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

From: Trond Myklebust
Date: Wed Aug 31 2016 - 11:03:00 EST

> On Aug 31, 2016, at 09:37, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Wednesday, August 31, 2016 1:17:48 PM CEST Trond Myklebust wrote:
>> What version of gcc are you using? Iâm 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.

Iâm not getting that error on gcc 6.1.1 for x86_64 with either âmake W=1â or âmake W=2â.
âmake W=3â does gives rise to one warning in nfs4_slot_get_seqid:

/home/trondmy/devel/kernel/linux/fs/nfs/nfs4session.c: In function ânfs4_slot_get_seqidâ:
/home/trondmy/devel/kernel/linux/fs/nfs/nfs4session.c:184:10: warning: conversion to âintâ from âlong intâ may alter its value [-Wconversion]
return PTR_ERR(slot);

(which is another false positive) but thatâs 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, Iâm 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.