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Tue, 1 Aug 2023 02:03:01 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface User-Agent: Cyrus-JMAP/3.9.0-alpha0-592-ga9d4a09b4b-fm-defalarms-20230725.001-ga9d4a09b Mime-Version: 1.0 Message-Id: <2d9c843f-e884-47d3-a825-6402db0a2cb8@app.fastmail.com> In-Reply-To: <87y1ivln1v.ffs@tglx> References: <20230721102237.268073801@infradead.org> <20230721105743.819362688@infradead.org> <87edkonjrk.ffs@tglx> <87mszcm0zw.ffs@tglx> <20230731192012.GA11704@hirez.programming.kicks-ass.net> <87a5vbn5r0.ffs@tglx> <20230731213341.GB51835@hirez.programming.kicks-ass.net> <87y1ivln1v.ffs@tglx> Date: Tue, 01 Aug 2023 08:02:21 +0200 From: "Arnd Bergmann" To: "Thomas Gleixner" , "Peter Zijlstra" Cc: "Jens Axboe" , linux-kernel@vger.kernel.org, "Ingo Molnar" , "Darren Hart" , dave@stgolabs.net, andrealmeid@igalia.com, "Andrew Morton" , urezki@gmail.com, "Christoph Hellwig" , "Lorenzo Stoakes" , linux-api@vger.kernel.org, linux-mm@kvack.org, Linux-Arch , malteskarupke@web.de Subject: Re: [PATCH v1 02/14] futex: Extend the FUTEX2 flags Content-Type: text/plain X-Rspam-User: X-Stat-Signature: jahk77xxgkkufkrjt84389gngama673h X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 370D62001C X-HE-Tag: 1690869787-821196 X-HE-Meta: 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 sjbWIbAu 5W8J1bd65Bvyi4sCrK3jCpqnTHdv+7jYRZQRgWjtesNNTg+O/4c5y2U7yvMuLFgyQyBHvAGSnr9mpOJee+hhlmwsJQtKp1T86Ow/BHYI7dYkJfsrJW9o5pUGra/+B50o1+8OO+xviRNkBKX4zCNl+cKE8y4gId9nmm/69s8wh0zM6JEE= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Aug 1, 2023, at 00:43, Thomas Gleixner wrote: > On Mon, Jul 31 2023 at 23:33, Peter Zijlstra wrote: >> On Mon, Jul 31, 2023 at 11:14:11PM +0200, Thomas Gleixner wrote: >>> --- a/include/uapi/linux/futex.h >>> +++ b/include/uapi/linux/futex.h >>> @@ -74,7 +74,12 @@ >>> struct futex_waitv { >>> __u64 val; >>> __u64 uaddr; >>> - __u32 flags; >>> + union { >>> + __u32 flags; >>> + __u32 size : 2, >>> + : 5, >>> + private : 1; >>> + }; >>> __u32 __reserved; >>> }; >> >> Durr, I'm not sure I remember if that does the right thing across >> architectures -- might just work. But I'm fairly sure this isn't the >> only case of a field in a flags thing in our APIs. Although obviously >> I can't find another case in a hurry :/ > > I know, but that doesn't make these things more readable and neither an > argument against doing it for futex2 :) ... > > Still that explicit bitfield does neither need comments nor does it > leave room for interpretation. It may be clear to the compiler, but without comments or looking up psABI documentation I certainly wouldn't know immediately which bits of the flags word overlay the bitfields for a given combination of __BIG_ENDIAN/__LITTLE_ENDIAN and __BIG_ENDIAN_BITFIELD/__LITTLE_ENDIAN_BITFIELD or architectures with unusual struct alignment requirements (m68k or arm-oabi). I'd prefer to completely avoid the bitfield here. Maybe having exclusive flags for each width would be less confusing, at the cost of needing two more flag bits and a slightly more complicated sanity check, or we could take an extra byte out of the __reserved field to store the length. Arnd