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Date: Thu, 6 Oct 2011 23:00:24 +0200
From: "Rafael J. Wysocki" <rjw@...k.pl>
To: Don Zickus <dzickus@...hat.com>
Cc: Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>,
Robert Richter <robert.richter@....com>,
Andrew Morton <akpm@...ux-foundation.org>,
seiji.aguchi@....com, vgoyal@...hat.com, mjg@...hat.com,
tony.luck@...el.com, gong.chen@...el.com, satoru.moriya@....com,
avi@...hat.com, Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [RFC][PATCH] x86, reboot: use NMI instead of REBOOT_VECTOR to stop cpus
On Thursday, October 06, 2011, Don Zickus wrote:
> On Wed, Sep 28, 2011 at 02:55:48PM -0400, Don Zickus wrote:
> > A recent discussion started talking about the locking on the pstore fs
> > and how it relates to the kmsg infrastructure. We noticed it was possible
> > for userspace to r/w to the pstore fs (grabbing the locks in the process)
> > and block the panic path from r/w to the same fs.
>
> Poke? Anyone have an opinion on this?
I think Matthew Garrett should have a look at this (now CCed).
> > The reason was the cpu with the lock could be doing work while the crashing
> > cpu is panic'ing. Busting those spinlocks might cause those cpus to step
> > on each other's data. Fine, fair enough.
> >
> > It was suggested it would be nice to serialize the panic path (ie stop
> > the other cpus) and have only one cpu running. This would allow us to
> > bust the spinlocks and not worry about another cpu stepping on the data.
> >
> > Of course, smp_send_stop() does this in the panic case. kmsg_dump() would
> > have to be moved to be called after it. Easy enough.
> >
> > The only problem is on x86 the smp_send_stop() function calls the
> > REBOOT_VECTOR. Any cpu with irqs disabled (which pstore and its backend
> > ERST would do), block this IPI and thus do not stop. This makes it
> > difficult to reliably log data to the pstore fs.
> >
> > The patch below switches from the REBOOT_VECTOR to NMI (and mimics what
> > kdump does). Switching to NMI allows us to deliver the IPI when irqs are
> > disabled, increasing the reliability of this function.
> >
> > However, Andi carefully noted that on some machines this approach does not
> > work because of broken BIOSes or whatever.
> >
> > I was hoping to get feedback on how much of a problem this really is. Are
> > there that many machines? I assume most modern machines have a reliable NMI
> > IPI mechanism (well on x86). Is this just a problem on 32-bit machines?
> > Early SMP machines?
> >
> > One idea I had was to create a blacklist of machines and have those machines
> > fallback to the original native_stop_other_cpus() that Andi wrote originally.
> > The hope was that list was small.
> >
> > Does anyone have any feedback whether this is a good idea or not? Perhaps I am
> > missing something? Perhaps I should approach this problem differently?
> >
> > [note] this patch sits on top of another NMI infrastructure change I have
> > submitted, so the nmi registeration might not apply cleanly without that patch.
> > However, for discussion purposes, I don't think that change is relevant, it is
> > more the idea/philosophy of this patch that I am worried about.
> >
> > Thanks,
> > Don
> > ---
> > arch/x86/kernel/smp.c | 56 +++++++++++++++++++++++++++++++++++++++++++++++-
> > 1 files changed, 54 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> > index 013e7eb..e98f0a1 100644
> > --- a/arch/x86/kernel/smp.c
> > +++ b/arch/x86/kernel/smp.c
> > @@ -28,6 +28,7 @@
> > #include <asm/mmu_context.h>
> > #include <asm/proto.h>
> > #include <asm/apic.h>
> > +#include <asm/nmi.h>
> > /*
> > * Some notes on x86 processor bugs affecting SMP operation:
> > *
> > @@ -147,6 +148,57 @@ void native_send_call_func_ipi(const struct cpumask *mask)
> > free_cpumask_var(allbutself);
> > }
> >
> > +static int stopping_cpu;
> > +
> > +static int smp_stop_nmi_callback(unsigned int val, struct pt_regs *regs)
> > +{
> > + /* We are registerd on stopping cpu too, avoid spurious NMI */
> > + if (raw_smp_processor_id() == stopping_cpu)
> > + return NMI_HANDLED;
> > +
> > + stop_this_cpu(NULL);
> > +
> > + return NMI_HANDLED;
> > +}
> > +
> > +static void native_nmi_stop_other_cpus(int wait)
> > +{
> > + unsigned long flags;
> > + unsigned long timeout;
> > +
> > + if (reboot_force)
> > + return;
> > +
> > + /*
> > + * Use an own vector here because smp_call_function
> > + * does lots of things not suitable in a panic situation.
> > + */
> > + if (num_online_cpus() > 1) {
> > + stopping_cpu = safe_smp_processor_id();
> > +
> > + if (register_nmi_handler(NMI_LOCAL, smp_stop_nmi_callback,
> > + NMI_FLAG_FIRST, "smp_stop"))
> > + return; /* return what? */
> > +
> > + /* sync above data before sending NMI */
> > + wmb();
> > +
> > + apic->send_IPI_allbutself(NMI_VECTOR);
> > +
> > + /*
> > + * Don't wait longer than a second if the caller
> > + * didn't ask us to wait.
> > + */
> > + timeout = USEC_PER_SEC;
> > + while (num_online_cpus() > 1 && (wait || timeout--))
> > + udelay(1);
> > + }
> > +
> > + local_irq_save(flags);
> > + disable_local_APIC();
> > + local_irq_restore(flags);
> > +}
> > +
> > /*
> > * this function calls the 'stop' function on all other CPUs in the system.
> > */
> > @@ -159,7 +211,7 @@ asmlinkage void smp_reboot_interrupt(void)
> > irq_exit();
> > }
> >
> > -static void native_stop_other_cpus(int wait)
> > +static void native_irq_stop_other_cpus(int wait)
> > {
> > unsigned long flags;
> > unsigned long timeout;
> > @@ -229,7 +281,7 @@ struct smp_ops smp_ops = {
> > .smp_prepare_cpus = native_smp_prepare_cpus,
> > .smp_cpus_done = native_smp_cpus_done,
> >
> > - .stop_other_cpus = native_stop_other_cpus,
> > + .stop_other_cpus = native_nmi_stop_other_cpus,
> > .smp_send_reschedule = native_smp_send_reschedule,
> >
> > .cpu_up = native_cpu_up,
> --
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