On Mon, Jan 28, 2013 at 5:04 AM, Liu Bo <bo.li.liu@xxxxxxxxxx> wrote:
> While running snapshot testscript created by Mitch and David,
> the race between autodefrag and snapshot deletion can lead to
> corruption of dead_root list so that we can get crash on
> btrfs_clean_old_snapshots().
>
> And besides autodefrag, scrub also do the same thing, ie. read
> root first and get inode.
>
> Here is the story(take autodefrag as an example):
> (1) when we delete a snapshot or subvolume, it will set its root's
> refs to zero and do a iput() on its own inode, and if this inode happens
> to be the only active in-meory one in root's inode rbtree, it will add
> itself to the global dead_roots list for later cleanup.
>
> (2) after (1), the autodefrag thread may read another inode for defrag
> and the inode is just in the deleted snapshot/subvolume, but all of these
> are without checking if the root is still valid(refs > 0). So the end up
> result is adding the deleted snapshot/subvolume's root to the global
> dead_roots list AGAIN.
>
> Fortunately, we already have a srcu lock to avoid the race, ie. subvol_srcu.
>
> So all we need to do is to take the lock to protect 'read root and get inode',
> since we synchronize to wait for the rcu grace period before adding something
> to the global dead_roots list.
>
> Reported-by: Mitch Harder <mitch.harder@xxxxxxxxxxxxxxxx>
> Signed-off-by: Liu Bo <bo.li.liu@xxxxxxxxxx>
I'm still seeing seeing issues with duplications in the dead_roots list.
I'm using a 3.7.4 kernel merged with the for-linus branch with the
following four patches:
[PATCH V5] Btrfs: snapshot-aware defrag
[PATCH] Btrfs: List Debugging for cleaning deleted
Non-functional patch to issue some trace_printk debugging.
[PATCH] [RFC] Btrfs: Check for duplicate dead root list
This is the patch discussed in the snapshot-aware defrag thread.
It checks for duplicate list entries, and dumps a backtrace
if it finds one.
Btrfs: fix race between snapshot deletion and getting inode
I've run into several backtraces similar to the following:
[ 3129.368196] btrfs: Duplicate dead root entry.
[ 3129.368199] ------------[ cut here ]------------
[ 3129.368220] WARNING: at fs/btrfs/transaction.c:893
btrfs_add_dead_root+0x73/0xbc [btrfs]()
[ 3129.368223] Hardware name: OptiPlex 745
[ 3129.368224] Modules linked in: ipv6 snd_hda_codec_analog
snd_hda_intel snd_hda_codec snd_hwdep snd_pcm snd_page_alloc iTCO_wdt
ppdev iTCO_vendor_support i2c_i801 parport_pc floppy tg3 sr_mod
microcode snd_timer snd lpc_ich serio_raw pcspkr parport ablk_helper
cryptd lrw xts gf128mul aes_x86_64 sha256_generic fuse xfs nfs lockd
sunrpc reiserfs btrfs zlib_deflate ext4 jbd2 ext3 jbd ext2 mbcache
sl811_hcd hid_generic xhci_hcd ohci_hcd uhci_hcd ehci_hcd
[ 3129.368268] Pid: 4309, comm: btrfs-endio-wri Tainted: G W
3.7.4-sad-v2+ #1
[ 3129.368271] Call Trace:
[ 3129.368278] [<ffffffff81030586>] warn_slowpath_common+0x83/0x9b
[ 3129.368282] [<ffffffff810305b8>] warn_slowpath_null+0x1a/0x1c
[ 3129.368297] [<ffffffffa0179e0b>] btrfs_add_dead_root+0x73/0xbc [btrfs]
[ 3129.368313] [<ffffffffa0187bef>] btrfs_destroy_inode+0x227/0x25b [btrfs]
[ 3129.368319] [<ffffffff8111393a>] destroy_inode+0x3b/0x54
[ 3129.368322] [<ffffffff81113a9c>] evict+0x149/0x151
[ 3129.368327] [<ffffffff81114322>] iput+0x12c/0x135
[ 3129.368342] [<ffffffffa01845e7>] relink_extent_backref+0x669/0x6af [btrfs]
[ 3129.368346] [<ffffffff815e9849>] ? __slab_free+0x17c/0x21b
[ 3129.368362] [<ffffffffa017c33d>] ? record_extent_backrefs+0xa3/0xa3 [btrfs]
[ 3129.368377] [<ffffffffa0184d9d>] ?
btrfs_finish_ordered_io+0x770/0x827 [btrfs]
[ 3129.368393] [<ffffffffa0184d6d>] btrfs_finish_ordered_io+0x740/0x827 [btrfs]
[ 3129.368409] [<ffffffffa0184e69>] finish_ordered_fn+0x15/0x17 [btrfs]
[ 3129.368424] [<ffffffffa019e7fd>] worker_loop+0x14c/0x493 [btrfs]
[ 3129.368439] [<ffffffffa019e6b1>] ? btrfs_queue_worker+0x258/0x258 [btrfs]
[ 3129.368443] [<ffffffff8104c750>] kthread+0xba/0xc2
[ 3129.368447] [<ffffffff8104c696>] ? kthread_freezable_should_stop+0x52/0x52
[ 3129.368451] [<ffffffff815f301c>] ret_from_fork+0x7c/0xb0
[ 3129.368455] [<ffffffff8104c696>] ? kthread_freezable_should_stop+0x52/0x52
[ 3129.368458] ---[ end trace 46705ba72c45db88 ]---
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