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 ...
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