Re: 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]

Â
On Thu, 16 Feb 2017, Alan Stern wrote:Â

> OnÂThu,Â16ÂFebÂ2017,ÂAjayÂKaherÂwrote:
>Â
>>Â>ÂOnÂThu,Â14ÂFebÂ2017,ÂAlanÂSternÂwrote:
>>Â>Â
>>Â>ÂIÂthinkÂAjay'sÂargumentÂisÂcorrectÂandÂaÂpatchÂisÂneeded.ÂÂButÂthis
>>Â>ÂpatchÂmissesÂtheÂraceÂbetweenÂinit_usb_class()ÂandÂrelease_usb_class().ÂÂ
>>Â
>>ÂThanksÂAlanÂforÂyourÂcomments,ÂinÂpatchÂv2ÂIÂhaveÂtakenÂcareÂfor
>>Ârelease_usb_class()Âalso.ÂPleaseÂreviewÂagain.
>>Â
>>Â>ÂTheÂbasicÂproblemÂisÂthatÂreferenceÂcountingÂdoesn'tÂworkÂwhenÂyouÂtry
>>Â>ÂtoÂuseÂtheÂsameÂglobalÂpointerÂ(usb_class)ÂtoÂreferÂtoÂmultiple
>>Â>ÂgenerationsÂofÂaÂdynamicallyÂallocatedÂentity.ÂÂWeÂhadÂtheÂsameÂsortÂof
>>Â>ÂproblemÂmanyÂyearsÂagoÂwithÂtheÂusb_interfaceÂstructureÂ(andÂwe
>>Â>ÂultimatelyÂfixedÂitÂbyÂcreatingÂaÂseparateÂusb_interface_cache
>>Â>Âstructure).
>>Â>ÂÂ
>>Â>ÂTheÂbestÂapproachÂhereÂwouldÂbeÂtoÂforgetÂaboutÂallÂtheÂreference
>>Â>Âcounting.ÂÂGetÂridÂofÂusb_classÂentirely,ÂandÂcreateÂtheÂ"usbmisc"
>>Â>ÂclassÂstructureÂjustÂonce,ÂwhenÂusbcoreÂinitializes.ÂÂOr,ÂifÂyou
>>Â>Âprefer,ÂuseÂaÂmutexÂtoÂprotectÂaÂroutineÂthatÂallocatesÂtheÂclass
>>Â>ÂstructureÂdynamically,ÂjustÂonce.ÂÂEitherÂway,Âdon'tÂdeallocateÂit
>>Â>ÂuntilÂusbcoreÂisÂunloaded.
>>Â
>>ÂusbmiscÂclassÂcreationÂshouldÂnotÂrequireÂeverytimeÂwhenÂUSBÂcore
>>Âinitializes.ÂSoÂbetterÂtoÂkeepÂusbmiscÂclassÂcreationÂasÂitÂis.Â
>>ÂAndÂtoÂpreventÂtheÂraceÂconditionsÂjustÂprotectÂitÂwithÂMutexÂlocking
>>ÂasÂperÂpatchÂv2.
>Â
> ThisÂisÂnotÂright.ÂÂWhatÂhappensÂifÂusb_register_dev()ÂrunsÂjustÂbefore
> release_usb_class()ÂcallsÂmutex_lock()?
Â
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;







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