Cache the results of the unwind to ensure the unwind is only performed
once, even when called by multiple tracers.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 include/linux/unwind_deferred_types.h |  8 +++++++-
 kernel/unwind/deferred.c              | 26 ++++++++++++++++++++------
 2 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h
index 9749824aea09..6f71a06329fb 100644
--- a/include/linux/unwind_deferred_types.h
+++ b/include/linux/unwind_deferred_types.h
@@ -2,8 +2,14 @@
 #ifndef _LINUX_UNWIND_USER_DEFERRED_TYPES_H
 #define _LINUX_UNWIND_USER_DEFERRED_TYPES_H
 
-struct unwind_task_info {
+struct unwind_cache {
 	unsigned long		*entries;
+	unsigned int		nr_entries;
+	u64			cookie;
+};
+
+struct unwind_task_info {
+	struct unwind_cache	cache;
 	u64			cookie;
 };
 
diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
index f0dbe4069247..2f38055cce48 100644
--- a/kernel/unwind/deferred.c
+++ b/kernel/unwind/deferred.c
@@ -56,6 +56,7 @@ static void unwind_deferred_task_work(struct callback_head *head)
 {
 	struct unwind_work *work = container_of(head, struct unwind_work, work);
 	struct unwind_task_info *info = &current->unwind_info;
+	struct unwind_cache *cache = &info->cache;
 	struct unwind_stacktrace trace;
 	u64 cookie;
 
@@ -73,17 +74,30 @@ static void unwind_deferred_task_work(struct callback_head *head)
 	if (!current->mm)
 		goto do_callback;
 
-	if (!info->entries) {
-		info->entries = kmalloc(UNWIND_MAX_ENTRIES * sizeof(long),
-					GFP_KERNEL);
-		if (!info->entries)
+	if (!cache->entries) {
+		cache->entries = kmalloc(UNWIND_MAX_ENTRIES * sizeof(long),
+					 GFP_KERNEL);
+		if (!cache->entries)
 			goto do_callback;
 	}
 
-	trace.entries = info->entries;
+	trace.entries = cache->entries;
+
+	if (cookie == cache->cookie) {
+		/*
+		 * The user stack has already been previously unwound in this
+		 * entry context.  Skip the unwind and use the cache.
+		 */
+		trace.nr = cache->nr_entries;
+		goto do_callback;
+	}
+
 	trace.nr = 0;
 	unwind_user(&trace, UNWIND_MAX_ENTRIES);
 
+	cache->cookie = cookie;
+	cache->nr_entries = trace.nr;
+
 do_callback:
 	work->func(work, &trace, cookie);
 	work->pending = 0;
@@ -174,5 +188,5 @@ void unwind_task_free(struct task_struct *task)
 {
 	struct unwind_task_info *info = &task->unwind_info;
 
-	kfree(info->entries);
+	kfree(info->cache.entries);
 }
-- 
2.48.1