linux-kernel - Re: [PATCH v2] reboot: Backup orderly

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Date:	Thu, 14 Jan 2016 11:09:13 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Keerthy <a0393675@...com>
Cc:	Keerthy <j-keerthy@...com>, linux-kernel@...r.kernel.org,
	edubezval@...il.com, grygorii.strashko@...com, nm@...com,
	linux-pm@...r.kernel.org, linux-omap@...r.kernel.org,
	joel@....id.au, akpm@...ux-foundation.org,
	linux-arm-kernel@...ts.infradead.org, peterz@...radead.org,
	dyoung@...hat.com, josh@...htriplett.org, mpe@...erman.id.au,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH v2] reboot: Backup orderly_poweroff


* Keerthy <a0393675@...com> wrote:

> Hi Ingo,
> 
> On Thursday 14 January 2016 02:35 PM, Ingo Molnar wrote:
> >
> >* Keerthy <j-keerthy@...com> wrote:
> >
> >>orderly_poweroff is triggered when a graceful shutdown
> >>of system is desired. This may be used in many critical states of the
> >>kernel such as when subsystems detects conditions such as critical
> >>temperature conditions. However, in certain conditions in system
> >>boot up sequences like those in the middle of driver probes being
> >>initiated, userspace will be unable to power off the system in a clean
> >>manner and leaves the system in a critical state. In cases like these,
> >>the /sbin/poweroff will return success (having forked off to attempt
> >>powering off the system. However, the system overall will fail to
> >>completely poweroff (since other modules will be probed) and the system
> >>is still functional with no userspace (since that would have shut itself
> >>off).
> >>
> >>However, there is no clean way of detecting such failure of userspace
> >>powering off the system. In such scenarios, it is necessary for a backup
> >>workqueue to be able to force a shutdown of the system when orderly
> >>shutdown is not successful after a configurable time period.
> >>
> >>Signed-off-by: Keerthy <j-keerthy@...com>
> >>Suggested-by: Eduardo Valentin <edubezval@...il.com>
> >>Reported-by: Nishanth Menon <nm@...com>
> >>---
> >>Links to previous discussion can be found here:
> >>
> >>http://www.spinics.net/lists/linux-omap/msg124925.html
> >>
> >>Boot tested on DRA7.
> >>
> >>changes in v2:
> >>
> >>	* Changed #ifdef to #if CONFIG_SHUTDOWN_BACKUP_DELAY_MS
> >>
> >>  arch/Kconfig    |  7 +++++++
> >>  kernel/reboot.c | 23 ++++++++++++++++++-----
> >>  2 files changed, 25 insertions(+), 5 deletions(-)
> >>
> >>Index: linux/arch/Kconfig
> >>===================================================================
> >>--- linux.orig/arch/Kconfig	2016-01-11 15:26:07.732173131 +0530
> >>+++ linux/arch/Kconfig	2016-01-11 15:26:07.728173205 +0530
> >>@@ -37,6 +37,18 @@
> >>  	def_bool y
> >>  	depends on PERF_EVENTS && HAVE_PERF_EVENTS_NMI && !PPC64
> >>
> >>+config SHUTDOWN_BACKUP_DELAY_MS
> >>+	int "Backup shutdown delay in milli-seconds"
> >>+	default 0
> >>+	help
> >>+	  The number of milliseconds to delay before backup workqueue
> >>+	  executes attempting to poweroff the system after the
> >>+	  orderly_poweroff function has failed to complete.
> >>+
> >>+	  If set to 0, the backup workqueue is not active. The value
> >>+	  should be conservatively configured based on userspace latencies
> >>+	  expected for a given system.
> >
> >I don't really understand this. In what circumstances can a reboot fail?
> >
> >I think that is what should be fixed: a reboot should never fail, instead of
> >introducing some sort of fragile timeout based method.
> 
> Here is the complete description of the scenario which was reported by Nishanth 
> who encountered the issue. The link has bootlogs and description of the exact 
> case which led to this patch.
> 
> http://www.spinics.net/lists/linux-omap/msg124923.html

it's a reply in the middle of a discussion ...

What I managed to decode is that this:

static int __orderly_poweroff(bool force)
{
        int ret;

        ret = run_cmd(poweroff_cmd);

        if (ret && force) {
                pr_warn("Failed to start orderly shutdown: forcing the issue\n");

                /*
                 * I guess this should try to kick off some daemon to sync and
                 * poweroff asap.  Or not even bother syncing if we're doing an
                 * emergency shutdown?
                 */
                emergency_sync();
                kernel_power_off();
        }

        return ret;
}

could fail to actually power the system off, if the run_cmd(poweroff_cmd) 
'succeeds', but due to a user-space bug it does not actually call the real 
poweroff system call?

Thanks,

	Ingo