Date: Mon, 10 Dec 2018 16:57:13 -0700 From: Ian Lepore <ian@freebsd.org> To: John Baldwin <jhb@FreeBSD.org>, Warner Losh <imp@bsdimp.com>, Kevin Bowling <kevin.bowling@kev009.com> Cc: Mateusz Guzik <mjguzik@gmail.com>, 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: <1544486233.1860.343.camel@freebsd.org> In-Reply-To: <45f85061-2633-852c-3cc0-41f64d51e4f0@FreeBSD.org> 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> <45f85061-2633-852c-3cc0-41f64d51e4f0@FreeBSD.org>
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On Mon, 2018-12-10 at 14:15 -0800, John Baldwin wrote: > On 12/8/18 7:43 PM, Warner Losh wrote: > > > > > > > > On Sat, Dec 8, 2018, 8:36 PM Kevin Bowling <kevin.bowling@kev009.co > > m <mailto:kevin.bowling@kev009.com> wrote: > > > > On Sat, Dec 8, 2018 at 12:09 AM Mateusz Guzik <mjguzik@gmail.co > > m <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. > On a single-core system all you need to implement 64-bit atomics in the kernel is to disable interrupts around using normal load/store operations on the values. Do we have any platforms that are SMP but don't have hardware primitives for 64-bit atomics? -- Ian
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