Re: [V6.1 PATCH 5/6] x86/nmi: Fix to save registers for crash dump on external NMI broadcast

Borislav Petkov
2015-12-11T18:04:09Z

On Thu, Dec 10, 2015 at 03:52:46PM +0900, Hidehiro Kawai wrote:
> Now, multiple CPUs can receive external NMI simultaneously by
> specifying "apic_extnmi=all" as an boot option. When we take a
> crash dump by using external NMI with this option, we fail to save
> register values into the crash dump. This happens as follows:
>
> CPU 0 CPU 1
> ================================ =============================
> receive an external NMI
> default_do_nmi() receive an external NMI
> spin_lock(&nmi_reason_lock) default_do_nmi()
> io_check_error() spin_lock(&nmi_reason_lock)
> panic() busy loop
> ...
> kdump_nmi_shootdown_cpus()
> issue NMI IPI -----------> blocked until IRET
> busy loop...
>
> Here, since CPU 1 is in NMI context, additional NMI from CPU 0
> is blocked until CPU 1 executes IRET. However, CPU 1 never
> executes IRET, so the NMI is not handled and the callback function
> to save registers is never called.
>
> To solve this issue, we check if the IPI for crash dumping was
> issued while waiting for nmi_reason_lock to be released, and if so,
> call its callback function directly. If the IPI is not issued (e.g.
> kdump is disabled), the actual behavior doesn't change.
>
> V6.1:
> - Reintroduce the UP version of run_crash_ipi_callback to fix build
> error for CONFIG_SMP=n and CONFIG_DEBUG_SPINLOCK=y case
>
> V6:
> - Separated from the former patch `panic/x86: Allow cpus to save
> registers even if they are looping in NMI context'
> - Fix comments
> - Remove unneeded UP version of poll_crash_ipi_and_calback
> - Rename poll_crash_ipi_and_callback to run_crash_ipi_callback
>
> 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/include/asm/reboot.h | 1 +
> arch/x86/kernel/nmi.c | 11 ++++++++++-
> arch/x86/kernel/reboot.c | 22 +++++++++++++++++-----
> 3 files changed, 28 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/reboot.h b/arch/x86/include/asm/reboot.h
> index a82c4f1..2cb1cc2 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 run_crash_ipi_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 5e00de7..cbfa0b5 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>
> @@ -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 while holding
> + * nmi_reason_lock. Check if the CPU issued the IPI for crash
> + * dumping, and if so, call its callback directly. If there is
> + * no CPU preparing crash dump, we simply loop here without doing
> + * special things.
> + */
> + while (!raw_spin_trylock(&nmi_reason_lock))
> + run_crash_ipi_callback(regs);

Added cpu_relax() here too:

+ while (!raw_spin_trylock(&nmi_reason_lock)) {
+ run_crash_ipi_callback(regs);
+ cpu_relax();
+ }

--
Regards/Gruss,
Boris.

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