From: Ajay Kaher <ajay.kaher@samsung.com>
To: Alan Stern <stern@rowland.harvard.edu>,
"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>
Cc: "linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
AMAN DEEP <aman.deep@samsung.com>,
HEMANSHU SRIVASTAVA <hemanshu.s@samsung.com>
Subject: FW: FW: RE: Re: 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
Date: Fri, 03 Mar 2017 14:49:47 +0000 [thread overview]
Message-ID: <20170303144947epcms5p2afa99fe4129042363253bf804e1451cc@epcms5p2> (raw)
In-Reply-To: <20170302074947epcms5p16d50cace2b286f0cabaec6ac31f2390b@epcms5p1>
[-- Attachment #1: Type: text/plain, Size: 4066 bytes --]
> On Thr, 2 Mar 2017, Ajay Kaher wrote:
>> On Wed, 1 Mar 2017, Alan Stern wrote:
>>> On Wed, 1 Mar 2017, Ajay Kaher wrote:
>>>> On Mon, 22 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.
>>>>>>
>>>>>
>>>>> 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?
>>>>
>>>> Access of usb_class->kref is only from either init_usb_class()
>>>> or destroy_usb_class(), and both these functions are now protected
>>>> with Mutex Locking in patch v3, so there is no chance of race condition
>>>> as per above scenarios.
>>>>
>>>>> 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.
>>>>
>>>> As per my understanding, I found to be one of the best possible solution
>>>> for this problem and this solutiuon don't have any side effect.
>>>
>>> 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.
>>
>> Under the circumstances, your patch is acceptable.
>>
>> If you really want to make the point crystal clear, you could replace
>> usb_class->kref with an ordinary integer counter. Then it would be
>> obvious that there are no references other than the ones taken by
>> init_usb_class() and released by destroy_usb_class().
>
> usb_class->kref is not accessible outside the file.c
> as usb_class is _static_ inside the file.c and
> pointer of usb_class->kref is not passed anywhere.
>
> Hence as you wanted, there are no references of usb_class->kref
> other than taken by init_usb_class() and released by destroy_usb_class().
Verified the code again, I hope my last comments clarifed the things
which came in your mind and helps you to accept the patch :)
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;
next prev parent reply other threads:[~2017-03-03 15:24 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CGME20170220192040epcas3p37ded8ece49ed9385df80d3b813a953bc@epcas3p3.samsung.com>
2017-02-22 12:57 ` 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
[not found] ` <CGME20170220192040epcas3p37ded8ece49ed9385df80d3b813a953bc@epcms5p3>
2017-03-01 11:12 ` FW: " Ajay Kaher
2017-03-01 15:15 ` Alan Stern
[not found] ` <CGME20170301151552epcas5p3ecf8f7dc55323b2ceaff8c8a9b9644c4@epcms5p1>
2017-03-02 7:49 ` FW: RE: " Ajay Kaher
[not found] ` <CGME20170301151552epcas5p3ecf8f7dc55323b2ceaff8c8a9b9644c4@epcms5p2>
2017-03-03 14:49 ` Ajay Kaher [this message]
2017-03-03 16:53 ` FW: " Alan Stern
[not found] ` <CGME20170303165352epcas5p2c408a284100b5053daf458a4ad4d9337@epcms5p7>
2017-03-07 4:35 ` Subject: [PATCH v4] " Ajay Kaher
2017-03-07 15:00 ` Alan Stern
[not found] ` <CGME20170307150103epcas2p348320fdfbb176978d084d8f2b7b9a049@epcms5p7>
2017-03-09 11:34 ` Ajay Kaher
2017-03-09 12:10 ` gregkh
[not found] ` <CGME20170307150103epcas2p348320fdfbb176978d084d8f2b7b9a049@epcms5p2>
2017-03-17 10:56 ` [PATCH v4] USB: " Ajay Kaher
2017-03-23 7:18 ` gregkh
2017-03-21 14:49 ` FW: " Ajay Kaher
2017-03-08 18:05 ` Subject: [PATCH v4] USB:Core: BugFix: " gregkh
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