ext4: notify sysfs on errors_count value change

After s_error_count is incremented, signal the change in the
corresponding sysfs attribute via sysfs_notify. This allows userspace to
poll() on changes to /sys/fs/ext4/*/errors_count.

[ Moved call of ext4_notify_error_sysfs() to flush_stashed_error_work()
  to avoid BUG's caused by calling sysfs_notify trying to sleep after
  being called from an invalid context. -- TYT ]
Signed-off-by: Jonathan Davies <jonathan.davies@nutanix.com>
Link: https://lore.kernel.org/r/20210611140209.28903-1-jonathan.davies@nutanix.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ext4.h

@@ -3624,6 +3624,7 @@ extern const struct inode_operations ext4_symlink_inode_operations;
 extern const struct inode_operations ext4_fast_symlink_inode_operations;
 
 /* sysfs.c */
+extern void ext4_notify_error_sysfs(struct ext4_sb_info *sbi);
 extern int ext4_register_sysfs(struct super_block *sb);
 extern void ext4_unregister_sysfs(struct super_block *sb);
 extern int __init ext4_init_sysfs(void);

fs/ext4/super.c

@@ -718,6 +718,7 @@ static void flush_stashed_error_work(struct work_struct *work)
 			goto write_directly;
 		}
 		jbd2_journal_stop(handle);
+		ext4_notify_error_sysfs(sbi);
 		return;
 	}
 write_directly:
@@ -726,6 +727,7 @@ static void flush_stashed_error_work(struct work_struct *work)
 	 * out and hope for the best.
 	 */
 	ext4_commit_super(sbi->s_sb);
+	ext4_notify_error_sysfs(sbi);
 }
 
 #define ext4_error_ratelimit(sb)					\

fs/ext4/sysfs.c

@@ -506,6 +506,11 @@ static struct kobj_type ext4_feat_ktype = {
 	.release	= (void (*)(struct kobject *))kfree,
 };
 
+void ext4_notify_error_sysfs(struct ext4_sb_info *sbi)
+{
+	sysfs_notify(&sbi->s_kobj, NULL, "errors_count");
+}
+
 static struct kobject *ext4_root;
 
 static struct kobject *ext4_feat;