linux-kernel - Re: [PATCH v4 08/25] kernel: Add combined power-off+restart handler call chain API

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Date:   Mon, 29 Nov 2021 01:36:24 +0100
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
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Subject: Re: [PATCH v4 08/25] kernel: Add combined power-off+restart handler
 call chain API

On Mon, Nov 29, 2021 at 12:53:51AM +0300, Dmitry Osipenko wrote:
> 29.11.2021 00:17, Michał Mirosław пишет:
> >> I'm having trouble with parsing this comment. Could you please try to
> >> rephrase it? I don't see how you could check whether power-off handler
> >> is available if you'll mix all handlers together.
> > If notify_call_chain() would be fixed to return NOTIFY_OK if any call
> > returned NOTIFY_OK, then this would be a clear way to gather the
> > answer if any of the handlers will attempt the final action (reboot or
> > power off).
> Could you please show a code snippet that implements your suggestion?

A rough idea is this:

 static int notifier_call_chain(struct notifier_block **nl,
 			       unsigned long val, void *v,
 			       int nr_to_call, int *nr_calls)
 {
-	int ret = NOTIFY_DONE;
+	int ret, result = NOTIFY_DONE;
 	struct notifier_block *nb, *next_nb;
 
 	nb = rcu_dereference_raw(*nl);
 
 	while (nb && nr_to_call) {
...
 		ret = nb->notifier_call(nb, val, v);
+
+		/* Assuming NOTIFY_STOP-carrying return is always greater than non-stopping one. */
+		if (result < ret)
+			result = ret;
... 
 	}
-	return ret;
+	return result;
 }

Then:

bool prepare_reboot()
{
	int ret = xx_notifier_call_chain(&shutdown_notifier, PREPARE_REBOOT, ...);
	return ret == NOTIFY_OK;
}

And the return value would signify whether the reboot will be attempted
when calling the chain for the REBOOT action. (Analogously for powering off.)

Best Regards
Michał Mirosław