linux-kernel - Re: [PATCH v2 01/47] kernel: Add support for poweroff handler call chain

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Date:	Tue, 21 Oct 2014 08:46:24 +0200
From:	Philippe Rétornaz <philippe.retornaz@...il.com>
To:	Guenter Roeck <linux@...ck-us.net>, linux-kernel@...r.kernel.org
CC:	linux-pm@...r.kernel.org, Alan Cox <gnomes@...rguk.ukuu.org.uk>,
	Alexander Graf <agraf@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Heiko Stuebner <heiko@...ech.de>,
	Lee Jones <lee.jones@...aro.org>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Romain Perier <romain.perier@...il.com>
Subject: Re: [PATCH v2 01/47] kernel: Add support for poweroff handler call
 chain

Hello

[...]
> - Use raw notifiers protected by spinlocks instead of atomic notifiers
[...]

> +/**
> + *	do_kernel_power_off - Execute kernel poweroff handler call chain
> + *
> + *	Calls functions registered with register_power_off_handler.
> + *
> + *	Expected to be called from machine_power_off as last step of
> + *	the poweroff sequence.
> + *
> + *	Powers off the system immediately if a poweroff handler function
> + *	has been registered. Otherwise does nothing.
> + */
> +void do_kernel_power_off(void)
> +{
> +	spin_lock(&power_off_handler_lock);
> +	raw_notifier_call_chain(&power_off_handler_list, 0, NULL);
> +	spin_unlock(&power_off_handler_lock);
> +}

I don't get it. You are still in atomic context inside the poweroff callback
since you lock it with a spinlock.

It does not change much from the atomic notifier which was doing exactly the
same thing but with RCU:

    rcu_read_lock();
    ret = notifier_call_chain(&nh->head, val, v, nr_to_call, nr_calls);
    rcu_read_unlock();

Why not using the blocking_notifier_* family ?
It will lock with a read-write semaphore under which you can sleep.

For instance, twl4030_power_off will sleep, since it is doing I2C access.
So you cannot call it in atomic context.

Thanks,

Philippe

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