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Message-ID: <1431603304-162571-17-git-send-email-long.wanglong@huawei.com>
Date: Thu, 14 May 2015 11:35:03 +0000
From: Wang Long <long.wanglong@...wei.com>
To: <rostedt@...dmis.org>, <jkosina@...e.cz>,
<gregkh@...uxfoundation.org>
CC: <stable@...r.kernel.org>, <wanglong@...qinren.net>,
<peifeiyue@...wei.com>, <linux-kernel@...r.kernel.org>,
<morgan.wang@...wei.com>, <paulmck@...ux.vnet.ibm.com>,
<pmladek@...e.cz>, <dzickus@...hat.com>, <x86@...nel.org>,
<sasha.levin@...cle.com>
Subject: [RFC PATCH 16/17] x86/nmi: Perform a safe NMI stack trace on all CPUs
From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
commit a9edc88093287183ac934be44f295f183b2c62dd upstream.
When trigger_all_cpu_backtrace() is called on x86, it will trigger an
NMI on each CPU and call show_regs(). But this can lead to a hard lock
up if the NMI comes in on another printk().
In order to avoid this, when the NMI triggers, it switches the printk
routine for that CPU to call a NMI safe printk function that records the
printk in a per_cpu seq_buf descriptor. After all NMIs have finished
recording its data, the seq_bufs are printed in a safe context.
Link: http://lkml.kernel.org/p/20140619213952.360076309@goodmis.org
Link: http://lkml.kernel.org/r/20141115050605.055232587@goodmis.org
Tested-by: Jiri Kosina <jkosina@...e.cz>
Acked-by: Jiri Kosina <jkosina@...e.cz>
Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Reviewed-by: Petr Mladek <pmladek@...e.cz>
[wanglong: backport to 3.10 stable
- adjust context
]
Signed-off-by: Wang Long <long.wanglong@...wei.com>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
---
arch/x86/kernel/apic/hw_nmi.c | 86 +++++++++++++++++++++++++++++++++++++++++--
1 file changed, 83 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
index a698d71..1eb5f90 100644
--- a/arch/x86/kernel/apic/hw_nmi.c
+++ b/arch/x86/kernel/apic/hw_nmi.c
@@ -18,6 +18,7 @@
#include <linux/nmi.h>
#include <linux/module.h>
#include <linux/delay.h>
+#include <linux/seq_buf.h>
#ifdef CONFIG_HARDLOCKUP_DETECTOR
u64 hw_nmi_get_sample_period(int watchdog_thresh)
@@ -29,12 +30,33 @@ u64 hw_nmi_get_sample_period(int watchdog_thresh)
#ifdef arch_trigger_all_cpu_backtrace
/* For reliability, we're prepared to waste bits here. */
static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;
+static cpumask_var_t printtrace_mask;
+
+#define NMI_BUF_SIZE 4096
+
+struct nmi_seq_buf {
+ unsigned char buffer[NMI_BUF_SIZE];
+ struct seq_buf seq;
+};
+
+/* Safe printing in NMI context */
+static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
/* "in progress" flag of arch_trigger_all_cpu_backtrace */
static unsigned long backtrace_flag;
+static void print_seq_line(struct nmi_seq_buf *s, int start, int end)
+{
+ const char *buf = s->buffer + start;
+
+ printk("%.*s", (end - start) + 1, buf);
+}
+
void arch_trigger_all_cpu_backtrace(void)
{
+ struct nmi_seq_buf *s;
+ int len;
+ int cpu;
int i;
if (test_and_set_bit(0, &backtrace_flag))
@@ -45,6 +67,15 @@ void arch_trigger_all_cpu_backtrace(void)
return;
cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
+ cpumask_copy(printtrace_mask, to_cpumask(backtrace_mask));
+ /*
+ * Set up per_cpu seq_buf buffers that the NMIs running on the other
+ * CPUs will write to.
+ */
+ for_each_cpu(cpu, to_cpumask(backtrace_mask)) {
+ s = &per_cpu(nmi_print_seq, cpu);
+ seq_buf_init(&s->seq, s->buffer, NMI_BUF_SIZE);
+ }
printk(KERN_INFO "sending NMI to all CPUs:\n");
apic->send_IPI_all(NMI_VECTOR);
@@ -56,10 +87,57 @@ void arch_trigger_all_cpu_backtrace(void)
mdelay(1);
}
+ /*
+ * Now that all the NMIs have triggered, we can dump out their
+ * back traces safely to the console.
+ */
+ for_each_cpu(cpu, printtrace_mask) {
+ int last_i = 0;
+
+ s = &per_cpu(nmi_print_seq, cpu);
+ len = seq_buf_used(&s->seq);
+ if (!len)
+ continue;
+
+ /* Print line by line. */
+ for (i = 0; i < len; i++) {
+ if (s->buffer[i] == '\n') {
+ print_seq_line(s, last_i, i);
+ last_i = i + 1;
+ }
+ }
+ /* Check if there was a partial line. */
+ if (last_i < len) {
+ print_seq_line(s, last_i, len - 1);
+ pr_cont("\n");
+ }
+ }
+
clear_bit(0, &backtrace_flag);
smp_mb__after_clear_bit();
}
+/*
+ * It is not safe to call printk() directly from NMI handlers.
+ * It may be fine if the NMI detected a lock up and we have no choice
+ * but to do so, but doing a NMI on all other CPUs to get a back trace
+ * can be done with a sysrq-l. We don't want that to lock up, which
+ * can happen if the NMI interrupts a printk in progress.
+ *
+ * Instead, we redirect the vprintk() to this nmi_vprintk() that writes
+ * the content into a per cpu seq_buf buffer. Then when the NMIs are
+ * all done, we can safely dump the contents of the seq_buf to a printk()
+ * from a non NMI context.
+ */
+static int nmi_vprintk(const char *fmt, va_list args)
+{
+ struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
+ unsigned int len = seq_buf_used(&s->seq);
+
+ seq_buf_vprintf(&s->seq, fmt, args);
+ return seq_buf_used(&s->seq) - len;
+}
+
static int __kprobes
arch_trigger_all_cpu_backtrace_handler(unsigned int cmd, struct pt_regs *regs)
{
@@ -68,12 +146,14 @@ arch_trigger_all_cpu_backtrace_handler(unsigned int cmd, struct pt_regs *regs)
cpu = smp_processor_id();
if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
- static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED;
+ printk_func_t printk_func_save = this_cpu_read(printk_func);
- arch_spin_lock(&lock);
+ /* Replace printk to write into the NMI seq */
+ this_cpu_write(printk_func, nmi_vprintk);
printk(KERN_WARNING "NMI backtrace for cpu %d\n", cpu);
show_regs(regs);
- arch_spin_unlock(&lock);
+ this_cpu_write(printk_func, printk_func_save);
+
cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
return NMI_HANDLED;
}
--
1.8.3.4
--
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