From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jakub Kicinski Subject: Re: [PATCH] sched/deadline: fix one-bit signed bitfields to be unsigned Date: Tue, 28 Nov 2017 20:07:53 -0800 Message-ID: <20171128200726.1c806eb7@cakuba.netronome.com> References: <20171128203619.71430-1-jeffrey.t.kirsher@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: mingo@redhat.com, peterz@infradead.org, Jacob Keller , linux-kernel@vger.kernel.org, netdev@vger.kernel.org, nhorman@redhat.com, sassmann@redhat.com, jogreene@redhat.com, luca abeni To: Jeff Kirsher Return-path: In-Reply-To: <20171128203619.71430-1-jeffrey.t.kirsher@intel.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Tue, 28 Nov 2017 12:36:19 -0800, Jeff Kirsher wrote: > From: Jacob Keller > > 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 > Cc: luca abeni > Tested-by: Andrew Bowers > Signed-off-by: Jeff Kirsher This is already in Linus's tree (I've been waiting for it to land as well :)) commit aa5222e92f8000ed3c1c38dddf11c83222aadfb3 Author: Dan Carpenter 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 Reported-by: Xin Long Signed-off-by: Dan Carpenter Reviewed-by: Luca Abeni Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: kernel-janitors@vger.kernel.org Cc: luca abeni Link: http://lkml.kernel.org/r/20171013070121.dzcncojuj2f4utij@mwanda Signed-off-by: Ingo Molnar