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

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Message-ID: <20171128200726.1c806eb7@cakuba.netronome.com>
Date:   Tue, 28 Nov 2017 20:07:53 -0800
From:   Jakub Kicinski <kubakici@...pl>
To:     Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Cc:     mingo@...hat.com, peterz@...radead.org,
        Jacob Keller <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 :))

commit aa5222e92f8000ed3c1c38dddf11c83222aadfb3
Author: Dan Carpenter <dan.carpenter@...cle.com>
Date:   Fri Oct 13 10:01:22 2017 +0300

    sched/deadline: Don't use dubious signed bitfields
    
    It doesn't cause a run-time bug, but these bitfields should be unsigned.
    When it's signed ->dl_throttled is set to either 0 or -1, instead of
    0 and 1 as expected.
    
    The sched.h file is included into tons of places so Sparse generates
    a flood of warnings like this:
    
      ./include/linux/sched.h:477:54: error: dubious one-bit signed bitfield
    
    Reported-by: Matthew Wilcox <willy@...radead.org>
    Reported-by: Xin Long <lucien.xin@...il.com>
    Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
    Reviewed-by: Luca Abeni <luca.abeni@...tannapisa.it>
    Cc: Linus Torvalds <torvalds@...ux-foundation.org>
    Cc: Peter Zijlstra <peterz@...radead.org>
    Cc: Thomas Gleixner <tglx@...utronix.de>
    Cc: kernel-janitors@...r.kernel.org
    Cc: luca abeni <luca.abeni@...tannapisa.it>
    Link: http://lkml.kernel.org/r/20171013070121.dzcncojuj2f4utij@mwanda
    Signed-off-by: Ingo Molnar <mingo@...nel.org>