linux-kernel - Re: [PATCH] x86: avoid calling arch_trigger_all_cpu

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Date:	Wed, 10 Nov 2010 09:39:50 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	DDD <dongdong.deng@...driver.com>
Cc:	Don Zickus <dzickus@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: avoid calling arch_trigger_all_cpu_backtrace() at
 the same time on SMP


* DDD <dongdong.deng@...driver.com> wrote:

> Ingo Molnar wrote:
> >* Don Zickus <dzickus@...hat.com> wrote:
> >
> >>From: Dongdong Deng <dongdong.deng@...driver.com>
> >>
> >>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@...driver.com>
> >>Reviewed-by: Bruce Ashfield <bruce.ashfield@...driver.com>
> >>CC: Thomas Gleixner <tglx@...utronix.de>
> >>CC: Ingo Molnar <mingo@...hat.com>
> >>CC: "H. Peter Anvin" <hpa@...or.com>
> >>CC: x86@...nel.org
> >>CC: linux-kernel@...r.kernel.org
> >>Signed-off-by: Don Zickus <dzickus@...hat.com>
> >>---
> >> arch/x86/kernel/apic/hw_nmi.c |   14 ++++++++++++++
> >> arch/x86/kernel/apic/nmi.c    |   14 ++++++++++++++
> >> 2 files changed, 28 insertions(+), 0 deletions(-)
> >>
> >>diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
> >>index cefd694..3aea0a5 100644
> >>--- a/arch/x86/kernel/apic/hw_nmi.c
> >>+++ b/arch/x86/kernel/apic/hw_nmi.c
> >>@@ -29,6 +29,16 @@ u64 hw_nmi_get_sample_period(void)
> >> void arch_trigger_all_cpu_backtrace(void)
> >> {
> >> 	int i;
> >>+	static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED;
> >
> >Please dont put statics into the middle of local variables - put
> >them into file scope in a visible way.
> 
> Got it. will change it.
> 
> >
> >>+	unsigned long flags;
> >>+
> >>+	local_irq_save(flags);
> >>+	if (!arch_spin_trylock(&lock))
> >>+		/*
> >>+		 * If there is already a trigger_all_cpu_backtrace()
> >>+		 * in progress, don't output double cpu dump infos.
> >>+		 */
> >>+		goto out_restore_irq;
> >> 	cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
> >>@@ -41,6 +51,10 @@ void arch_trigger_all_cpu_backtrace(void)
> >> 			break;
> >> 		mdelay(1);
> >> 	}
> >>+
> >>+	arch_spin_unlock(&lock);
> >>+out_restore_irq:
> >>+	local_irq_restore(flags);
> >> }
> >> static int __kprobes
> >>diff --git a/arch/x86/kernel/apic/nmi.c b/arch/x86/kernel/apic/nmi.c
> >>index a43f71c..5fa8a13 100644
> >>--- a/arch/x86/kernel/apic/nmi.c
> >>+++ b/arch/x86/kernel/apic/nmi.c
> >>@@ -552,6 +552,16 @@ int do_nmi_callback(struct pt_regs *regs, int cpu)
> >> void arch_trigger_all_cpu_backtrace(void)
> >> {
> >> 	int i;
> >>+	static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED;
> >>+	unsigned long flags;
> >>+
> >>+	local_irq_save(flags);
> >>+	if (!arch_spin_trylock(&lock))
> >>+		/*
> >>+		 * If there is already a trigger_all_cpu_backtrace()
> >>+		 * in progress, don't output double cpu dump infos.
> >>+		 */
> >>+		goto out_restore_irq;
> >> 	cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
> >>@@ -564,4 +574,8 @@ void arch_trigger_all_cpu_backtrace(void)
> >> 			break;
> >> 		mdelay(1);
> >> 	}
> >>+
> >>+	arch_spin_unlock(&lock);
> >>+out_restore_irq:
> >>+	local_irq_restore(flags);
> >
> >This spinlock is never actually used as a spinlock - it's a "in
> >progress" flag. Why not use a flag and test_and_set_bit()?
> 
> Yep, it's a "in progress" flag, I will change to use a flag to
> replace the spinlock.

Thanks.

> >Also, the irq disabling really needed, will this code ever be called with irqs on?
> 
> This code could be called with irqs on.
> 
> If the arch_trigger_all_cpu_backtrace() was triggered by
> "spin_lock()"'s spin_lock_debug detector, it is possible that
> the irq is enabled, thus we have to save and disable it here.

Ok - please document that fact in the code.

Thanks,

	Ingo
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