linux-kernel - Re: [PATCH] bitfield.h: annotate type_replace_bits functions with __must

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Date:   Thu, 17 Sep 2020 07:24:00 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Vinod Koul <vkoul@...nel.org>
Cc:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        davem@...emloft.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bitfield.h: annotate type_replace_bits functions with
 __must_check

On Thu, Sep 17, 2020 at 09:34:59AM +0530, Vinod Koul wrote:
> On 16-09-20, 16:33, Srinivas Kandagatla wrote:
> > 
> > 
> > On 16/09/2020 16:20, Greg KH wrote:
> > > On Wed, Sep 16, 2020 at 04:03:33PM +0100, Srinivas Kandagatla wrote:
> > > > usage of apis like u32_replace_bits() without actually catching the return
> > > > value could hide problems without any warning!
> > > > 
> > > > Found this with recent usage of this api in SoundWire!
> > > > Having __must_check annotation would really catch this issues in future!
> > > > 
> > > > Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
> > > > ---
> > > >   include/linux/bitfield.h | 2 +-
> > > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> > > > index 4e035aca6f7e..eb4f69253946 100644
> > > > --- a/include/linux/bitfield.h
> > > > +++ b/include/linux/bitfield.h
> > > > @@ -131,7 +131,7 @@ static __always_inline __##type type##_encode_bits(base v, base field)	\
> > > >   		__field_overflow();					\
> > > >   	return to((v & field_mask(field)) * field_multiplier(field));	\
> > > >   }									\
> > > > -static __always_inline __##type type##_replace_bits(__##type old,	\
> > > > +static __always_inline __must_check __##type type##_replace_bits(__##type old, \
> > > >   					base val, base field)		\
> > > >   {									\
> > > >   	return (old & ~to(field)) | type##_encode_bits(val, field);	\
> > > > -- 
> > > > 2.21.0
> > > > 
> > > 
> > > Don't add __must_check to things that if merged will instantly cause
> > > build warnings to the system, that's just rude :(
> > Currently there are not many users for this api, found only one instance in
> > drivers/net/ipa/ipa_table.c which is also fixed with
> > https://lkml.org/lkml/2020/9/10/1062
> > 
> > > 
> > > Fix up everything first, and then try to make this type of change.
> > > 
> > > But why does this function have to be checked?
> > As this function would return updated value, this check is more to with
> > using the return value!
> > 
> > It is easy for someone to ignore this return value assuming that the new
> > value is already updated! So this check should help!
> > 
> > TBH, This is what happened when we(vkoul and me) tried use this api :-)
> 
> So the only user of this has been moved to *p_replace_bits(), looking
> back I think we should remove *_replace_bits (no users atm) and
> duplicate of *p_replace_bits(). If not adding this patch would be
> sensible thing to do
> 
> Somehow I feel former is a better idea ;-)

Yes, please remove it if there is no users.

thanks,

greg k-h