linux-kernel - Re: [RFC 01/32] fs: introduce new 'struct inode

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Date:	Sat, 31 May 2014 18:15:30 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Andreas Schwab <schwab@...ux-m68k.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	"Joseph S. Myers" <joseph@...esourcery.com>,
	John Stultz <john.stultz@...aro.org>,
	Christoph Hellwig <hch@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ley Foon Tan <lftan@...era.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC 01/32] fs: introduce new 'struct inode_time'

On Sat, May 31, 2014 at 4:54 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Saturday 31 May 2014 10:39:02 Andreas Schwab wrote:
>> Geert Uytterhoeven <geert@...ux-m68k.org> writes:
>>
>> > Hi Arnd,
>> >
>> > On Fri, May 30, 2014 at 10:01 PM, Arnd Bergmann <arnd@...db.de> wrote:
>> >> + * The variant using bit fields is less efficient to access, but
>> >> + * small and has a wider range as the 32-bit one, plus it keeps
>> >> + * the signedness of the original timespec.
>> >> + */
>> >> +struct inode_time {
>> >> +       long long       tv_sec  : 34;
>> >> +       int             tv_nsec : 30;
>> >> +};
>> >
>> > Don't you need 31 bits for tv_nsec, to accommodate for the sign bit?
>> > I know you won't really store negative numbers there, but storing a large
>> > positive number will become negative on read out, won't it?
>>
>> Only if the int bitfield is signed.  Bitfields are weird, aren't they?

According to 6.7.2#5 (thanks for the reference), this is implementation defined.

> It was a mistake on my side, as I didn't know about that rule and
> meant write 'unsigned int' really. Also, I always have a bad feeling

IC, but the comment said "plus it keeps the signedness".
So it doesn't keep the signedness for the tv_nsec field.

> about using bitfields in general.

Hehe...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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