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[PATCH 0/3] Shared NMI backtracing support for ARM/x86

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  [PATCH 0/3] Shared NMI backtracing support for ARM/x86 Russell King - ARM Linux <linux@arm.linux.org.uk> - 2015-07-15 22:40 +0200
    [PATCH 2/3] nmi: x86: convert to generic nmi handler Russell King <rmk+kernel@arm.linux.org.uk> - 2015-07-15 22:40 +0200
      Re: [PATCH 2/3] nmi: x86: convert to generic nmi handler Thomas Gleixner <tglx@linutronix.de> - 2015-07-16 13:10 +0200
    [PATCH 1/3] nmi: create generic NMI backtrace implementation Russell King <rmk+kernel@arm.linux.org.uk> - 2015-07-15 22:40 +0200
      Re: [PATCH 1/3] nmi: create generic NMI backtrace implementation Thomas Gleixner <tglx@linutronix.de> - 2015-07-16 13:10 +0200

#1185032 — [PATCH 0/3] Shared NMI backtracing support for ARM/x86

Back in September, I stumbled across a single CPU IRQs-off lockup of an
ARM SMP system, and decided to hack something together based on a much
older hacky implementation used with StrongARM CPUs from early 2000s.

This resulted in a copy of the x86 NMI backtrace code into ARM as it was
back then, and feedback indicated that it wasn't a good time to push
such an effort forward, as printk() in NMI context is dodgy.

Over time, the x86 code has had this problem addressed, and last week
I updated the patch which I've been carrying in my tree to move the
shared code out of arch/x86 into lib/ rather than duplicating it, and
switch the ARM implementation to use it.

Discussing this with Thomas Gliexner, he agreed to give it a test over
last weekend, and he has reported to me this evening "no explosion so
far".  Since then, I've made a change to add the NOKPROBE_SYMBOL() to
the generic handler as per the x86 original code.

I'm aware that there are other competing implementations out there -
Daniel has one based on my patch from September time, but I don't think
that goes far enough with code sharing.  I'm also partially aware of
an implementation from Petr too.

 arch/arm/include/asm/irq.h    |   5 ++
 arch/arm/kernel/smp.c         |  18 +++++
 arch/x86/kernel/apic/hw_nmi.c | 133 ++--------------------------------
 include/linux/nmi.h           |   6 ++
 lib/Makefile                  |   2 +-
 lib/nmi_backtrace.c           | 162 ++++++++++++++++++++++++++++++++++++++++++
 6 files changed, 196 insertions(+), 130 deletions(-)

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#1185034 — [PATCH 2/3] nmi: x86: convert to generic nmi handler

Convert x86 to use the generic nmi handler code which can be shared
between architectures.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
---
 arch/x86/kernel/apic/hw_nmi.c | 133 ++----------------------------------------
 1 file changed, 4 insertions(+), 129 deletions(-)

diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
index 6873ab925d00..045e424fb368 100644
--- a/arch/x86/kernel/apic/hw_nmi.c
+++ b/arch/x86/kernel/apic/hw_nmi.c
@@ -28,146 +28,21 @@ u64 hw_nmi_get_sample_period(int watchdog_thresh)
 #endif
 
 #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_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)
+static void nmi_raise_cpu_backtrace(cpumask_t *mask)
 {
-	const char *buf = s->buffer + start;
-
-	printk("%.*s", (end - start) + 1, buf);
+	apic->send_IPI_mask(mask, NMI_VECTOR);
 }
 
 void arch_trigger_all_cpu_backtrace(bool include_self)
 {
-	struct nmi_seq_buf *s;
-	int len;
-	int cpu;
-	int i;
-	int this_cpu = get_cpu();
-
-	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.
-		 */
-		put_cpu();
-		return;
-	}
-
-	cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
-	if (!include_self)
-		cpumask_clear_cpu(this_cpu, to_cpumask(backtrace_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);
-	}
-
-	if (!cpumask_empty(to_cpumask(backtrace_mask))) {
-		pr_info("sending NMI to %s CPUs:\n",
-			(include_self ? "all" : "other"));
-		apic->send_IPI_mask(to_cpumask(backtrace_mask), NMI_VECTOR);
-	}
-
-	/* Wait for up to 10 seconds for all CPUs to do the backtrace */
-	for (i = 0; i < 10 * 1000; i++) {
-		if (cpumask_empty(to_cpumask(backtrace_mask)))
-			break;
-		mdelay(1);
-		touch_softlockup_watchdog();
-	}
-
-	/*
-	 * 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_atomic();
-	put_cpu();
-}
-
-/*
- * 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;
+	nmi_trigger_all_cpu_backtrace(include_self, nmi_raise_cpu_backtrace);
 }
 
 static int
 arch_trigger_all_cpu_backtrace_handler(unsigned int cmd, struct pt_regs *regs)
 {
-	int cpu;
-
-	cpu = smp_processor_id();
-
-	if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
-		printk_func_t printk_func_save = this_cpu_read(printk_func);
-
-		/* 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);
-		this_cpu_write(printk_func, printk_func_save);
-
-		cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
+	if (nmi_cpu_backtrace(regs))
 		return NMI_HANDLED;
-	}
 
 	return NMI_DONE;
 }
-- 
2.1.0

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#1185729 — Re: [PATCH 2/3] nmi: x86: convert to generic nmi handler

On Wed, 15 Jul 2015, Russell King wrote:

> Convert x86 to use the generic nmi handler code which can be shared
> between architectures.
> 
> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

Reviewed-and-tested-by: Thomas Gleixner <tglx@linutronix.de>
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#1185037 — [PATCH 1/3] nmi: create generic NMI backtrace implementation

x86s NMI backtrace implementation (for arch_trigger_all_cpu_backtrace())
is fairly generic in nature - the only architecture specific bits are
the act of raising the NMI to other CPUs, and reporting the status of
the NMI handler.

These are fairly simple to factor out, and produce a generic
implementation which can be shared between ARM and x86.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
---
 include/linux/nmi.h |   6 ++
 lib/Makefile        |   2 +-
 lib/nmi_backtrace.c | 162 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 169 insertions(+), 1 deletion(-)
 create mode 100644 lib/nmi_backtrace.c

diff --git a/include/linux/nmi.h b/include/linux/nmi.h
index f94da0e65dea..5791e3229068 100644
--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h
@@ -49,6 +49,12 @@ static inline bool trigger_allbutself_cpu_backtrace(void)
 	arch_trigger_all_cpu_backtrace(false);
 	return true;
 }
+
+/* generic implementation */
+void nmi_trigger_all_cpu_backtrace(bool include_self,
+				   void (*raise)(cpumask_t *mask));
+bool nmi_cpu_backtrace(struct pt_regs *regs);
+
 #else
 static inline bool trigger_all_cpu_backtrace(void)
 {
diff --git a/lib/Makefile b/lib/Makefile
index 6897b527581a..392169c5bc4e 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -13,7 +13,7 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
 	 sha1.o md5.o irq_regs.o argv_split.o \
 	 proportions.o flex_proportions.o ratelimit.o show_mem.o \
 	 is_single_threaded.o plist.o decompress.o kobject_uevent.o \
-	 earlycpio.o seq_buf.o
+	 earlycpio.o seq_buf.o nmi_backtrace.o
 
 obj-$(CONFIG_ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS) += usercopy.o
 lib-$(CONFIG_MMU) += ioremap.o
diff --git a/lib/nmi_backtrace.c b/lib/nmi_backtrace.c
new file mode 100644
index 000000000000..88d3d32e5923
--- /dev/null
+++ b/lib/nmi_backtrace.c
@@ -0,0 +1,162 @@
+/*
+ *  NMI backtrace support
+ *
+ * Gratuitously copied from arch/x86/kernel/apic/hw_nmi.c by Russell King,
+ * with the following header:
+ *
+ *  HW NMI watchdog support
+ *
+ *  started by Don Zickus, Copyright (C) 2010 Red Hat, Inc.
+ *
+ *  Arch specific calls to support NMI watchdog
+ *
+ *  Bits copied from original nmi.c file
+ */
+#include <linux/cpumask.h>
+#include <linux/delay.h>
+#include <linux/kprobes.h>
+#include <linux/nmi.h>
+#include <linux/seq_buf.h>
+
+#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_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 nmi_trigger_all_cpu_backtrace(bool include_self,
+				   void (*raise)(cpumask_t *mask))
+{
+	struct nmi_seq_buf *s;
+	int i, cpu, this_cpu = get_cpu();
+
+	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.
+		 */
+		put_cpu();
+		return;
+	}
+
+	cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
+	if (!include_self)
+		cpumask_clear_cpu(this_cpu, to_cpumask(backtrace_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);
+	}
+
+	if (!cpumask_empty(to_cpumask(backtrace_mask))) {
+		pr_info("Sending NMI to %s CPUs:\n",
+			(include_self ? "all" : "other"));
+		raise(to_cpumask(backtrace_mask));
+	}
+
+	/* Wait for up to 10 seconds for all CPUs to do the backtrace */
+	for (i = 0; i < 10 * 1000; i++) {
+		if (cpumask_empty(to_cpumask(backtrace_mask)))
+			break;
+		mdelay(1);
+		touch_softlockup_watchdog();
+	}
+
+	/*
+	 * 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 len, 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_atomic();
+	put_cpu();
+}
+
+/*
+ * 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;
+}
+
+bool nmi_cpu_backtrace(struct pt_regs *regs)
+{
+	int cpu = smp_processor_id();
+
+	if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
+		printk_func_t printk_func_save = this_cpu_read(printk_func);
+
+		/* Replace printk to write into the NMI seq */
+		this_cpu_write(printk_func, nmi_vprintk);
+		pr_warn("NMI backtrace for cpu %d\n", cpu);
+		show_regs(regs);
+		this_cpu_write(printk_func, printk_func_save);
+
+		cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
+		return true;
+	}
+
+	return false;
+}
+NOKPROBE_SYMBOL(nmi_cpu_backtrace);
+#endif
-- 
2.1.0

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#1185722 — Re: [PATCH 1/3] nmi: create generic NMI backtrace implementation

On Wed, 15 Jul 2015, Russell King wrote:

> x86s NMI backtrace implementation (for arch_trigger_all_cpu_backtrace())
> is fairly generic in nature - the only architecture specific bits are
> the act of raising the NMI to other CPUs, and reporting the status of
> the NMI handler.
> 
> These are fairly simple to factor out, and produce a generic
> implementation which can be shared between ARM and x86.
> 
> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
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