FW: RE: 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]

> On Mon, 22 Feb 2017, Ajay Kaher wrote:
>Â
>>ÂOnÂMon,Â20ÂFebÂ2017,ÂAjayÂKaherÂwrote:
>>Â
>>>ÂAlan,ÂasÂperÂmyÂunderstandingÂIÂhaveÂshiftedÂtheÂlockÂfrom
>>>Ârelease_usb_class()ÂtoÂdestroy_usb_class()ÂinÂpatchÂv3.Â
>>>ÂIfÂitÂisÂnotÂright,ÂpleaseÂexplainÂinÂdetailÂwhichÂraceÂcondition
>>>ÂIÂhaveÂmissedÂandÂalsoÂshareÂyourÂsuggestions.
>>>Â
>>Â
>>ÂHaveÂyouÂconsideredÂwhatÂwouldÂhappenÂifÂdestroy_usb_class()Âran,ÂbutÂ
>>ÂsomeÂotherÂCPUÂwasÂstillÂholdingÂaÂreferenceÂtoÂusb_class?ÂÂAndÂwhatÂifÂ
>>ÂtheÂlastÂreferenceÂgetsÂdroppedÂlaterÂon,ÂwhileÂinit_usb_class()ÂisÂ
>>Ârunning?
>Â
> AccessÂofÂusb_class->krefÂisÂonlyÂfromÂeitherÂinit_usb_class()
> orÂdestroy_usb_class(),ÂandÂbothÂtheseÂfunctionsÂareÂnowÂprotected
> withÂMutexÂLockingÂinÂpatchÂv3,ÂsoÂthereÂisÂnoÂchanceÂofÂraceÂcondition
> asÂperÂaboveÂscenarios.
>Â
>>ÂMaybeÂthat'sÂnotÂpossibleÂhere,ÂbutÂitÂisÂpossibleÂinÂgeneralÂforÂ
>>ÂrefcountedÂobjects.ÂÂSoÂyes,ÂthisÂcodeÂisÂprobablyÂokay,ÂbutÂitÂisn'tÂ
>>ÂgoodÂform.
>Â
> AsÂperÂmyÂunderstanding,ÂIÂfoundÂtoÂbeÂoneÂofÂtheÂbestÂpossibleÂsolution
> forÂthisÂproblemÂandÂthisÂsolutiuonÂdon'tÂhaveÂanyÂsideÂeffect.

Alan, I had shared modified Patch v3 as per your inputs to prevent
the race condition during simultaneously calling of init_usb_class().
If you think there is scope to improve the patch, please share your inputs.

thanks,
ajayÂkaher


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;

Â