[PATCH 3.19-rc2 v13 5/5] ARM: Fix on-demand backtrace triggered by IRQ

[Daniel Thompson]

Currently if arch_trigger_all_cpu_backtrace() is called with interrupts
disabled and on a platform the delivers IPI_CPU_BACKTRACE using regular
IRQ requests the system will wedge for ten seconds waiting for the
current CPU to react to a masked interrupt.

This patch resolves this issue by calling directly into the backtrace
dump code instead of generating an IPI.

Signed-off-by: Daniel Thompson <daniel.thompson at linaro.org>
Cc: Steven Rostedt <rostedt at goodmis.org>
---
 arch/arm/kernel/smp.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index 12667eb68198..644f654f7a7e 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -767,7 +767,10 @@ void ipi_cpu_backtrace(struct pt_regs *regs)
 		/* 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);
+		if (regs != NULL)
+			show_regs(regs);
+		else
+			dump_stack();
 		this_cpu_write(printk_func, printk_func_save);
 
 		cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
@@ -812,6 +815,16 @@ void arch_trigger_all_cpu_backtrace(bool include_self)
 		seq_buf_init(&s->seq, s->buffer, NMI_BUF_SIZE);
 	}
 
+	/*
+	 * If irqs are disabled on the current processor then, if
+	 * IPI_CPU_BACKTRACE is delivered using IRQ, we will won't be able to
+	 * react to IPI_CPU_BACKTRACE until we leave this function. We avoid
+	 * the potential timeout (not to mention the failure to print useful
+	 * information) by calling the backtrace directly.
+	 */
+	if (include_self && irqs_disabled())
+		ipi_cpu_backtrace(in_interrupt() ? get_irq_regs() : NULL);
+
 	if (!cpumask_empty(to_cpumask(backtrace_mask))) {
 		pr_info("Sending NMI to %s CPUs:\n",
 			(include_self ? "all" : "other"));
-- 
1.9.3