* Re: Subject: [PATCH v3] USB:Core: BugFix: Proper handling of Race Condition when two USB class drivers try to call init_usb_class simultaneously
[not found] <CGME20170216145246epcas4p3bf3b910ecb1892e52fc473e485689cb6@epcas4p3.samsung.com>
@ 2017-02-20 9:23 ` Ajay Kaher
2017-02-20 19:20 ` Alan Stern
0 siblings, 1 reply; 2+ messages in thread
From: Ajay Kaher @ 2017-02-20 9:23 UTC (permalink / raw)
To: Alan Stern
Cc: gregkh, linux-usb, linux-kernel, AMAN DEEP, HEMANSHU SRIVASTAVA
[-- Attachment #1: Type: text/plain, Size: 3286 bytes --]
On Thu, 16 Feb 2017, Alan Stern wrote:
> On Thu, 16 Feb 2017, Ajay Kaher wrote:
>
>> > On Thu, 14 Feb 2017, Alan Stern wrote:
>> >
>> > I think Ajay's argument is correct and a patch is needed. But this
>> > patch misses the race between init_usb_class() and release_usb_class().
>>
>> Thanks Alan for your comments, in patch v2 I have taken care for
>> release_usb_class() also. Please review again.
>>
>> > The basic problem is that reference counting doesn't work when you try
>> > to use the same global pointer (usb_class) to refer to multiple
>> > generations of a dynamically allocated entity. We had the same sort of
>> > problem many years ago with the usb_interface structure (and we
>> > ultimately fixed it by creating a separate usb_interface_cache
>> > structure).
>> >
>> > The best approach here would be to forget about all the reference
>> > counting. Get rid of usb_class entirely, and create the "usbmisc"
>> > class structure just once, when usbcore initializes. Or, if you
>> > prefer, use a mutex to protect a routine that allocates the class
>> > structure dynamically, just once. Either way, don't deallocate it
>> > until usbcore is unloaded.
>>
>> usbmisc class creation should not require everytime when USB core
>> initializes. So better to keep usbmisc class creation as it is.
>> And to prevent the race conditions just protect it with Mutex locking
>> as per patch v2.
>
> This is not right. What happens if usb_register_dev() runs just before
> release_usb_class() calls mutex_lock()?
Alan, as per my understanding I have shifted the lock from
release_usb_class() to destroy_usb_class() in patch v3.
If it is not right, please explain in detail which race condition
I have missed and also share your suggestions.
thanks,
ajay kaher
Signed-off-by: Ajay Kaher
---
drivers/usb/core/file.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/drivers/usb/core/file.c b/drivers/usb/core/file.c
index 822ced9..a12d184 100644
--- a/drivers/usb/core/file.c
+++ b/drivers/usb/core/file.c
@@ -27,6 +27,7 @@
#define MAX_USB_MINORS 256
static const struct file_operations *usb_minors[MAX_USB_MINORS];
static DECLARE_RWSEM(minor_rwsem);
+static DEFINE_MUTEX(init_usb_class_mutex);
static int usb_open(struct inode *inode, struct file *file)
{
@@ -109,8 +110,10 @@ static void release_usb_class(struct kref *kref)
static void destroy_usb_class(void)
{
+ mutex_lock(&init_usb_class_mutex);
if (usb_class)
kref_put(&usb_class->kref, release_usb_class);
+ mutex_unlock(&init_usb_class_mutex);
}
int usb_major_init(void)
@@ -171,7 +174,10 @@ int usb_register_dev(struct usb_interface *intf,
if (intf->minor >= 0)
return -EADDRINUSE;
+ mutex_lock(&init_usb_class_mutex);
retval = init_usb_class();
+ mutex_unlock(&init_usb_class_mutex);
+
if (retval)
return retval;
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: Subject: [PATCH v3] USB:Core: BugFix: Proper handling of Race Condition when two USB class drivers try to call init_usb_class simultaneously
2017-02-20 9:23 ` Subject: [PATCH v3] USB:Core: BugFix: Proper handling of Race Condition when two USB class drivers try to call init_usb_class simultaneously Ajay Kaher
@ 2017-02-20 19:20 ` Alan Stern
0 siblings, 0 replies; 2+ messages in thread
From: Alan Stern @ 2017-02-20 19:20 UTC (permalink / raw)
To: Ajay Kaher
Cc: gregkh, linux-usb, linux-kernel, AMAN DEEP, HEMANSHU SRIVASTAVA
On Mon, 20 Feb 2017, Ajay Kaher wrote:
> Alan, as per my understanding I have shifted the lock from
> release_usb_class() to destroy_usb_class() in patch v3.
> If it is not right, please explain in detail which race condition
> I have missed and also share your suggestions.
>
> thanks,
> ajay kaher
>
> Signed-off-by: Ajay Kaher
>
> ---
>
> drivers/usb/core/file.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/drivers/usb/core/file.c b/drivers/usb/core/file.c
> index 822ced9..a12d184 100644
> --- a/drivers/usb/core/file.c
> +++ b/drivers/usb/core/file.c
> @@ -27,6 +27,7 @@
> #define MAX_USB_MINORS 256
> static const struct file_operations *usb_minors[MAX_USB_MINORS];
> static DECLARE_RWSEM(minor_rwsem);
> +static DEFINE_MUTEX(init_usb_class_mutex);
>
> static int usb_open(struct inode *inode, struct file *file)
> {
> @@ -109,8 +110,10 @@ static void release_usb_class(struct kref *kref)
>
> static void destroy_usb_class(void)
> {
> + mutex_lock(&init_usb_class_mutex);
> if (usb_class)
> kref_put(&usb_class->kref, release_usb_class);
> + mutex_unlock(&init_usb_class_mutex);
> }
>
> int usb_major_init(void)
> @@ -171,7 +174,10 @@ int usb_register_dev(struct usb_interface *intf,
> if (intf->minor >= 0)
> return -EADDRINUSE;
>
> + mutex_lock(&init_usb_class_mutex);
> retval = init_usb_class();
> + mutex_unlock(&init_usb_class_mutex);
> +
> if (retval)
> return retval;
Have you considered what would happen if destroy_usb_class() ran, but
some other CPU was still holding a reference to usb_class? And what if
the last reference gets dropped later on, while init_usb_class() is
running?
Maybe that's not possible here, but it is possible in general for
refcounted objects. So yes, this code is probably okay, but it isn't
good form.
Alan Stern
^ permalink raw reply [flat|nested] 2+ messages in thread
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2017-02-20 9:23 ` Subject: [PATCH v3] USB:Core: BugFix: Proper handling of Race Condition when two USB class drivers try to call init_usb_class simultaneously Ajay Kaher
2017-02-20 19:20 ` Alan Stern
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