linux-kernel - Re: [PATCH v2 2/2] PM / sleep: don't suspend parent when async child suspend

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Message-ID: <50971906.K6xak2t6Z6@vostro.rjw.lan>
Date:   Wed, 02 Nov 2016 04:51:08 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Brian Norris <briannorris@...omium.org>,
        Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Doug Anderson <dianders@...omium.org>,
        Brian Norris <computersforpeace@...il.com>,
        Jeffy Chen <jeffy.chen@...k-chips.com>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        Chuansheng Liu <chuansheng.liu@...el.com>
Subject: Re: [PATCH v2 2/2] PM / sleep: don't suspend parent when async child suspend_{noirq,late} fails

On Tuesday, November 01, 2016 12:04:28 AM Dmitry Torokhov wrote:
> On Mon, Oct 31, 2016 at 10:25 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> > On Thursday, October 27, 2016 09:05:34 AM Brian Norris wrote:
> >> Consider two devices, A and B, where B is a child of A, and B utilizes
> >> asynchronous suspend (it does not matter whether A is sync or async). If
> >> B fails to suspend_noirq() or suspend_late(), or is interrupted by a
> >> wakeup (pm_wakeup_pending()), then it aborts and sets the async_error
> >> variable. However, device A does not (immediately) check the async_error
> >> variable; it may continue to run its own suspend_noirq()/suspend_late()
> >> callback. This is bad.
> >>
> >> We can resolve this problem by checking the async_error flag after
> >> waiting for children to suspend, using the same logic for the noirq and
> >> late suspend cases as we already do for __device_suspend().
> >>
> >> It's easy to observe this erroneous behavior by, for example, forcing a
> >> device to sleep a bit in its suspend_noirq() (to ensure the parent is
> >> waiting for the child to complete), then return an error, and watch the
> >> parent suspend_noirq() still get called. (Or similarly, fake a wakeup
> >> event at the right (or is it wrong?) time.)
> >>
> >> Fixes: de377b397272 ("PM / sleep: Asynchronous threads for suspend_late")
> >> Fixes: 28b6fd6e3779 ("PM / sleep: Asynchronous threads for suspend_noirq")
> >> Reported-by: Jeffy Chen <jeffy.chen@...k-chips.com>
> >> Signed-off-by: Brian Norris <briannorris@...omium.org>
> >> Reviewed-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> >> ---
> >> v2: s/early/late/ in commit message
> >>
> >>  drivers/base/power/main.c | 6 ++++++
> >>  1 file changed, 6 insertions(+)
> >>
> >> diff --git a/drivers/base/power/main.c b/drivers/base/power/main.c
> >> index c58563581345..eaf6b53463a5 100644
> >> --- a/drivers/base/power/main.c
> >> +++ b/drivers/base/power/main.c
> >> @@ -1040,6 +1040,9 @@ static int __device_suspend_noirq(struct device *dev, pm_message_t state, bool a
> >>
> >>       dpm_wait_for_children(dev, async);
> >>
> >> +     if (async_error)
> >> +             goto Complete;
> >> +
> >
> > This is a second chech for async_error in this routine and is the first one
> > really needed after adding this?
> 
> There is really no point in waiting for children to be suspended if
> error has already been signalled; that's what first check achieves.
> The 2nd check ensures that we abort suspend if any of the children
> failed to suspend.
> 
> I'd say both checks are needed (well, 1st is helpful, 2nd is essential).

OK, fair enough.

Thanks,
Rafael