Messages in this thread | | | Date | Wed, 13 Aug 2014 15:57:38 -0400 | From | Prarit Bhargava <> | Subject | Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2] |
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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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