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SubjectRe: [PATCH v2 01/47] kernel: Add support for poweroff handler call chain
Le 21/10/2014 15:29, Guenter Roeck a écrit :
> 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.
>> 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.

You're right and it even disable all others CPUs (if any).
I don't understand why it needs to disable local interrupts since the
code path to pm_power_off is simply doing:

syscall -> migrate to reboot cpu -> disable local interrupt -> disable
others cpu -> pm_power_off().

I don't understand why we cannot re-enable interrupts right before
And it looks like that all pm_power_off callbacks which can sleep are
I still wonder how i2c communication can works without local interrupts
... it looks
like somebody is re-enabling them (or the code was never run)

> 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 ?

I don't think inventing a new locking mechanism is a good solution.
We need first to know for sure if we can sleep or not in pm_power_off.
If yes then we need to re-enable local interrupts and we can use a mutex.
If not then the atomic notifier is fine and a lots of drivers are wrong.


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