Re: [PATCH 3/3] vfs: fix filesystem_sync vs write race on rw=>ro remount v2

[Dmitry Monakhov <dmonakhov@openvz.org>]

Jan Kara <jack@suse.cz> writes:

> On Tue 02-03-10 17:04:26, Dmitry Monakhov wrote:
>> Jan Kara <jack@suse.cz> writes:
>> 
>> > On Sat 27-02-10 03:25:49, Dmitry Monakhov wrote:
>> >> +/**
>> >> + * Check whenever is it possible to remount given sb to readonly.
>> >> + * @sb : super block in question
>> >> + *
>> >> + * Caller is responsible to set ST_REMOUNT_RO state before the call.
>> >> + */
>> >> +int fs_may_remount_ro(struct super_block *sb)
>> >> +{
>> >> +	struct vfsmount *mnt;
>> >> +	int ret = 1;
>> >> +	spin_lock(&vfsmount_lock);
>> >> +	list_for_each_entry(mnt, &sb->s_vfsmount, mnt_sb_list) {
>> >> +		ret = !mnt_check_writers(mnt, 0);
>> >> +		if (ret)
>> >> +			break;
>> >> +	}
>> >> +	spin_unlock(&vfsmount_lock);
>> >> +	/*
>> >> +	 * If new writer appears after we have checked all vfsmounts.
>> >> +	 * Then ST_REMOUNT_RO bit will be cleared.
>> >> +	 */
>> >> +	if (!test_bit(ST_REMOUNT_RO, &sb->s_state))
>> >> +		ret = 0;
>> >> +	return ret;
>> >> +}
>> >   This misses the case when the superblock as unlinked-but-open files.
>> > In such case we have to fail remount RO as well. The original
>> > fs_may_remount_ro checks for that..
>> Since file is opened for write one of vfsmnt struct would have non
>> zero write count. So -EBUSY will be returned from fs_may_remount_ro()
>   But file can be open for reading only...
Ohh.. i see. what is the reason to fail RO remount due to unlinked
files? Is this because not all filesystem has orphan list?
Shame on such FS. Seems that i have to also grab write count
in unlink path if i_nlink becomes zero and drop on inode release.