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

[Philippe RÃtornaz]

Le 21/10/2014 15:29, Guenter Roeck a Ãcrit :
> On 10/20/2014 11:46 PM, Philippe RÃtornaz 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 
pm_power_off().
And it looks like that all pm_power_off callbacks which can sleep are 
broken.
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.

Thanks,

Philippe
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