[PATCH] arm64: break while loop if task had been rescheduled

[Tengfei Fan]

While printing a task's backtrace and this task isn't
current task, it is possible that task's fp and fp+8
have the same value, so cannot break the while loop.
This can break while loop if this task had been
rescheduled during print this task's backtrace.

Signed-off-by: Tengfei Fan <tengfeif@xxxxxxxxxxxxxx>
---
 arch/arm64/kernel/traps.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index 2975598..9df6e02 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -103,6 +103,9 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
 {
 	struct stackframe frame;
 	int skip = 0;
+	long cur_state = 0;
+	unsigned long cur_sp = 0;
+	unsigned long cur_fp = 0;
 
 	pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk);
 
@@ -127,6 +130,9 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
 		 */
 		frame.fp = thread_saved_fp(tsk);
 		frame.pc = thread_saved_pc(tsk);
+		cur_state = tsk->state;
+		cur_sp = thread_saved_sp(tsk);
+		cur_fp = frame.fp;
 	}
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 	frame.graph = 0;
@@ -134,6 +140,23 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
 
 	printk("Call trace:\n");
 	do {
+		if (tsk != current && (cur_state != tsk->state
+			/*
+			 * We would not be printing backtrace for the task
+			 * that has changed state from uninterruptible to
+			 * running before hitting the do-while loop but after
+			 * saving the current state. If task is in running
+			 * state before saving the state, then we may print
+			 * wrong call trace or end up in infinite while loop
+			 * if *(fp) and *(fp+8) are same. While the situation
+			 * will stop print when that task schedule out.
+			 */
+			|| cur_sp != thread_saved_sp(tsk)
+			|| cur_fp != thread_saved_fp(tsk))) {
+			printk("The task:%s had been rescheduled!\n",
+				tsk->comm);
+			break;
+		}
 		/* skip until specified stack frame */
 		if (!skip) {
 			dump_backtrace_entry(frame.pc);
-- 
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