[PATCH v4] f2fs: fix to avoid race during access gc_thread pointer

[Chao Yu]

Thread A			Thread B		Thread C
- f2fs_remount
 - stop_gc_thread
				- f2fs_sbi_store
							- issue_discard_thread
   sbi->gc_thread = NULL;
				  sbi->gc_thread->gc_wake = 1
							  access sbi->gc_thread->gc_urgent

Previously, we allocate memory for sbi->gc_thread based on background
gc thread mount option, the memory can be released if we turn off
that mount option, but still there are several places access gc_thread
pointer without considering race condition, result in NULL point
dereference.

In order to fix this issue, introduce gc_rwsem to exclude those operations.

Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
---
v4:
- use introduced sbi.gc_rwsem lock instead of sb.s_umount.
 fs/f2fs/f2fs.h    |  1 +
 fs/f2fs/gc.c      |  4 ++++
 fs/f2fs/segment.c | 11 ++++++++++-
 fs/f2fs/super.c   | 19 ++++++++++++++-----
 fs/f2fs/sysfs.c   | 14 +++++++++++---
 5 files changed, 40 insertions(+), 9 deletions(-)

diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 80490a7991a7..e238d0ea0be7 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -1191,6 +1191,7 @@ struct f2fs_sb_info {
 
 	/* for cleaning operations */
 	struct mutex gc_mutex;			/* mutex for GC */
+	struct rw_semaphore gc_rwsem;		/* rw semaphore for gc_thread */
 	struct f2fs_gc_kthread	*gc_thread;	/* GC thread */
 	unsigned int cur_victim_sec;		/* current victim section num */
 
diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
index 9bb2ddbbed1e..b74714be7be7 100644
--- a/fs/f2fs/gc.c
+++ b/fs/f2fs/gc.c
@@ -187,17 +187,21 @@ static void select_policy(struct f2fs_sb_info *sbi, int gc_type,
 		p->max_search = dirty_i->nr_dirty[type];
 		p->ofs_unit = 1;
 	} else {
+		down_read(&sbi->gc_rwsem);
 		p->gc_mode = select_gc_type(sbi->gc_thread, gc_type);
+		up_read(&sbi->gc_rwsem);
 		p->dirty_segmap = dirty_i->dirty_segmap[DIRTY];
 		p->max_search = dirty_i->nr_dirty[DIRTY];
 		p->ofs_unit = sbi->segs_per_sec;
 	}
 
 	/* we need to check every dirty segments in the FG_GC case */
+	down_read(&sbi->gc_rwsem);
 	if (gc_type != FG_GC &&
 			(sbi->gc_thread && !sbi->gc_thread->gc_urgent) &&
 			p->max_search > sbi->max_victim_search)
 		p->max_search = sbi->max_victim_search;
+	up_read(&sbi->gc_rwsem);
 
 	/* let's select beginning hot/small space first in no_heap mode*/
 	if (test_opt(sbi, NOHEAP) &&
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 320cc1c57246..33d146939048 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -174,11 +174,18 @@ bool need_SSR(struct f2fs_sb_info *sbi)
 	int node_secs = get_blocktype_secs(sbi, F2FS_DIRTY_NODES);
 	int dent_secs = get_blocktype_secs(sbi, F2FS_DIRTY_DENTS);
 	int imeta_secs = get_blocktype_secs(sbi, F2FS_DIRTY_IMETA);
+	bool gc_urgent = false;
 
 	if (test_opt(sbi, LFS))
 		return false;
+
+	down_read(&sbi->gc_rwsem);
 	if (sbi->gc_thread && sbi->gc_thread->gc_urgent)
-		return true;
+		gc_urgent = true;
+	up_read(&sbi->gc_rwsem);
+
+	if (gc_urgent)
+		return false;
 
 	return free_sections(sbi) <= (node_secs + 2 * dent_secs + imeta_secs +
 			SM_I(sbi)->min_ssr_sections + reserved_sections(sbi));
@@ -1421,8 +1428,10 @@ static int issue_discard_thread(void *data)
 		if (dcc->discard_wake)
 			dcc->discard_wake = 0;
 
+		down_read(&sbi->gc_rwsem);
 		if (sbi->gc_thread && sbi->gc_thread->gc_urgent)
 			init_discard_policy(&dpolicy, DPOLICY_FORCE, 1);
+		up_read(&sbi->gc_rwsem);
 
 		sb_start_intwrite(sbi->sb);
 
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index c8e5fe5d71fe..294be9e92aee 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -1476,15 +1476,23 @@ static int f2fs_remount(struct super_block *sb, int *flags, char *data)
 	 * option. Also sync the filesystem.
 	 */
 	if ((*flags & SB_RDONLY) || !test_opt(sbi, BG_GC)) {
+		down_write(&sbi->gc_rwsem);
 		if (sbi->gc_thread) {
 			stop_gc_thread(sbi);
 			need_restart_gc = true;
 		}
-	} else if (!sbi->gc_thread) {
-		err = start_gc_thread(sbi);
-		if (err)
-			goto restore_opts;
-		need_stop_gc = true;
+		up_write(&sbi->gc_rwsem);
+	} else {
+		down_write(&sbi->gc_rwsem);
+		if (!sbi->gc_thread) {
+			err = start_gc_thread(sbi);
+			if (err) {
+				up_write(&sbi->gc_rwsem);
+				goto restore_opts;
+			}
+			need_stop_gc = true;
+		}
+		up_write(&sbi->gc_rwsem);
 	}
 
 	if (*flags & SB_RDONLY ||
@@ -2802,6 +2810,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 	}
 
 	init_rwsem(&sbi->cp_rwsem);
+	init_rwsem(&sbi->gc_rwsem);
 	init_waitqueue_head(&sbi->cp_wait);
 	init_sb_info(sbi);
 
diff --git a/fs/f2fs/sysfs.c b/fs/f2fs/sysfs.c
index 6d8d8f41e517..34c542dbc459 100644
--- a/fs/f2fs/sysfs.c
+++ b/fs/f2fs/sysfs.c
@@ -173,6 +173,7 @@ static ssize_t f2fs_sbi_store(struct f2fs_attr *a,
 	unsigned long t;
 	unsigned int *ui;
 	ssize_t ret;
+	bool gc_entry = (a->struct_type == GC_THREAD);
 
 	ptr = __struct_ptr(sbi, a->struct_type);
 	if (!ptr)
@@ -248,16 +249,23 @@ static ssize_t f2fs_sbi_store(struct f2fs_attr *a,
 	if (!strcmp(a->attr.name, "trim_sections"))
 		return -EINVAL;
 
-	*ui = t;
-
-	if (!strcmp(a->attr.name, "iostat_enable") && *ui == 0)
+	if (!strcmp(a->attr.name, "iostat_enable") && t == 0)
 		f2fs_reset_iostat(sbi);
+
+	if (gc_entry)
+		down_read(&sbi->gc_rwsem);
+
 	if (!strcmp(a->attr.name, "gc_urgent") && t == 1 && sbi->gc_thread) {
 		sbi->gc_thread->gc_wake = 1;
 		wake_up_interruptible_all(&sbi->gc_thread->gc_wait_queue_head);
 		wake_up_discard_thread(sbi, true);
 	}
 
+	*ui = t;
+
+	if (gc_entry)
+		up_read(&sbi->gc_rwsem);
+
 	return count;
 }
 
-- 
2.17.0.391.g1f1cddd558b5