Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2]

[Prarit Bhargava <prarit@redhat.com>]

On 08/05/2014 06:51 PM, Saravana Kannan wrote:

> 
> I definitely have a fix for this and the original race you reported. It's
> basically reverting that commit you reverted + a fix for the deadlock. That's
> the only way to fix the scaling_governor issue.
> 
> You fix the deadlock by moving the governor attribute group removing to the
> framework code and doing it before STOP+EXIT to governor without holding the
> policy lock. And the reverse for INIT+STOP.
> 

I'm still not convinced of the deadlock so I did a bit of additional research
and am pretty close to saying that this is a false positive from the lockdep
code in the kernfs area.

A few things that have caused me concern about the lockdep splat we're seeing:

1.  The splat occurs when we hit __kernfs_remove+0x25b/0x360 which resolves to

        if (kernfs_lockdep(kn)) {
                rwsem_acquire(&kn->dep_map, 0, 0, _RET_IP_);  <<< RIGHT HERE
                if (atomic_read(&kn->active) != KN_DEACTIVATED_BIAS)
                        lock_contended(&kn->dep_map, _RET_IP_);
        }

ie) the *ONLY* way we hit a "deadlock" in this code is if we have LOCKDEP
configured in the kernfs.

It should be noted, that having kernfs_lockdep() always return 0 [1], results in
NO additional lockdep warnings.

Additionally the splat contains

[  107.428421]        CPU0                    CPU1
[  107.433482]        ----                    ----
[  107.438544]   lock(&policy->rwsem);
[  107.442459]                                lock(s_active#98);
[  107.448916]                                lock(&policy->rwsem);
[  107.455650]   lock(s_active#98);

which also points to the situation above (s_active is the default naming used in
the kernfs lockdep code).

In short -- there is no deadlock here.  It only happens in the lockdep code
itself, not because lockdep has identified a real problem.

2.  I then started asking myself why this was occurring.  The reason appears to
be that the attribute for scaling_governor is deleting other sysfs attributes
and that got me to wondering if there were other areas where this occurred and I
remembered it does!  In the USB code writing and reading to the  bConfiguration
of a device may lead to the removal of "down stream" attributes.  The reading
and writing of bConfiguration occurs in
drivers/usb/core/sysfs.c:79


/* configuration value is always present, and r/w */
usb_actconfig_show(bConfigurationValue, "%u\n");

static ssize_t bConfigurationValue_store(struct device *dev,
                                         struct device_attribute *attr,
                                         const char *buf, size_t count)
{
        struct usb_device       *udev = to_usb_device(dev);
        int                     config, value;

        if (sscanf(buf, "%d", &config) != 1 || config < -1 || config > 255)
                return -EINVAL;
        usb_lock_device(udev);
        value = usb_set_configuration(udev, config);
        usb_unlock_device(udev);
        return (value < 0) ? value : count;
}

... and the next lines are IMO important here:

static DEVICE_ATTR_IGNORE_LOCKDEP(bConfigurationValue, S_IRUGO | S_IWUSR,
                bConfigurationValue_show, bConfigurationValue_store);

FWIW, it isn't *exactly* the same ... but commit
356c05d58af05d582e634b54b40050c73609617b explains a similarity between what is
happening with our lockdep splat and the lockdep issues seen in USB.

3.  I came across this from an earlier discussion ...

https://lkml.org/lkml/2010/1/29/306

"We get false positives when the code of a sysfs attribute
synchronously removes other sysfs attributes.  In general that is not
safe due to hotplug etc, but there are specific instances of static
sysfs entries like the pm_core where it appears to be safe."

...


So ... the question that I have is: is this lockdep splat "real"?  It seems to
only occur because we enable the lockdep code in kernfs, that is it occurs as a
side-effect and doesn't appear to be "real" to me.

I only offer this in an effort to keep work to a minimum ;)

P.

[1] It wasn't that simple.  There are some other changes that have to be made.
But you get the idea ...