linux-kernel - RE: [PATCH 1/2] Staging: fsl-mc: include: mc: Kernel type 's16' preferred over 'int16

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Date:   Mon, 14 Nov 2016 14:55:06 +0000
From:   Stuart Yoder <stuart.yoder@....com>
To:     Dan Carpenter <dan.carpenter@...cle.com>,
        Shiva Kerdel <shiva@...ev.nl>
CC:     "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nipun Gupta <nipun.gupta@....com>,
        "treding@...dia.com" <treding@...dia.com>,
        "Laurentiu Tudor" <laurentiu.tudor@....com>
Subject: RE: [PATCH 1/2] Staging: fsl-mc: include: mc: Kernel type 's16'
 preferred over 'int16_t'



> -----Original Message-----
> From: Dan Carpenter [mailto:dan.carpenter@...cle.com]
> Sent: Monday, November 14, 2016 4:06 AM
> To: Stuart Yoder <stuart.yoder@....com>
> Cc: Shiva Kerdel <shiva@...ev.nl>; devel@...verdev.osuosl.org; gregkh@...uxfoundation.org; linux-
> kernel@...r.kernel.org; Nipun Gupta <nipun.gupta@....com>; treding@...dia.com; Laurentiu Tudor
> <laurentiu.tudor@....com>
> Subject: Re: [PATCH 1/2] Staging: fsl-mc: include: mc: Kernel type 's16' preferred over 'int16_t'
> 
> On Fri, Nov 11, 2016 at 02:52:31PM +0000, Stuart Yoder wrote:
> > > > diff --git a/drivers/staging/fsl-mc/include/mc-bus.h b/drivers/staging/fsl-mc/include/mc-bus.h
> > > > index e915574..c7cad87 100644
> > > > --- a/drivers/staging/fsl-mc/include/mc-bus.h
> > > > +++ b/drivers/staging/fsl-mc/include/mc-bus.h
> > > > @@ -42,8 +42,8 @@ struct msi_domain_info;
> > > >   */
> > > >  struct fsl_mc_resource_pool {
> > > >  	enum fsl_mc_pool_type type;
> > > > -	int16_t max_count;
> > > > -	int16_t free_count;
> > > > +	s16 max_count;
> > >
> > > My understanding is that this has to be signed because the design of
> > > this driver is that we keep adding devices until the the counter
> > > overflows.  After that there are a couple tests for
> > > "if (WARN_ON(res_pool->max_count < 0)) " which prevent the driver from
> > > working again.
> > >
> > > This all seems pretty horrible.
> >
> > Can you elaborate?
> >
> > The resource pools managed by this driver are populated by hardware objects
> > discovered when the fsl-mc bus probes a DPRC/container.
> >
> > The number of potential objects discovered of a given type is in the hundreds,
> > so a signed 16-bit number is order of magnitudes larger than anything we will
> > ever encounter.
> >
> > Would you feel better about this if max_count was an int?
> 
> Yeah.
> 
> >
> > The max_count reflects the total number of objects discovered.  If that is
> > exceeded we display a warning, because something is horribly wrong.  Nothing
> > stops working, the allocator simply refuses to add anything else to the
> > free list.
> 
> I didn't look at this carefully...  Anyway we can't remove devices
> either.  If we just had an upper bound instead of overflowing the s16
> then we could still remove devices.
> 
> >
> > The only reason max_count is there at all is as an internal check against
> > bugs and resource leaks.  If the driver is being removed and a resource
> > pool is being freed, max_count must be zero...i.e. all objects should have
> > been removed.  If not, there is a leak somewhere.  So, it's a sanity check.
> >
> 
> Just use a normal upper bound with a #define instead of an magic number
> hidden and then disguised as an integer overflow.

Ok, agree that it would be clearer like that.

Shiva, can you respin this patch and just make both max_count and free_count
to be of type "int".

I will get Dan's suggestion sent as a separate patch...to #define the upper bound
instead of relying on integer overflow.

Thanks,
Stuart