Re: [PATCH] fs: fix filesystem_sync vs write race on rw=>ro remount

[Dmitry Monakhov <dmonakhov@openvz.org>]

Al Viro <viro@ZenIV.linux.org.uk> writes:

> On Sun, Jan 24, 2010 at 02:41:15PM +0300, Dmitry Monakhov wrote:
>> Currently on rw=>ro remount we have following race
>> | mount /mnt -oremount,ro | write-task |
>> |-------------------------+------------|
>> |                         | open(RDWR) |
>> | shrink_dcache_sb(sb);   |            |
>> | sync_filesystem(sb);    |            |
>> |                         | write()    |
>> |                         | close()    |
>> | fs_may_remount_ro(sb)   |            |
>> | sb->s_flags = new_flags |            |
>> Later writeback or sync() will result in error due to MS_RDONLY flag
>> In case of ext4 this result in jbd2_start failure on writeback
>> ext4_da_writepages: jbd2_start: 1024 pages, ino 1431; err -30 
>> In fact all others are affected by this error but it is not visible
>> because the skip s_flags check on writeback. For example ext3 check
>> (s_flags & MS_RDONLY) only if page has no buffers during journal start.
>> 
>> In order to prevent the race we have to block new writers before
>> fs_may_remount_ro() and sync_filesystem(). Let's introduce new
>> sb->s_flags MS_RO_REMOUNT flag for this purpose. But suddenly we have
>> no available space in MS_XXX bits, let's share this bit with MS_REMOUNT.
>> This is possible because MS_REMOUNT used only for passing arguments
>> from flags to sys_mount() and never used in sb->s_flags.
>
> It's not a solution.  You get an _attempted_ remount ro making writes
> fail, even if it's going to be unsuccessful.  No go...
We have two options for new writers:
1) Fail it via -EROFS
   Yes, remount may fail, but it is really unlikely.
2) Defer(block) new writers on until we complete or fail remount
   for example like follows. Do you like second solution ?
diff --git a/fs/namespace.c b/fs/namespace.c
index c768f73..daf3c5a 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -196,6 +196,15 @@ int __mnt_is_readonly(struct vfsmount *mnt)
 		return 1;
 	if (mnt->mnt_sb->s_flags & MS_RDONLY)
 		return 1;
+	if (mnt->mnt_sb->s_flags & MS_RO_REMOUNT) {
+		int ret = 0;
+		/* Serialize against remount */
+		down_read(&mnt->mnt_sb->s_umount);
+		if (mnt->mnt_sb->s_flags & MS_RDONLY)
+			ret = 1;
+		up_read(&mnt->mnt_sb->s_umount);
+		return ret;
+	}
 	return 0;
 }
 EXPORT_SYMBOL_GPL(__mnt_is_readonly);