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SubjectRe: [PATCH v2 1/1] leds: remove PAGE_SIZE limit of /sys/class/leds/<led>/trigger
Hi!

> > > down_read(&triggers_list_lock);
> > > down_read(&led_cdev->trigger_lock);
> > >
> > > - if (!led_cdev->trigger)
> > > - len += scnprintf(buf+len, PAGE_SIZE - len, "[none] ");
> > > + len = led_trigger_format(NULL, 0, true, led_cdev);
> > > + data = kvmalloc(len + 1, GFP_KERNEL);
> >
> > Why kvmalloc() and not just kmalloc()? How big is this buffer you are
> > expecting to have here?
>
> The ledtrig-cpu supports upto 9999 cpus. If all these cpus were available,
> the buffer size would be 78,890 bytes.
> (for i in `seq 0 9999`;do echo -n " cpu$i"; done | wc -c)
>
> The intention of this kvmalloc() allocation is to avoid costly allocation
> if possible.

Sounds good.

> > > - else
> > > - len += scnprintf(buf+len, PAGE_SIZE - len, "%s ",
> > > - trig->name);
> > > - }
> > > up_read(&led_cdev->trigger_lock);
> > > up_read(&triggers_list_lock);
> >
> > Two locks? Why not 3? 5? How about just 1? :)
>
> I don't touch these locks in this patch :)

Nor should you :-).

> > Just return -ENOMEM above if !data, which makes this much simpler.
>
> We are holding the two locks, so we need to release them before return.
> Which one do you prefer?
>
> ...
> data = kvmalloc(len + 1, GFP_KERNEL);
> if (data)
> len = led_trigger_format(data, len + 1, false, led_cdev);
> else
> len = -ENOMEM;
>
> up_read(&led_cdev->trigger_lock);
> up_read(&triggers_list_lock);
>
> if (len < 0)
> return len;
>
> vs.
>
> ...
> data = kvmalloc(len + 1, GFP_KERNEL);
> if (!data) {
> up_read(&led_cdev->trigger_lock);
> up_read(&triggers_list_lock);
> return -ENOMEM;
> }
> len = led_trigger_format(data, len + 1, false, led_cdev);
>
> up_read(&led_cdev->trigger_lock);
> up_read(&triggers_list_lock);

Second one is better. Using goto to jump to error path doing cleanups
is also acceptable.

Thanks,
Pavel

--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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