Date: Wed, 20 Feb 2013 14:52:59 -0700 From: Ian Lepore <ian@FreeBSD.org> To: John Baldwin <jhb@FreeBSD.org> Cc: Konstantin Belousov <kostikbel@gmail.com>, freebsd-current@FreeBSD.org, Svatopluk Kraus <onwahe@gmail.com> Subject: Re: [patch] i386 pmap sysmaps_pcpu[] atomic access Message-ID: <1361397179.1185.13.camel@revolution.hippie.lan> In-Reply-To: <201302201432.10502.jhb@freebsd.org> References: <CAFHCsPUVTM9jfrnzY72YsPszLWkg-UaJcycTR4xXcS%2BfPzS1Vg@mail.gmail.com> <201302201022.29294.jhb@freebsd.org> <20130220192739.GM2598@kib.kiev.ua> <201302201432.10502.jhb@freebsd.org>
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On Wed, 2013-02-20 at 14:32 -0500, John Baldwin wrote: > On Wednesday, February 20, 2013 2:27:39 pm Konstantin Belousov wrote: > > On Wed, Feb 20, 2013 at 10:22:29AM -0500, John Baldwin wrote: > > > On Wednesday, February 20, 2013 7:31:08 am Svatopluk Kraus wrote: > > > > On Tue, Feb 19, 2013 at 7:51 PM, Konstantin Belousov > > > > <kostikbel@gmail.com> wrote: > > > > > On Mon, Feb 18, 2013 at 11:18:16PM +0100, Svatopluk Kraus wrote: > > > > >> On Mon, Feb 18, 2013 at 9:36 PM, Konstantin Belousov > > > > >> <kostikbel@gmail.com> wrote: > > > > >> Well, I'm taking a part on porting FreeBSD to ARM11mpcore. UP case was > > > > >> simple. SMP case is more complex and rather new for me. Recently, I > > > > >> was solving a problem with PCPU stuff. For example, PCPU_GET is > > > > >> implemented by one instruction on i386 arch. So, a need of atomicity > > > > >> with respect to interrupts can be overlooked. On load-store archs, the > > > > >> implementation which works in SMP case is not so simple. And what > > > > >> works in UP case (single PCPU), not works in SMP case. Believe me, > > > > >> mysterious and sporadic 'mutex not owned' assertions and others ones > > > > >> caused by curthreads mess, it takes a while ... > > > > > Note that PCPU_GET() is not needed to be atomic. The reason is that the code > > > > > which uses its result would not be atomic (single-instruction or whatever, see > > > > > below). Thus, either the preemption should not matter since the action with > > > > > the per-cpu data is advisory, as is in the case of i386 pmap you noted. > > > > > or some external measures should be applied in advance to the containing > > > > > region (which you proposed, by bracing with sched_pin()). > > > > > > > > So, it's advisory in the case of i386 pmap... Well, if you can live > > > > with that, I can too. > > > > > > > > > > > > > > Also, note that it is not interrupts but preemption which is concern. > > > > > > > > Yes and no. In theory, yes, a preemption is a concern. In FreeBSD, > > > > however, sched_pin() and critical_enter() and their counterparts are > > > > implemented with help of curthread. And curthread definition falls to > > > > PCPU_GET(curthread) if not defined in other way. So, curthread should > > > > be atomic with respect to interrupts and in general, PCPU_GET() should > > > > be too. Note that spinlock_enter() definitions often (always) use > > > > curthread too. Anyhow, it's defined in MD code and can be defined for > > > > each arch separately. > > > > > > curthread is a bit magic. :) If you perform a context switch during an > > > interrupt (which will change 'curthread') you also change your register state. > > > When you resume, the register state is also restored. This means that while > > > something like 'PCPU_GET(cpuid)' might be stale after you read it, 'curthread' > > > never is. However, it is true that actually reading curthread has to be > > > atomic. If your read of curthread looks like: > > > > > > mov <pcpu reg>, r0 > > > add <offset of pc_curthread>, r0 > > > ld r0, r1 > > > > > > Then that will indeed break. Alpha used a fixed register for 'pcpu_reg' > > > (as does ia64 IIRC). OTOH, you might also be able to depend on the fact that > > > pc_curthread is the first thing in 'struct pcpu' (and always will be, you could > > > add a CTASSERT to future-proof). In that case, you can remove the 'add' > > > instruction and instead just do: > > > > > > ld <pcpu reg>, r1 > > > > > > which is fine. > > > > I just looked at the arm pcpu.h, and indeed the curthread falls > > back to the default implementation from sys/pcpu.h, which is > > get_pcpu()->pc_curthread. This looks buggy for SMP, does our arm port > > support any multi-cpu configuration ? > > Not in HEAD. There is a projects branch for armv6 I think that does. > There isn't anything I've heard of that makes it all the way to single user mode yet with multiple cores enabled. -- Ian
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