Benjamin Herrenschmidt <benh@kernel.crashing.org> 于2019年5月2日周四 下午2:25写道：

> > > The basic idea yes, the whole bool *locked is horrid though.
> > > Wouldn't it
> > > work to have a get_device_parent_locked that always returns with
> > > the mutex held,
> > > or just move the mutex to the caller or something simpler like this
> > > ?
> > >
> >
> > Greg and Rafael, do you have any suggestions for this? Or you also
> > agree with Ben?
>
> Ping guys ? This is worth fixing...

I also agree with you. But Greg and Rafael seem to be high latency right now.

From your suggestions, I think introduce get_device_parent_locked() may easy
to fix. So, do you agree with the fix of the following code snippet
(You can also
view attachments)?

I introduce a new function named get_device_parent_locked_if_glue_dir() which
always returns with the mutex held only when we live in glue dir. We should call
unlock_if_glue_dir() to release the mutex. The
get_device_parent_locked_if_glue_dir()
and unlock_if_glue_dir() should be called in pairs.

---
drivers/base/core.c | 44 ++++++++++++++++++++++++++++++++++++--------
1 file changed, 36 insertions(+), 8 deletions(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 4aeaa0c92bda..5112755c43fa 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -1739,8 +1739,9 @@ class_dir_create_and_add(struct class *class,
struct kobject *parent_kobj)
static DEFINE_MUTEX(gdp_mutex);
-static struct kobject *get_device_parent(struct device *dev,
-                    struct device *parent)
+static struct kobject *__get_device_parent(struct device *dev,
+                    struct device *parent,
+                    bool lock)
{
   if (dev->class) {
       struct kobject *kobj = NULL;
@@ -1779,14 +1780,16 @@ static struct kobject
*get_device_parent(struct device *dev,
           }
       spin_unlock(&dev->class->p->glue_dirs.list_lock);
       if (kobj) {
-           mutex_unlock(&gdp_mutex);
+           if (!lock)
+               mutex_unlock(&gdp_mutex);
           return kobj;
       }
       /* or create a new class-directory at the parent device */
       k = class_dir_create_and_add(dev->class, parent_kobj);
       /* do not emit an uevent for this simple "glue" directory */
-       mutex_unlock(&gdp_mutex);
+       if (!lock)
+           mutex_unlock(&gdp_mutex);
       return k;
   }
@@ -1799,6 +1802,19 @@ static struct kobject *get_device_parent(struct
device *dev,
   return NULL;
}
+static inline struct kobject *get_device_parent(struct device *dev,
+                       struct device *parent)
+{
+   return __get_device_parent(dev, parent, false);
+}
+
+static inline struct kobject *
+get_device_parent_locked_if_glue_dir(struct device *dev,
+                struct device *parent)
+{
+   return __get_device_parent(dev, parent, true);
+}
+
static inline bool live_in_glue_dir(struct kobject *kobj,
                struct device *dev)
{
@@ -1831,6 +1847,16 @@ static void cleanup_glue_dir(struct device
*dev, struct kobject *glue_dir)
   mutex_unlock(&gdp_mutex);
}
+static inline void unlock_if_glue_dir(struct device *dev,
+                struct kobject *glue_dir)
+{
+   /* see if we live in a "glue" directory */
+   if (!live_in_glue_dir(glue_dir, dev))
+       return;
+
+   mutex_unlock(&gdp_mutex);
+}
+
static int device_add_class_symlinks(struct device *dev)
{
   struct device_node *of_node = dev_of_node(dev);
@@ -2040,7 +2066,7 @@ int device_add(struct device *dev)
   pr_debug("device: '%s': %s\n", dev_name(dev), __func__);
   parent = get_device(dev->parent);
-   kobj = get_device_parent(dev, parent);
+   kobj = get_device_parent_locked_if_glue_dir(dev, parent);
   if (IS_ERR(kobj)) {
       error = PTR_ERR(kobj);
       goto parent_error;
@@ -2055,10 +2081,12 @@ int device_add(struct device *dev)
   /* first, register with generic layer. */
   /* we require the name to be set before, and pass NULL */
   error = kobject_add(&dev->kobj, dev->kobj.parent, NULL);
-   if (error) {
-       glue_dir = get_glue_dir(dev);
+
+   glue_dir = get_glue_dir(dev);
+   unlock_if_glue_dir(dev, glue_dir);
+
+   if (error)
       goto Error;
-   }
   /* notify platform of device entry */
   error = device_platform_notify(dev, KOBJ_ADD);
--