linux-ext4 - Re: [PATCH] fs: fix filesystem_sync vs write race on rw=>ro remount

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Date:	Mon, 25 Jan 2010 02:01:17 +0300
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: fix filesystem_sync vs write race on rw=>ro remount

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

> On Mon, Jan 25, 2010 at 12:15:51AM +0300, Dmitry Monakhov wrote:
>
>> > 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 ?
>
> Umm...  I wonder what the locking implications would be...  Frankly,
> I suspect that what we really want is this:
> 	* per-superblock write count of some kind, bumped when we decide
> that writeback is inevitable and dropped when we are done with it (the
> same thing goes for async part of unlink(), etc.)
> 	* fs_may_remount_ro() checking that write count
> So basically we try to push those short-term writers to completion and
> if new ones had come while we'd been doing that (or some are really
> stuck) we fail remount with -EBUSY.
>
> As a short-term solution the second patch would do probably (-stable and .33),
> but in the next cycle I'd rather see something addressing the real problem.
> fs_may_remount_ro() in its current form is really broken by design - it
> should not scan any lists (which is where your race comes from, BTW)
This is not actually true. The race happens not only because
fs_may_remount_ro() is not atomic, but because we have two stages
1) fs_may_remount_ro()
2) sync_filesystem()
Even when we make first stage atomic, we still have race between
second stage and new writers.
BTW: Your idea about per-sb counter may be useful here but
it must be not reference count, but it may be used like i_version
For example:
mnt_want_write()
{
   mnt->mnt_sb->s_wr_count++;
}
mnt_drop_write()
{
   mnt->mnt_sb->s_wr_count++;
}
do_remount_sb {
    cur = mnt->mnt_sb->s_wr_count;
    if (fs_may_remount_ro())
         return -EBUSY;
    sync_filesystem()
    if (cur != mnt->mnt_sb->s_wr_count)
         return -EBUSY;
}


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