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Re: assertion failure with latest xfs

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>, Lachlan McIlroy <lachlan@xxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
Subject: Re: assertion failure with latest xfs
From: Lachlan McIlroy <lachlan@xxxxxxx>
Date: Wed, 29 Oct 2008 11:43:31 +1100
In-reply-to: <20081023222126.GA18495@disturbed>
References: <49003EFF.4090404@xxxxxxx> <20081023173149.GA30316@xxxxxxxxxxxxx> <20081023222126.GA18495@disturbed>
Reply-to: lachlan@xxxxxxx
User-agent: Thunderbird 2.0.0.17 (X11/20080914)
Dave Chinner wrote:
On Thu, Oct 23, 2008 at 01:31:49PM -0400, Christoph Hellwig wrote:
On Thu, Oct 23, 2008 at 07:08:15PM +1000, Lachlan McIlroy wrote:
Just encountered this after pulling in the latest changes.  We are trying to
initialise an inode that should have an i_count of 1 but instead it is 2.  I
was running XFSQA test 167 when it happened.
I think the assert is incorrect.  The inode has been added to the radix
tree in xfs_iget_cache_miss, and starting from that point an igrab can
kick in from the sync code and bump the refcount.

Actually, it was put there for a reason. The generic code doesn't
allow new inodes to be found in the cache until the I_LOCK flag is
cleared. This is done by calling wait_on_inode() after a successful
lookup (which waits on I_LOCK) and unlock_new_inode() clears the
I_LOCK|I_NEW bits and wakes anyone who was waiting on that inode via
wake_up_inode().  So the assert was put there to catch potential
races in lookup where a second process does a successful igrab()
before the inode is fully initialised.

I think the race is in dealing with cache hits and recycling a
XFS_IRECLAIMABLE inode. We set the XFS_INEW flag there under
the radix tree read lock, which means we can have parallel lookups
on the same inode that goes:

        thread 1                                thread 2
        test XFS_INEW
                -> not set
        test XFS_IRECLAIMABLE
                -> set
                                                test XFS_INEW
                                                        -> not set
        set XFS_INEW
        clear XFS_IRECLAIMABLE
                                                test XFS_IRECLAIMABLE
                                                        -> not set
        xfs_setup_inode()
                -> i_state = I_NEW|I_LOCK
                                                igrab(inode)
                                                        -> I_CLEAR not set
                                                        -> refcount = 2
                -> inode_add_to_lists
                -> assert(refcount == 1)
                .....
                -> clear XFS_INEW
                -> unlock_new_inode()
                        -> clear I_NEW|I_LOCK

I thought I'd handled this race with the ordering of setting/clearing
XFS_INEW/XFS_IRECLAIMABLE. Clearly not. I'll add a comment to this
ordering because it is key to actually detecting the race condition
so we can handle it.

Hmmmm - there's also another bug in xfs_iget_cache_hit() - we don't
drop the reference we got if we found an unlinked inode after the
igrab() (the ENOENT case). I'll fix that as well.

Patch below that I'm currently running through xfsqa.

I gave this patch a go and it still asserted at the same place running
the same test.


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