RE: [V5 PATCH 2/4] panic/x86: Allow cpus to save registers even if they are looping in NMI context

河合英宏 / KAWAI，HIDEHIRO
2015-11-25T05:52:09Z

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> On Fri, Nov 20, 2015 at 06:36:46PM +0900, Hidehiro Kawai wrote:
> > nmi_shootdown_cpus(), a subroutine of crash_kexec(), sends NMI IPI
> > to non-panic cpus to stop them while saving their register
>
> ...to stop them and save their register...

Thanks for the correction.

> > information and doing some cleanups for crash dumping. So if a
> > non-panic cpus is infinitely looping in NMI context, we fail to
>
> That should be CPU. Please use "CPU" instead of "cpu" in all your text
> in your next submission.

OK, I'll fix that.

> > save its register information and lose the information from the
> > crash dump.
> >
> > `Infinite loop in NMI context' can happen:
> >
> > a. when a cpu panics on NMI while another cpu is processing panic
> > b. when a cpu received an external or unknown NMI while another
> > cpu is processing panic on NMI
> >
> > In the case of a, it loops in panic_smp_self_stop(). In the case
> > of b, it loops in raw_spin_lock() of nmi_reason_lock.
>
> Please describe those two cases more verbosely - it takes slow people
> like me a while to figure out what exactly can happen.

a. when a cpu panics on NMI while another cpu is processing panic
Ex.
CPU 0 CPU 1
================= =================
panic()
panic_cpu <-- 0
check panic_cpu
crash_kexec()
receive an unknown NMI
unknown_nmi_error()
nmi_panic()
panic()
check panic_cpu
panic_smp_self_stop()
infinite loop in NMI context

b. when a cpu received an external or unknown NMI while another
cpu is processing panic on NMI
Ex.
CPU 0 CPU 1
====================== ==================
receive an unknown NMI
unknown_nmi_error()
nmi_panic() receive an unknown NMI
panic_cpu <-- 0 unknown_nmi_error()
panic() nmi_panic()
check panic_cpu panic
crash_kexec() check panic_cpu
panic_smp_self_stop()
infinite loop in NMI context

> > This can
> > happen on some servers which broadcasts NMIs to all CPUs when a dump
> > button is pushed.
> >
> > To save registers in these case too, this patch does following things:
> >
> > 1. Move the timing of `infinite loop in NMI context' (actually
> > done by panic_smp_self_stop()) outside of panic() to enable us to
> > refer pt_regs
>
> I can't parse that sentence. And I really tried :-\
> panic_smp_self_stop() is still in panic().

panic_smp_self_stop() is still used when a CPU in normal context
should go into infinite loop. Only when a CPU is in NMI context,
nmi_panic_self_stop() is called and the CPU loops infinitely
without entering panic().

I'll try to revise this sentense.

> > 2. call a callback of nmi_shootdown_cpus() directly to save
> > registers and do some cleanups after setting waiting_for_crash_ipi
> > which is used for counting down the number of cpus which handled
> > the callback
> >
> > V5:
> > - Use WRITE_ONCE() when setting crash_ipi_done to 1 so that the
> > compiler doesn't change the instruction order
> > - Support the case of b in the above description
> > - Add poll_crash_ipi_and_callback()
> >
> > V4:
> > - Rewrite the patch description
> >
> > V3:
> > - Newly introduced
> >
> > Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Eric Biederman <ebiederm@xmission.com>
> > Cc: Vivek Goyal <vgoyal@redhat.com>
> > Cc: Michal Hocko <mhocko@kernel.org>
> > ---
> > arch/x86/include/asm/reboot.h | 1 +
> > arch/x86/kernel/nmi.c | 17 +++++++++++++----
> > arch/x86/kernel/reboot.c | 28 ++++++++++++++++++++++++++++
> > include/linux/kernel.h | 12 ++++++++++--
> > kernel/panic.c | 10 ++++++++++
> > kernel/watchdog.c | 2 +-
> > 6 files changed, 63 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/reboot.h b/arch/x86/include/asm/reboot.h
> > index a82c4f1..964e82f 100644
> > --- a/arch/x86/include/asm/reboot.h
> > +++ b/arch/x86/include/asm/reboot.h
> > @@ -25,5 +25,6 @@ void __noreturn machine_real_restart(unsigned int type);
> >
> > typedef void (*nmi_shootdown_cb)(int, struct pt_regs*);
> > void nmi_shootdown_cpus(nmi_shootdown_cb callback);
> > +void poll_crash_ipi_and_callback(struct pt_regs *regs);
> >
> > #endif /* _ASM_X86_REBOOT_H */
> > diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
> > index 5131714..74a1434 100644
> > --- a/arch/x86/kernel/nmi.c
> > +++ b/arch/x86/kernel/nmi.c
> > @@ -29,6 +29,7 @@
> > #include <asm/mach_traps.h>
> > #include <asm/nmi.h>
> > #include <asm/x86_init.h>
> > +#include <asm/reboot.h>
> >
> > #define CREATE_TRACE_POINTS
> > #include <trace/events/nmi.h>
> > @@ -231,7 +232,7 @@ pci_serr_error(unsigned char reason, struct pt_regs *regs)
> > #endif
> >
> > if (panic_on_unrecovered_nmi)
> > - nmi_panic("NMI: Not continuing");
> > + nmi_panic(regs, "NMI: Not continuing");
> >
> > pr_emerg("Dazed and confused, but trying to continue\n");
> >
> > @@ -256,7 +257,7 @@ io_check_error(unsigned char reason, struct pt_regs *regs)
> > show_regs(regs);
> >
> > if (panic_on_io_nmi) {
> > - nmi_panic("NMI IOCK error: Not continuing");
> > + nmi_panic(regs, "NMI IOCK error: Not continuing");
> >
> > /*
> > * If we return from nmi_panic(), it means we have received
> > @@ -305,7 +306,7 @@ unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
> >
> > pr_emerg("Do you have a strange power saving mode enabled?\n");
> > if (unknown_nmi_panic || panic_on_unrecovered_nmi)
> > - nmi_panic("NMI: Not continuing");
> > + nmi_panic(regs, "NMI: Not continuing");
> >
> > pr_emerg("Dazed and confused, but trying to continue\n");
> > }
> > @@ -357,7 +358,15 @@ static void default_do_nmi(struct pt_regs *regs)
> > }
> >
> > /* Non-CPU-specific NMI: NMI sources can be processed on any CPU */
> > - raw_spin_lock(&nmi_reason_lock);
> > +
> > + /*
> > + * Another CPU may be processing panic routines with holding
>
> while

I'll fix it.

> > + * nmi_reason_lock. Check IPI issuance from the panicking CPU
> > + * and call the callback directly.
>
> This is one strange sentence. Does it mean something like:
>
> "Check if the panicking CPU issued the IPI and, if so, call the crash
> callback directly."
>
> ?

Yes. Thanks for the suggestion.

> > + */
> > + while (!raw_spin_trylock(&nmi_reason_lock))
> > + poll_crash_ipi_and_callback(regs);
>
> Waaait a minute: so if we're getting NMIs broadcasted on every core but
> we're *not* crash dumping, we will run into here too. This can't be
> right. :-\

As Steven commented, poll_crash_ipi_and_callback() does nothing
if we're not crash dumping. But yes, this is confusing. I'll add
more detailed comment, or change the logic a bit if I come up with
better one.

> > +
> > reason = x86_platform.get_nmi_reason();
> >
> > if (reason & NMI_REASON_MASK) {
> > diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
> > index 02693dd..44c5f5b 100644
> > --- a/arch/x86/kernel/reboot.c
> > +++ b/arch/x86/kernel/reboot.c
> > @@ -718,6 +718,7 @@ static int crashing_cpu;
> > static nmi_shootdown_cb shootdown_callback;
> >
> > static atomic_t waiting_for_crash_ipi;
> > +static int crash_ipi_done;
> >
> > static int crash_nmi_callback(unsigned int val, struct pt_regs *regs)
> > {
> > @@ -780,6 +781,9 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback)
> >
> > smp_send_nmi_allbutself();
> >
> > + /* Kick cpus looping in nmi context. */
> > + WRITE_ONCE(crash_ipi_done, 1);
> > +
> > msecs = 1000; /* Wait at most a second for the other cpus to stop */
> > while ((atomic_read(&waiting_for_crash_ipi) > 0) && msecs) {
> > mdelay(1);
> > @@ -788,9 +792,33 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback)
> >
> > /* Leave the nmi callback set */
> > }
> > +
> > +/*
> > + * Wait for the timing of IPI for crash dumping, and then call its callback
>
> "Wait for the crash dumping IPI to complete... "

So, I think "Wait for the crash dumping IPI to be issued..." is better.
"complete" would be ambiguous in this context.

> > + * directly. This function is used when we have already been in NMI handler.
> > + */
> > +void poll_crash_ipi_and_callback(struct pt_regs *regs)
>
> Why "poll"? We won't return from crash_nmi_callback() if we're not the
> crashing CPU.

This function polls that crash IPI has been issued by checking
crash_ipi_done, then invokes the callback. This is different
from so-called "poll" functions. Do you have some good name?

> > +{
> > + if (crash_ipi_done)
> > + crash_nmi_callback(0, regs); /* Shouldn't return */
> > +}
> > +
> > +/* Override the weak function in kernel/panic.c */
> > +void nmi_panic_self_stop(struct pt_regs *regs)
> > +{
> > + while (1) {
> > + poll_crash_ipi_and_callback(regs);
> > + cpu_relax();
> > + }
> > +}
> > +
> > #else /* !CONFIG_SMP */
> > void nmi_shootdown_cpus(nmi_shootdown_cb callback)
> > {
> > /* No other CPUs to shoot down */
> > }
> > +
> > +void poll_crash_ipi_and_callback(struct pt_regs *regs)
> > +{
> > +}
> > #endif
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index 480a4fd..728a31b 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -255,6 +255,7 @@ extern long (*panic_blink)(int state);
> > __printf(1, 2)
> > void panic(const char *fmt, ...)
> > __noreturn __cold;
> > +void nmi_panic_self_stop(struct pt_regs *);
> > extern void oops_enter(void);
> > extern void oops_exit(void);
> > void print_oops_end_marker(void);
> > @@ -450,12 +451,19 @@ extern atomic_t panic_cpu;
> > /*
> > * A variant of panic() called from NMI context.
> > * If we've already panicked on this cpu, return from here.
> > + * If another cpu already panicked, loop in nmi_panic_self_stop() which
> > + * can provide architecture dependent code such as saving register states
> > + * for crash dump.
> > */
> > -#define nmi_panic(fmt, ...) \
> > +#define nmi_panic(regs, fmt, ...) \
> > do { \
> > + int old_cpu; \
> > int this_cpu = raw_smp_processor_id(); \
> > - if (atomic_cmpxchg(&panic_cpu, -1, this_cpu) != this_cpu) \
> > + old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu); \
> > + if (old_cpu == -1) \
> > panic(fmt, ##__VA_ARGS__); \
> > + else if (old_cpu != this_cpu) \
> > + nmi_panic_self_stop(regs); \
>
> Same here - this is assuming that broadcasting NMIs to all cores
> automatically means there's a crash dump in progress:
>
> nmi_panic_self_stop() -> poll_crash_ipi_and_callback()
>
> and this cannot be right.
>
> > } while (0)
> >
> > /*
> > diff --git a/kernel/panic.c b/kernel/panic.c
> > index 24ee2ea..4fce2be 100644
> > --- a/kernel/panic.c
> > +++ b/kernel/panic.c
> > @@ -61,6 +61,16 @@ void __weak panic_smp_self_stop(void)
> > cpu_relax();
> > }
> >
> > +/*
> > + * Stop ourself in NMI context if another cpu has already panicked.
>
> "ourselves"

Thanks. I'll fix it.

Regards,

--
Hidehiro Kawai
Hitachi, Ltd. Research & Development Group

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