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

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Date:	Tue, 21 Oct 2014 06:29:30 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Philippe Rétornaz <philippe.retornaz@...il.com>,
	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

On 10/20/2014 11:46 PM, Philippe Rétornaz wrote:
> 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.
>

Learning something new all the time. I assumed that spin_lock (unlike
spin_lock_irqsafe) would not run in atomic context.

I did not want to use a sleeping lock because I am not sure if that
works for all architectures; some disable (local) interrupts before
calling the function (eg arm and arm64), and I don't want to change
that semantics.

I have another idea how to get there without changing the lock situation
while executing the call chain - just set a flag indicating that it is
running and execute it without lock. Would that work ?

Guenter

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