linux-kernel - Re: [PATCH 1/2] NFSv4.1: work around -Wmaybe-uninitialized warning

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Message-ID: <DC57A918-AB14-483B-87EF-A88469193855@primarydata.com>
Date:   Wed, 31 Aug 2016 15:02:42 +0000
From:   Trond Myklebust <trondmy@...marydata.com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     Schumaker Anna <anna.schumaker@...app.com>,
        "List Linux Network Devel Mailing" <netdev@...r.kernel.org>,
        List Linux NFS Mailing <linux-nfs@...r.kernel.org>,
        List Linux Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] NFSv4.1: work around -Wmaybe-uninitialized warning

> On Aug 31, 2016, at 09:37, Arnd Bergmann <arnd@...db.de> wrote:
> 
> On Wednesday, August 31, 2016 1:17:48 PM CEST Trond Myklebust wrote:
>> What version of gcc are you using? I’m unable to reproduce with gcc 6.1.1..
> 
> This is also on 6.1.1 for ARM. Note that 6e8d666e9253 ("Disable
> "maybe-uninitialized" warning globally") turned off those warnings, so
> unless you explicitly pass -Wmaybe-uninitialized (e.g. by building with
> "make W=1"), you won't get it.
> 

I’m not getting that error on gcc 6.1.1 for x86_64 with either “make W=1” or “make W=2”.
“make W=3” does gives rise to one warning in nfs4_slot_get_seqid:

/home/trondmy/devel/kernel/linux/fs/nfs/nfs4session.c: In function ‘nfs4_slot_get_seqid’:
/home/trondmy/devel/kernel/linux/fs/nfs/nfs4session.c:184:10: warning: conversion to ‘int’ from ‘long int’ may alter its value [-Wconversion]
   return PTR_ERR(slot);
          ^~~~~~~~~~~~~

(which is another false positive) but that’s all...

> The reason I'm still sending the patches for this warning is that
> we do get a number of valid ones (this was the only false positive
> out of the seven such warnings since last week).

There is a Zen-like quality to IS_ERR() when it casts a const pointer to an unsigned long, back to a non-const pointer, and then back to an unsigned long before comparing it to another unsigned long cast constant negative integer. However, I’m not sure the C99 standard would agree that a positive test result implies we can assume that a simple cast of the same pointer to a signed long will result in a negative, non-zero valued errno.

I suspect that if we really want to fix these false negatives, we should probably address that issue.

Cheers
  Trond