On 08/13/2014 12:57 PM, Prarit Bhargava wrote:
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.
This seems VERY different from our situation. I don't see an equivalent
of a policy lock that's grabbed from both threads, but in opposite order.
If I'm not mistaken, the sysfs entry here uses some wait/complete pair
to wait for something. But that's an equivalent of a semaphore with max
count of 1. Lockdep just seems to be making it obvious by adding
semaphore calls.
So, a semaphore equivalent deadlock with another semaphore. I believe
this is a read deadlock.
-Saravana
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