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 14:52:25 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Joshua Thompson <funaho@...ai.org>,
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        Linus Walleij <linus.walleij@...aro.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
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        Vincent Chen <deanbo422@...il.com>,
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Subject: Re: [PATCH v4 08/25] kernel: Add combined power-off+restart handler
 call chain API

29.11.2021 03:36, Michał Mirosław пишет:
> 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.)

If you started to execute call chain, then you began the power-off /
restart sequence, this is a point of no return. Sorry, I still don't
understand what you're trying to achieve.

The approach of having separate call chains is simple and intuitive, I
don't see reasons to change it.