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Date: Tue, 8 May 2012 14:53:11 -0400 (EDT)
From: Alan Stern <stern@...land.harvard.edu>
To: Tejun Heo <tj@...nel.org>
cc: "Eric W. Biederman" <ebiederm@...ssion.com>,
Peter Zijlstra <peterz@...radead.org>,
Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Lockdep false positive in sysfs
On Mon, 7 May 2012, Tejun Heo wrote:
> On Mon, May 07, 2012 at 05:51:52PM -0400, Alan Stern wrote:
> > I guess in the end it's a question of balance. Which has more
> > overhead, adding a few function calls here and there, or adding a new
> > flags field to every struct attribute?
>
> Yes, and there are different types of overheads. I'm happy to trade
> some runtime memory overhead under debugging mode for lower code
> complexity. Lock proving is pretty expensive anyway. I don't think
> there's much point in trying to optimize some bytes from struct
> attributes.
Okay, then what do you think about this approach? It does seem smaller
and simpler than the previous attempt.
And I did try to avoid unnecessary bloat; if lockdep isn't being used
then the extra attribute flag isn't present.
Alan Stern
Index: usb-3.4/include/linux/sysfs.h
===================================================================
--- usb-3.4.orig/include/linux/sysfs.h
+++ usb-3.4/include/linux/sysfs.h
@@ -27,6 +27,7 @@ struct attribute {
const char *name;
umode_t mode;
#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ bool ignore_lockdep:1;
struct lock_class_key *key;
struct lock_class_key skey;
#endif
@@ -80,6 +81,17 @@ struct attribute_group {
#define __ATTR_NULL { .attr = { .name = NULL } }
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+#define __ATTR_IGNORE_LOCKDEP(_name,_mode,_show,_store) { \
+ .attr = {.name = __stringify(_name), .mode = _mode, \
+ .ignore_lockdep = true }, \
+ .show = _show, \
+ .store = _store, \
+}
+#else
+#define __ATTR_IGNORE_LOCKDEP __ATTR
+#endif
+
#define attr_name(_attr) (_attr).attr.name
struct file;
Index: usb-3.4/fs/sysfs/dir.c
===================================================================
--- usb-3.4.orig/fs/sysfs/dir.c
+++ usb-3.4/fs/sysfs/dir.c
@@ -132,6 +132,24 @@ static void sysfs_unlink_sibling(struct
rb_erase(&sd->s_rb, &sd->s_parent->s_dir.children);
}
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+
+/* Test for attributes that want to ignore lockdep for read-locking */
+static bool ignore_lockdep(struct sysfs_dirent *sd)
+{
+ return sysfs_type(sd) == SYSFS_KOBJ_ATTR &&
+ sd->s_attr.attr->ignore_lockdep;
+}
+
+#else
+
+static inline bool ignore_lockdep(struct sysfs_dirent *sd)
+{
+ return true;
+}
+
+#endif
+
/**
* sysfs_get_active - get an active reference to sysfs_dirent
* @sd: sysfs_dirent to get an active reference to
@@ -155,15 +173,17 @@ struct sysfs_dirent *sysfs_get_active(st
return NULL;
t = atomic_cmpxchg(&sd->s_active, v, v + 1);
- if (likely(t == v)) {
- rwsem_acquire_read(&sd->dep_map, 0, 1, _RET_IP_);
- return sd;
- }
+ if (likely(t == v))
+ break;
if (t < 0)
return NULL;
cpu_relax();
}
+
+ if (likely(!ignore_lockdep(sd)))
+ rwsem_acquire_read(&sd->dep_map, 0, 1, _RET_IP_);
+ return sd;
}
/**
@@ -180,7 +200,8 @@ void sysfs_put_active(struct sysfs_diren
if (unlikely(!sd))
return;
- rwsem_release(&sd->dep_map, 1, _RET_IP_);
+ if (likely(!ignore_lockdep(sd)))
+ rwsem_release(&sd->dep_map, 1, _RET_IP_);
v = atomic_dec_return(&sd->s_active);
if (likely(v != SD_DEACTIVATED_BIAS))
return;
Index: usb-3.4/include/linux/device.h
===================================================================
--- usb-3.4.orig/include/linux/device.h
+++ usb-3.4/include/linux/device.h
@@ -503,6 +503,9 @@ ssize_t device_store_int(struct device *
#define DEVICE_INT_ATTR(_name, _mode, _var) \
struct dev_ext_attribute dev_attr_##_name = \
{ __ATTR(_name, _mode, device_show_ulong, device_store_ulong), &(_var) }
+#define DEVICE_ATTR_IGNORE_LOCKDEP(_name, _mode, _show, _store) \
+ struct device_attribute dev_attr_##_name = \
+ __ATTR_IGNORE_LOCKDEP(_name, _mode, _show, _store)
extern int device_create_file(struct device *device,
const struct device_attribute *entry);
Index: usb-3.4/drivers/usb/core/sysfs.c
===================================================================
--- usb-3.4.orig/drivers/usb/core/sysfs.c
+++ usb-3.4/drivers/usb/core/sysfs.c
@@ -73,7 +73,7 @@ set_bConfigurationValue(struct device *d
return (value < 0) ? value : count;
}
-static DEVICE_ATTR(bConfigurationValue, S_IRUGO | S_IWUSR,
+static DEVICE_ATTR_IGNORE_LOCKDEP(bConfigurationValue, S_IRUGO | S_IWUSR,
show_bConfigurationValue, set_bConfigurationValue);
/* String fields */
@@ -595,7 +595,7 @@ static ssize_t usb_dev_authorized_store(
return result < 0? result : size;
}
-static DEVICE_ATTR(authorized, 0644,
+static DEVICE_ATTR_IGNORE_LOCKDEP(authorized, 0644,
usb_dev_authorized_show, usb_dev_authorized_store);
/* "Safely remove a device" */
@@ -618,7 +618,7 @@ static ssize_t usb_remove_store(struct d
usb_unlock_device(udev);
return rc;
}
-static DEVICE_ATTR(remove, 0200, NULL, usb_remove_store);
+static DEVICE_ATTR_IGNORE_LOCKDEP(remove, 0200, NULL, usb_remove_store);
static struct attribute *dev_attrs[] = {
--
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