[PATCH 2/3] stacktrace: add save_stack_trace_tsk_safe()

[Josh Poimboeuf]

It isn't possible to get the stack of a running task (other than
current) because we don't have the stack pointer and the stack can be
inconsistent anyway.  Add a safe stack saving facility which only saves
the stack of the task if it's sleeping or if it's the current task.

Suggested-by: Jiri Kosina <jkosina@xxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 include/linux/stacktrace.h |  2 ++
 kernel/stacktrace.c        | 22 ++++++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/include/linux/stacktrace.h b/include/linux/stacktrace.h
index 669045a..898f0fc 100644
--- a/include/linux/stacktrace.h
+++ b/include/linux/stacktrace.h
@@ -20,6 +20,8 @@ extern void save_stack_trace_regs(struct pt_regs *regs,
 				  struct stack_trace *trace);
 extern void save_stack_trace_tsk(struct task_struct *tsk,
 				struct stack_trace *trace);
+extern void save_stack_trace_tsk_safe(struct task_struct *tsk,
+				      struct stack_trace *trace);
 
 extern void print_stack_trace(struct stack_trace *trace, int spaces);
 extern int snprint_stack_trace(char *buf, size_t size,
diff --git a/kernel/stacktrace.c b/kernel/stacktrace.c
index b6e4c16..49cdd7f 100644
--- a/kernel/stacktrace.c
+++ b/kernel/stacktrace.c
@@ -57,6 +57,28 @@ int snprint_stack_trace(char *buf, size_t size,
 }
 EXPORT_SYMBOL_GPL(snprint_stack_trace);
 
+static void __save_stack_trace_tsk_safe(struct task_struct *tsk,
+					void *data)
+{
+	struct stack_trace *trace = data;
+
+	if (tsk != current && tsk->on_cpu)
+		return;
+
+	save_stack_trace_tsk(tsk, trace);
+}
+
+/*
+ * If the task is sleeping, safely get its stack.  Otherwise, do nothing, since
+ * the stack of a running task is undefined.
+ */
+void save_stack_trace_tsk_safe(struct task_struct *tsk,
+			       struct stack_trace *trace)
+{
+	sched_task_call(__save_stack_trace_tsk_safe, tsk, trace);
+}
+EXPORT_SYMBOL_GPL(save_stack_trace_tsk_safe);
+
 /*
  * Architectures that do not implement save_stack_trace_tsk or
  * save_stack_trace_regs get this weak alias and a once-per-bootup warning
-- 
2.1.0

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