Re: Subject: [PATCH v4] USB:Core: BugFix: Proper handling of Race Condition when two USB class drivers try to call init_usb_class simultaneously

[Alan Stern]

On Tue, 7 Mar 2017, Ajay Kaher wrote:

> > OnÂFri,Â3ÂMarÂ2017,ÂAjayÂKaherÂwrote:
> >Â
> > >Â>Âusb_class->krefÂisÂnotÂaccessibleÂoutsideÂtheÂfile.c
> > >Â>ÂasÂusb_classÂisÂ_static_ÂinsideÂtheÂfile.cÂand
> > >Â>ÂpointerÂofÂusb_class->krefÂisÂnotÂpassedÂanywhere.
> > >Â>Â
> > >Â>ÂHenceÂasÂyouÂwanted,ÂthereÂareÂnoÂreferencesÂofÂusb_class->kref
> > >Â>ÂotherÂthanÂtakenÂbyÂinit_usb_class()ÂandÂreleasedÂbyÂdestroy_usb_class().
> > >Â
> > >ÂVerifiedÂtheÂcodeÂagain,ÂIÂhopeÂmyÂlastÂcommentsÂclarifedÂtheÂthings
> > >ÂwhichÂcameÂinÂyourÂmindÂandÂhelpsÂyouÂtoÂacceptÂtheÂpatchÂ:)
> > Â
> > YourÂmainÂpointÂisÂthatÂusb_class->krefÂisÂaccessedÂfromÂonlyÂtwo
> > points,ÂbothÂofÂwhichÂareÂprotectedÂbyÂtheÂnewÂmutex.ÂÂThisÂmeansÂthere
> > isÂnoÂreasonÂforÂtheÂvalueÂtoÂbeÂaÂstructÂkrefÂatÂall.ÂÂYouÂshould
> > changeÂitÂtoÂanÂintÂ(andÂchangeÂitsÂname).ÂÂLeavingÂitÂasÂaÂkrefÂwill
> > makeÂreadersÂwonderÂwhyÂitÂneedsÂtoÂbeÂupdatedÂatomically.
> 
> At many places in Linux kernel, instances of Kref have been used within
> Mutex, SpinLock and donât have any side effect.
> 
> Making to int and handle (i.e. get/put) it within file.c seems
> not good as we have Kref. Instead, we can have non_atomic version of kref.
> We can discuss about non_atomic kref in another thread, if you are interested.

Okay.

> > Also,ÂwhyÂdoesÂdestroy_usb_class()ÂhaveÂthatÂ"ifÂ(usb_class)Â"test?Â
> > Isn'tÂitÂtrueÂthatÂusb_classÂcanÂneverÂbeÂNULLÂthere?
> 
> Removed in Patch v4.
> 
> thanks,
> ajayÂkaher
> Â
> ÂÂ
> Signed-off-by:ÂAjayÂKaher
> Â
> ---
> 
>  drivers/usb/core/file.c |    9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/core/file.c b/drivers/usb/core/file.c
> index 822ced9..422ce7b 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 +110,9 @@ static void release_usb_class(struct kref *kref)
> 
>  static void destroy_usb_class(void)
>  {
> -       if (usb_class)
> -               kref_put(&usb_class->kref, release_usb_class);
> +       mutex_lock(&init_usb_class_mutex);
> +       kref_put(&usb_class->kref, release_usb_class);
> +       mutex_unlock(&init_usb_class_mutex);
>  }
> 
>  int usb_major_init(void)
> @@ -171,7 +173,10 @@ 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;

Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>

Alan Stern