linux-kernel - [PATCH 2/3] x86: Avoid calling arch_trigger_all_cpu

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Date:	Tue,  4 Jan 2011 22:38:08 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	x86@...nel.org
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Dongdong Deng <dongdong.deng@...driver.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, fweisbec@...il.com,
	Don Zickus <dzickus@...hat.com>
Subject: [PATCH 2/3] x86: Avoid calling arch_trigger_all_cpu_backtrace() at the same time

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 taken as
unknown reason NMI interrupts.

This patch uses a test_and_set to avoid the problem, and stop the
arch_trigger_all_cpu_backtrace() from calling to avoid dumping a
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
CC: fweisbec@...il.com
Signed-off-by: Don Zickus <dzickus@...hat.com>
---
 arch/x86/kernel/apic/hw_nmi.c |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
index 2b40a60..72ec29e 100644
--- a/arch/x86/kernel/apic/hw_nmi.c
+++ b/arch/x86/kernel/apic/hw_nmi.c
@@ -28,10 +28,20 @@ u64 hw_nmi_get_sample_period(void)
 /* For reliability, we're prepared to waste bits here. */
 static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;
 
+/* "in progress" flag of arch_trigger_all_cpu_backtrace */
+static unsigned long backtrace_flag;
+
 void arch_trigger_all_cpu_backtrace(void)
 {
 	int i;
 
+	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.
+		 */
+		return;
+
 	cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
 
 	printk(KERN_INFO "sending NMI to all CPUs:\n");
@@ -43,6 +53,9 @@ void arch_trigger_all_cpu_backtrace(void)
 			break;
 		mdelay(1);
 	}
+
+	clear_bit(0, &backtrace_flag);
+	smp_mb__after_clear_bit();
 }
 
 static int __kprobes
-- 
1.7.3.4

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