Re: [syzbot] Re: [syzbot] [f2fs?] KASAN: null-ptr-deref Write in f2fs_stop_gc_thread

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Subject: Re: [syzbot] [f2fs?] KASAN: null-ptr-deref Write in f2fs_stop_gc_thread
Author: lizhi.xu@windriver.com

before thread stop and free gc_thread, set sbi->gc_thread to NULL, 
and add lock for reentry f2fs_stop_gc_thread.

#syz test: upstream 2c9b3512402e

diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
index 6066c6eecf41..462f8957ea9a 100644
--- a/fs/f2fs/gc.c
+++ b/fs/f2fs/gc.c
@@ -203,14 +203,20 @@ int f2fs_start_gc_thread(struct f2fs_sb_info *sbi)
 
 void f2fs_stop_gc_thread(struct f2fs_sb_info *sbi)
 {
-	struct f2fs_gc_kthread *gc_th = sbi->gc_thread;
+	struct f2fs_gc_kthread *gc_th;
+	static DEFINE_MUTEX(gct_mutex);
 
+	mutex_lock(&gct_mutex);
+	gc_th = sbi->gc_thread;
 	if (!gc_th)
-		return;
+		goto unlock;
+
+	sbi->gc_thread = NULL;
 	kthread_stop(gc_th->f2fs_gc_task);
 	wake_up_all(&gc_th->fggc_wq);
 	kfree(gc_th);
-	sbi->gc_thread = NULL;
+unlock:
+	mutex_unlock(&gct_mutex);
 }
 
 static int select_gc_type(struct f2fs_sb_info *sbi, int gc_type)