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

Michal Hocko
2015-11-24T12:45:07Z

On Fri 20-11-15 18:36:44, 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>

I've finally seen testing results for these patches and managed to look
at them again.

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> 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");
> +
> + /*
> + * 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;
> +
> +/*
> + * 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.
> */
> - if (!spin_trylock(&panic_lock))
> + this_cpu = raw_smp_processor_id();
> + old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
> + if (old_cpu != -1 && old_cpu != this_cpu)
> panic_smp_self_stop();
>
> console_verbose();
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 18f34cf..b9be18f 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -351,7 +351,7 @@ static void watchdog_overflow_callback(struct perf_event *event,
> trigger_allbutself_cpu_backtrace();
>
> if (hardlockup_panic)
> - panic("Hard LOCKUP");
> + nmi_panic("Hard LOCKUP");
>
> __this_cpu_write(hard_watchdog_warn, true);
> return;
>

--
Michal Hocko
SUSE Labs