netdev - Re: [PATCH] rds: ib: force endiannes annotation

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Date:   Mon, 29 Apr 2019 13:18:36 +0200
From:   Nicholas Mc Guire <der.herr@...r.at>
To:     Edward Cree <ecree@...arflare.com>
Cc:     Nicholas Mc Guire <hofrat@...dl.org>,
        Santosh Shilimkar <santosh.shilimkar@...cle.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, rds-devel@....oracle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rds: ib: force endiannes annotation

On Mon, Apr 29, 2019 at 12:00:06PM +0100, Edward Cree wrote:
> On 29/04/2019 07:09, Nicholas Mc Guire wrote:
> > diff --git a/net/rds/ib_recv.c b/net/rds/ib_recv.c
> > index 7055985..a070a2d 100644
> > --- a/net/rds/ib_recv.c
> > +++ b/net/rds/ib_recv.c
> > @@ -824,7 +824,7 @@ static void rds_ib_cong_recv(struct rds_connection *conn,
> >  	}
> >  
> >  	/* the congestion map is in little endian order */
> > -	uncongested = le64_to_cpu(uncongested);
> > +	uncongested = le64_to_cpu((__force __le64)uncongested);
> >  
> >  	rds_cong_map_updated(map, uncongested);
> >  }
> Again, a __force cast doesn't seem necessary here.  It looks like the
>  code is just using the wrong types; if all of src, dst and uncongested
>  were __le64 instead of uint64_t, and the last two lines replaced with
>  rds_cong_map_updated(map, le64_to_cpu(uncongested)); then the semantics
>  would be kept with neither sparse errors nor __force.
> 
> __force is almost never necessary and mostly just masks other bugs or
>  endianness confusion in the surrounding code.  Instead of adding a
>  __force, either fix the code to be sparse-clean or leave the sparse
>  warning in place so that future developers know there's something not
>  right.
>
changing uncongested to __le64 is not an option here - it would only move
the sparse warnings to those other locatoins where the ports that 
became uncongested are being or'ed into uncongested.

I'm not using __force as the prime way to silence sparse - I try to find
an alternative first - the problem is in line 805
                for (k = 0; k < to_copy; k += 8) {
                        /* Record ports that became uncongested, ie
                         * bits that changed from 0 to 1. */
                        uncongested |= ~(*src) & *dst;
                        *dst++ = *src++;
                }
And in this case the endianness handling does seem right.

But ok with me to leave it in as it is - if you think that the __force
here is not justified.

thanks for your comments and notably the explainations !

thx!
hofrat
alternative