[PATCH 3/3] livepatch: Cleanup message handling in klp_try_switch_task()

[Petr Mladek <pmladek@suse.com>]

WARN_ON_ONCE() could not be called safely under rq lock because
of console deadlock issues. Fortunately, simple printk_deferred()
is enough because the warning is printed from a well defined
location and context.

Also klp_try_switch_task() is called under klp_mutex.
Therefore, the buffer for debug messages could be static.

Signed-off-by: Petr Mladek <pmladek@suse.com>
---
 kernel/livepatch/transition.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index 9c89ae8b337a..e8183d18227f 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -254,6 +254,7 @@ static int klp_check_stack_func(struct klp_func *func,
 static int klp_check_stack(struct task_struct *task, char *err_buf)
 {
 	static unsigned long entries[MAX_STACK_ENTRIES];
+	static int enosys_warned;
 	struct stack_trace trace;
 	struct klp_object *obj;
 	struct klp_func *func;
@@ -263,8 +264,16 @@ static int klp_check_stack(struct task_struct *task, char *err_buf)
 	trace.nr_entries = 0;
 	trace.max_entries = MAX_STACK_ENTRIES;
 	trace.entries = entries;
+
 	ret = save_stack_trace_tsk_reliable(task, &trace);
-	WARN_ON_ONCE(ret == -ENOSYS);
+	if (ret == -ENOSYS) {
+		if (!enosys_warned) {
+			printk_deferred(KERN_WARNING "%s: save_stack_trace_tsk_reliable() not supported on this architecture.\n",
+					__func__);
+			enosys_warned = 1;
+		}
+		return ret;
+	}
 	if (ret) {
 		snprintf(err_buf, STACK_ERR_BUF_SIZE,
 			 "%s: %s:%d has an unreliable stack\n",
@@ -297,11 +306,11 @@ static int klp_check_stack(struct task_struct *task, char *err_buf)
  */
 static bool klp_try_switch_task(struct task_struct *task)
 {
+	static char err_buf[STACK_ERR_BUF_SIZE];
 	struct rq *rq;
 	struct rq_flags flags;
 	int ret;
 	bool success = false;
-	char err_buf[STACK_ERR_BUF_SIZE];
 
 	err_buf[0] = '\0';
 
@@ -336,15 +345,15 @@ static bool klp_try_switch_task(struct task_struct *task)
 	task_rq_unlock(rq, task, &flags);
 
 	/*
-	 * Due to console deadlock issues, pr_debug() can't be used while
-	 * holding the task rq lock.  Instead we have to use a temporary buffer
+	 * printk_deferred() need to be used under rq lock to avoid
+	 * console deadlock issues.  But it does not support the dynamic
+	 * debug feature.  Instead we have to use a temporary buffer
 	 * and print the debug message after releasing the lock.
 	 */
 	if (err_buf[0] != '\0')
 		pr_debug("%s", err_buf);
 
 	return success;
-
 }
 
 /*
-- 
2.16.4