RE: [V5 PATCH 1/4] panic/x86: Fix re-entrance problem due to panic on NMI

河合英宏 / KAWAI，HIDEHIRO
2015-11-24T04:06:13Z

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Hi,

> On Fri, Nov 20, 2015 at 06:36:44PM +0900, Hidehiro Kawai wrote:
> > If panic on NMI happens just after panic() on the same CPU, panic()
> > is recursively called. As the result, it stalls after failing to
> > acquire panic_lock.
> >
> > To avoid this problem, don't call panic() in NMI context if
> > we've already entered panic().
> >
> > V4:
> > - Improve comments in io_check_error() and panic()
> >
> > V3:
> > - Introduce nmi_panic() macro to reduce code duplication
> > - In the case of panic on NMI, don't return from NMI handlers
> > if another cpu already panicked
> >
> > V2:
> > - Use atomic_cmpxchg() instead of current spin_trylock() to
> > exclude concurrent accesses to the panic routines
> > - Don't introduce no-lock version of panic()
> >
> > 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: Michal Hocko <mhocko@kernel.org>
> > ---
> > arch/x86/kernel/nmi.c | 16 ++++++++++++----
> > include/linux/kernel.h | 13 +++++++++++++
> > kernel/panic.c | 15 ++++++++++++---
> > kernel/watchdog.c | 2 +-
> > 4 files changed, 38 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
> > index 697f90d..5131714 100644
> > --- a/arch/x86/kernel/nmi.c
> > +++ b/arch/x86/kernel/nmi.c
> > @@ -231,7 +231,7 @@ pci_serr_error(unsigned char reason, struct pt_regs *regs)
> > #endif
> >
> > if (panic_on_unrecovered_nmi)
> > - panic("NMI: Not continuing");
> > + nmi_panic("NMI: Not continuing");
> >
> > pr_emerg("Dazed and confused, but trying to continue\n");
> >
> > @@ -255,8 +255,16 @@ io_check_error(unsigned char reason, struct pt_regs *regs)
> > reason, smp_processor_id());
> > show_regs(regs);
> >
> > - if (panic_on_io_nmi)
> > - panic("NMI IOCK error: Not continuing");
> > + if (panic_on_io_nmi) {
> > + nmi_panic("NMI IOCK error: Not continuing");
>
> Btw, that panic_on_io_nmi seems undocumented in
> Documentation/sysctl/kernel.txt. Care to document it, please, as a
> separate patch?

Sure. I'll post a documentation patch for it in a separate patch.
Because panic_on_io_nmi has been available since relatively old days,
I didn't check it.

> > +
> > + /*
> > + * If we return from nmi_panic(), it means we have received
> > + * NMI while processing panic(). So, simply return without
> > + * a delay and re-enabling NMI.
> > + */
> > + return;
> > + }
> >
> > /* Re-enable the IOCK line, wait for a few seconds */
> > reason = (reason & NMI_REASON_CLEAR_MASK) | NMI_REASON_CLEAR_IOCHK;
> > @@ -297,7 +305,7 @@ 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)
> > - panic("NMI: Not continuing");
> > + nmi_panic("NMI: Not continuing");
> >
> > pr_emerg("Dazed and confused, but trying to continue\n");
> > }
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index 350dfb0..480a4fd 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -445,6 +445,19 @@ extern int sysctl_panic_on_stackoverflow;
> >
> > extern bool crash_kexec_post_notifiers;
> >
> > +extern atomic_t panic_cpu;
>
> This needs a comment explaining what this variable is and what it
> denotes.

OK, I'll do that in the next version.

>
> > +
> > +/*
> > + * A variant of panic() called from NMI context.
> > + * If we've already panicked on this cpu, return from here.
> > + */
> > +#define nmi_panic(fmt, ...) \
> > + do { \
> > + int this_cpu = raw_smp_processor_id(); \
> > + if (atomic_cmpxchg(&panic_cpu, -1, this_cpu) != this_cpu) \
> > + panic(fmt, ##__VA_ARGS__); \
> > + } while (0)
> > +
> > /*
> > * Only to be used by arch init code. If the user over-wrote the default
> > * CONFIG_PANIC_TIMEOUT, honor it.
> > diff --git a/kernel/panic.c b/kernel/panic.c
> > index 4579dbb..24ee2ea 100644
> > --- a/kernel/panic.c
> > +++ b/kernel/panic.c
> > @@ -61,6 +61,8 @@ void __weak panic_smp_self_stop(void)
> > cpu_relax();
> > }
> >
> > +atomic_t panic_cpu = ATOMIC_INIT(-1);
> > +
> > /**
> > * panic - halt the system
> > * @fmt: The text string to print
> > @@ -71,17 +73,17 @@ void __weak panic_smp_self_stop(void)
> > */
> > void panic(const char *fmt, ...)
> > {
> > - static DEFINE_SPINLOCK(panic_lock);
> > static char buf[1024];
> > va_list args;
> > long i, i_next = 0;
> > int state = 0;
> > + int old_cpu, this_cpu;
> >
> > /*
> > * Disable local interrupts. This will prevent panic_smp_self_stop
> > * from deadlocking the first cpu that invokes the panic, since
> > * there is nothing to prevent an interrupt handler (that runs
> > - * after the panic_lock is acquired) from invoking panic again.
> > + * after setting panic_cpu) from invoking panic again.
> > */
> > local_irq_disable();
> >
> > @@ -94,8 +96,15 @@ void panic(const char *fmt, ...)
> > * multiple parallel invocations of panic, all other CPUs either
> > * stop themself or will wait until they are stopped by the 1st CPU
> > * with smp_send_stop().
> > + *
> > + * `old_cpu == -1' means this is the 1st CPU which comes here, so
> > + * go ahead.
> > + * `old_cpu == this_cpu' means we came from nmi_panic() which sets
> > + * panic_cpu to this cpu. In this case, this is also the 1st CPU.
>
> I'd prefer that -1 to be
>
> #define INVALID_CPU_NUM -1
>
> or
>
> #define UNDEFINED_CPU_NUM -1
>
> or so instead of a naked number.

OK, I'll use a macro.

Regards,

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

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