue(deferred_wq);
> >
> > here?
> >
> >         Arnd
> 
> It's fine to me. Since both of them are flushing workqueue.
> 
> Tested-by: <haojian.zhuang@...aro.org>

Hahaha. I just came to the same conclusion. I'll craft a proper patch
and send it off.

g.

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