Date: Mon, 10 Dec 2018 14:15:20 -0800 From: John Baldwin <jhb@FreeBSD.org> To: Warner Losh <imp@bsdimp.com>, Kevin Bowling <kevin.bowling@kev009.com> Cc: Mateusz Guzik <mjguzik@gmail.com>, Ian Lepore <ian@freebsd.org>, svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers <src-committers@freebsd.org>, scottl@freebsd.org Subject: Re: svn commit: r341682 - head/sys/sys Message-ID: <45f85061-2633-852c-3cc0-41f64d51e4f0@FreeBSD.org> In-Reply-To: <CANCZdfoXuKxuOztCU7Xoavak5b6KYB9bBQonssWUABgQabBBUQ@mail.gmail.com> References: <201812071205.wB7C5BvA038350@repo.freebsd.org> <1544206201.1860.288.camel@freebsd.org> <CAGudoHGUqiByb37wcbhMD3eWmxFrX5=BKMiF3bk6Ptr2WWggAg@mail.gmail.com> <CAK7dMtBj26Fa-eS3WcKezjnrweDJ9RgZvBpbiRK-9S-A4Svb8Q@mail.gmail.com> <CANCZdfoXuKxuOztCU7Xoavak5b6KYB9bBQonssWUABgQabBBUQ@mail.gmail.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On 12/8/18 7:43 PM, Warner Losh wrote: > > > On Sat, Dec 8, 2018, 8:36 PM Kevin Bowling <kevin.bowling@kev009.com <mailto:kevin.bowling@kev009.com> wrote: > > On Sat, Dec 8, 2018 at 12:09 AM Mateusz Guzik <mjguzik@gmail.com <mailto:mjguzik@gmail.com>> wrote: > > > > > Fully satisfying solution would be that all architectures get 64-bit > > ops, even if in the worst case they end up taking a lock. Then > > subsystems would not have to ifdef on anything. However, there > > was some opposition to this proposal and I don't think this is > > important enough to push. > > Mateusz, > > Who is opposing this particular polyfill solution? Scott Long brought > up a situation in driver development where this would be useful as > well. The polyfills lower the cognitive load and #ifdef soup which > are the right call here regardless of performance on toy ports. > > > I don't recall seeing the opposition either. It would have to be a global lock for all 64bit atomics.... but I think it would only be 2 atomics on those architectures. It would have to be a spin lock, so in the case of unrl you would be trading an operation on one of N regular mutexes for a single spin lock that was also contested by other things. This would be pretty crappy. For drivers that aren't actually used on platforms without 32-bit atomics we can simply not build them in sys/modules/Makefile or not put them in GENERIC. For something in the core kernel like unrl I think we will have to do what Mateusz has done here. -- John Baldwin
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?45f85061-2633-852c-3cc0-41f64d51e4f0>