[PATCH] ext4: make concurrent DEFAULT shutdown waits killable

[diasyzhangqing@gmail.com]

From: Qing Zhang <diasyzhang@tencent.com>

Concurrent DEFAULT shutdown callers can all pass the shutdown flag
check before the first caller finishes bdev_freeze().  They then wait
uninterruptibly on bd_fsfreeze_mutex and may perform redundant
freeze/thaw operations, triggering hung-task reports if the initial
freeze takes long enough.

Serialize DEFAULT shutdown attempts with a per-superblock mutex
acquired killably.  Re-check the shutdown flag after acquiring it to
skip the freeze after a successful attempt, or retry if the previous
attempt failed.

Leave LOGFLUSH and NOLOGFLUSH unchanged.  The ->shutdown callback uses
NOLOGFLUSH with s_umount held, so it must not acquire a mutex held
across a freeze.  This does not shorten the initial freeze or prevent
hung-task reports for the active freezer and other freeze-blocked
tasks.

Fixes: 783d94854499 ("ext4: add EXT4_IOC_GOINGDOWN ioctl")
Reported-by: syzbot+d4ecb9a1e5b3dcf8baec@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d4ecb9a1e5b3dcf8baec
Signed-off-by: Qing Zhang <diasyzhang@tencent.com>
---

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
 fs/ext4/ext4.h  |  1 +
 fs/ext4/ioctl.c | 14 +++++++++++++-
 fs/ext4/super.c |  1 +
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index f38fec3167fb..c4e44e3d58cc 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1647,6 +1647,7 @@ struct ext4_sb_info {
 	/* Journaling */
 	struct journal_s *s_journal;
 	unsigned long s_ext4_flags;		/* Ext4 superblock flags */
+	struct mutex s_shutdown_lock;	/* Serializes DEFAULT shutdown requests */
 	struct mutex s_orphan_lock;	/* Protects on disk list changes */
 	struct list_head s_orphan;	/* List of orphaned inodes in on disk
 					   list */
diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index e8e579498c99..3019b16320a0 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -818,16 +818,28 @@ int ext4_force_shutdown(struct super_block *sb, u32 flags)
 	if (ext4_forced_shutdown(sb))
 		return 0;
 
+	if (flags == EXT4_GOING_FLAGS_DEFAULT) {
+		if (mutex_lock_killable(&sbi->s_shutdown_lock))
+			return -EINTR;
+		if (ext4_forced_shutdown(sb)) {
+			mutex_unlock(&sbi->s_shutdown_lock);
+			return 0;
+		}
+	}
+
 	ext4_msg(sb, KERN_ALERT, "shut down requested (%d)", flags);
 	trace_ext4_shutdown(sb, flags);
 
 	switch (flags) {
 	case EXT4_GOING_FLAGS_DEFAULT:
 		ret = bdev_freeze(sb->s_bdev);
-		if (ret)
+		if (ret) {
+			mutex_unlock(&sbi->s_shutdown_lock);
 			return ret;
+		}
 		set_bit(EXT4_FLAGS_SHUTDOWN, &sbi->s_ext4_flags);
 		bdev_thaw(sb->s_bdev);
+		mutex_unlock(&sbi->s_shutdown_lock);
 		break;
 	case EXT4_GOING_FLAGS_LOGFLUSH:
 		/*
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 9cae52471dee..a1d4fc5c0a68 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -5547,6 +5547,7 @@ static int __ext4_fill_super(struct fs_context *fc, struct super_block *sb)
 
 	INIT_LIST_HEAD(&sbi->s_orphan); /* unlinked but open files */
 	mutex_init(&sbi->s_orphan_lock);
+	mutex_init(&sbi->s_shutdown_lock);
 
 	spin_lock_init(&sbi->s_bdev_wb_lock);
 
-- 
2.43.7