Re: [patch] net: moxa: fix an error code

Arnd Bergmann
2016-03-02T11:36:05Z

On Wednesday 02 March 2016 14:21:29 Dan Carpenter wrote:
> On Wed, Mar 02, 2016 at 11:52:29AM +0100, Arnd Bergmann wrote:
> > Did you find more of these?
> >
> > it doesn't matter much either way, but if you do multiple such patches,
>
> One or two. I already sent the fixes. I think it was applied.
>
> > I'd suggest using a single PTR_ERR_OR_ZERO() instead of IS_ERR()+PTR_ERR().
> >
> > I have found a couple of drivers in which that leads to better object
> > code, and avoids a warning about a possibly uninitialized variable
> > when the function gets inlined into another one (which won't happen
> > for this driver).
>
> Huh? I sent one where I could have done that but I deliberately didn't
> because I wanted the uninitialized warning if I made a mistake. It
> sounds like you're working around a GCC bug...

The uninitialized warning here is about a type mismatch preventing
gcc from noticing that two conditions are the same, I'm not sure
if this is a bug in gcc, or required by the C standard.

I don't think there is a way in which you would hide a correct
warning about an uninitialized warning.

Have a look at
https://git.kernel.org/cgit/linux/kernel/git/gfs2/linux-gfs2.git/commit/fs/gfs2?h=for-next&id\acfdc3071432a07713e4d007c2811e0224490b0

in which get_leaf_nr() uses the IS_ERR()/PTR_ERR() combo to return
an error from a pointer, or return success when the pointer was set,
followed by a warning about the use of the pointer in another
function. My original patch avoided the warning by using IS_ERR_VALUE()
in the caller, but in retrospect, IS_ERR_OR_ZERO() would have been
a nicer solution:

@@ -783,12 +783,15 @@ static int get_leaf_nr(struct gfs2_inode *dip, u32 index,
u64 *leaf_out)
{
__be64 *hash;
+ int error;

hash = gfs2_dir_get_hash_table(dip);
- if (IS_ERR(hash))
- return PTR_ERR(hash);
- *leaf_out = be64_to_cpu(*(hash + index));
- return 0;
+ error = PTR_ERR_OR_ZERO(hash);
+
+ if (!error)
+ *leaf_out = be64_to_cpu(*(hash + index));
+
+ return error;
}

and I've used that elsewhere now when I ran into this kind of
false positive warning.

Arnd