linux-kernel - RE: [PATCH] sched/deadline: fix one-bit signed bitfields to be unsigned

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Message-ID: <02874ECE860811409154E81DA85FBB5882AFAB96@ORSMSX115.amr.corp.intel.com>
Date:   Thu, 30 Nov 2017 00:12:09 +0000
From:   "Keller, Jacob E" <jacob.e.keller@...el.com>
To:     Jakub Kicinski <kubakici@...pl>,
        "Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>
CC:     "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "nhorman@...hat.com" <nhorman@...hat.com>,
        "sassmann@...hat.com" <sassmann@...hat.com>,
        "jogreene@...hat.com" <jogreene@...hat.com>,
        luca abeni <luca.abeni@...tannapisa.it>
Subject: RE: [PATCH] sched/deadline: fix one-bit signed bitfields to be
 unsigned

> -----Original Message-----
> From: Jakub Kicinski [mailto:kubakici@...pl]
> Sent: Tuesday, November 28, 2017 8:08 PM
> To: Kirsher, Jeffrey T <jeffrey.t.kirsher@...el.com>
> Cc: mingo@...hat.com; peterz@...radead.org; Keller, Jacob E
> <jacob.e.keller@...el.com>; linux-kernel@...r.kernel.org;
> netdev@...r.kernel.org; nhorman@...hat.com; sassmann@...hat.com;
> jogreene@...hat.com; luca abeni <luca.abeni@...tannapisa.it>
> Subject: Re: [PATCH] sched/deadline: fix one-bit signed bitfields to be unsigned
> 
> On Tue, 28 Nov 2017 12:36:19 -0800, Jeff Kirsher wrote:
> > From: Jacob Keller <jacob.e.keller@...el.com>
> >
> > Commit 799ba82de01e ("sched/deadline: Use C bitfields for the state
> > flags", 2017-10-10) introduced the use of C bitfields for these
> > variables. However, sparse complains about them:
> >
> > ./include/linux/sched.h:476:62: error: dubious one-bit signed bitfield
> > ./include/linux/sched.h:477:62: error: dubious one-bit signed bitfield
> > ./include/linux/sched.h:478:62: error: dubious one-bit signed bitfield
> > ./include/linux/sched.h:479:62: error: dubious one-bit signed bitfield
> >
> > This is because a one-bit signed bitfield can only hold the values 0 and
> > -1, which can cause problems if the program expects to be able to
> > represent the value positive 1.
> >
> > In practice, this may not cause a bug since -1 would be considered
> > "true" in logical tests, however we should avoid the practice anyways.
> >
> > Fixes: 799ba82de01e ("sched/deadline: Use C bitfields for the state flags", 2017-
> 10-10)
> > Signed-off-by: Jacob Keller <jacob.e.keller@...el.com>
> > Cc: luca abeni <luca.abeni@...tannapisa.it>
> > Tested-by: Andrew Bowers <andrewx.bowers@...el.com>
> > Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
> 
> This is already in Linus's tree (I've been waiting for it to land as
> well :))
> 

Excellent.

Regards,
Jake