Re: [V3 PATCH] x86: avoid calling arch_trigger_all_cpu_backtrace()at the same time on SMP

From: Don Zickus
Date: Thu Nov 11 2010 - 22:40:27 EST


On Fri, Nov 12, 2010 at 10:38:13AM +0800, DDD wrote:
> Dongdong Deng wrote:
> >The spin_lock_debug/rcu_cpu_stall detector uses
> >trigger_all_cpu_backtrace() to dump cpu backtrace.
> >Therefore it is possible that trigger_all_cpu_backtrace()
> >could be called at the same time on different CPUs, which
> >triggers and 'unknown reason NMI' warning. The following case
> >illustrates the problem:
> >
> > CPU1 CPU2 ... CPU N
> > trigger_all_cpu_backtrace()
> > set "backtrace_mask" to cpu mask
> > |
> >generate NMI interrupts generate NMI interrupts ...
> > \ | /
> > \ | /
> > The "backtrace_mask" will be cleaned by the first NMI interrupt
> > at nmi_watchdog_tick(), then the following NMI interrupts generated
> >by other cpus's arch_trigger_all_cpu_backtrace() will be took as
> >unknown reason NMI interrupts.
> >
> >This patch uses a lock to avoid the problem, and stop the
> >arch_trigger_all_cpu_backtrace() calling to avoid dumping double cpu
> >backtrace info when there is already a trigger_all_cpu_backtrace()
> >in progress.
> >
> >Signed-off-by: Dongdong Deng <dongdong.deng@xxxxxxxxxxxxx>
> >Reviewed-by: Bruce Ashfield <bruce.ashfield@xxxxxxxxxxxxx>
> >CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >CC: Ingo Molnar <mingo@xxxxxxxxxx>
> >CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
> >CC: x86@xxxxxxxxxx
> >CC: linux-kernel@xxxxxxxxxxxxxxx
> >Signed-off-by: Don Zickus <dzickus@xxxxxxxxxx>
> >---
> > arch/x86/kernel/apic/hw_nmi.c | 23 +++++++++++++++++++++++
> > arch/x86/kernel/apic/nmi.c | 23 +++++++++++++++++++++++
> > 2 files changed, 46 insertions(+), 0 deletions(-)
> >
> >diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
> >index cefd694..bfdab3b 100644
> >--- a/arch/x86/kernel/apic/hw_nmi.c
> >+++ b/arch/x86/kernel/apic/hw_nmi.c
> >@@ -26,9 +26,27 @@ u64 hw_nmi_get_sample_period(void)
> > }
> > #ifdef ARCH_HAS_NMI_WATCHDOG
> >+/* "in progress" flag of arch_trigger_all_cpu_backtrace */
> >+static unsigned long backtrace_flag;
> >+
> > void arch_trigger_all_cpu_backtrace(void)
> > {
> > int i;
> >+ unsigned long flags;
> >+
> >+ /*
> >+ * Have to disable irq here, as the
> >+ * arch_trigger_all_cpu_backtrace() could be
> >+ * triggered by "spin_lock()" with irqs on.
> >+ */
> >+ local_irq_save(flags);
> >+
> >+ if (test_and_set_bit(0, &backtrace_flag))
> >+ /*
> >+ * If there is already a trigger_all_cpu_backtrace() in progress
> >+ * (backtrace_flag == 1), don't output double cpu dump infos.
> >+ */
> >+ goto out_restore_irq;
> > cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
> >@@ -41,6 +59,11 @@ void arch_trigger_all_cpu_backtrace(void)
> > break;
> > mdelay(1);
> > }
> >+
> >+ clear_bit(0, &backtrace_flag);
>
> Hi Don,
>
> The clear_bit() on x86 (arch/x86/include/asm/bitops.h) does not
> contain a memory barrier, Could you add a memory barrier ops here
> when you pick up this patch?
>
> + clear_bit(0, &backtrace_flag);
> + smp_mb__after_clear_bit();
>
>

Ok. Thanks.

Cheers,
Don
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