On Tue, Aug 27, 2013 at 04:51:55PM +0100, Filipe David Borba Manana wrote:
> If there's an ongoing transaction when the uuid scan kthread attempts
> to create one, the kthread will block, waiting for that transaction to
> finish while it's keeping locks on the tree root, and in turn the existing
> transaction is waiting for those locks to be free.
>
> The stack trace reported by the kernel follows.
>
> [36700.671601] INFO: task btrfs-uuid:15480 blocked for more than 120 seconds.
> [36700.671602] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [36700.671602] btrfs-uuid D 0000000000000000 0 15480 2 0x00000000
> [36700.671604] ffff880710bd5b88 0000000000000046 ffff8803d36ba850 0000000000030000
> [36700.671605] ffff8806d76dc530 ffff880710bd5fd8 ffff880710bd5fd8 ffff880710bd5fd8
> [36700.671607] ffff8808098ac530 ffff8806d76dc530 ffff880710bd5b98 ffff8805e4508e40
> [36700.671608] Call Trace:
> [36700.671610] [<ffffffff816f36b9>] schedule+0x29/0x70
> [36700.671620] [<ffffffffa05a3bdf>] wait_current_trans.isra.33+0xbf/0x120 [btrfs]
> [36700.671623] [<ffffffff81066760>] ? add_wait_queue+0x60/0x60
> [36700.671629] [<ffffffffa05a5b06>] start_transaction+0x3d6/0x530 [btrfs]
> [36700.671636] [<ffffffffa05bb1f4>] ? btrfs_get_token_32+0x64/0xf0 [btrfs]
> [36700.671642] [<ffffffffa05a5fbb>] btrfs_start_transaction+0x1b/0x20 [btrfs]
> [36700.671649] [<ffffffffa05c8a81>] btrfs_uuid_scan_kthread+0x211/0x3d0 [btrfs]
> [36700.671655] [<ffffffffa05c8870>] ? __btrfs_open_devices+0x2a0/0x2a0 [btrfs]
> [36700.671657] [<ffffffff81065fa0>] kthread+0xc0/0xd0
> [36700.671659] [<ffffffff81065ee0>] ? flush_kthread_worker+0xb0/0xb0
> [36700.671661] [<ffffffff816fcd1c>] ret_from_fork+0x7c/0xb0
> [36700.671662] [<ffffffff81065ee0>] ? flush_kthread_worker+0xb0/0xb0
> [36700.671663] INFO: task btrfs:15481 blocked for more than 120 seconds.
> [36700.671664] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [36700.671665] btrfs D 0000000000000000 0 15481 15212 0x00000004
> [36700.671666] ffff880248cbf4c8 0000000000000086 ffff8803d36ba700 ffff8801dbd5c280
> [36700.671668] ffff880807815c40 ffff880248cbffd8 ffff880248cbffd8 ffff880248cbffd8
> [36700.671669] ffff8805e86a0000 ffff880807815c40 ffff880248cbf4d8 ffff8801dbd5c280
> [36700.671670] Call Trace:
> [36700.671672] [<ffffffff816f36b9>] schedule+0x29/0x70
> [36700.671679] [<ffffffffa05d9b0d>] btrfs_tree_lock+0x6d/0x230 [btrfs]
> [36700.671680] [<ffffffff81066760>] ? add_wait_queue+0x60/0x60
> [36700.671685] [<ffffffffa0582829>] btrfs_search_slot+0x999/0xb00 [btrfs]
> [36700.671691] [<ffffffffa05bd9de>] ? btrfs_lookup_first_ordered_extent+0x5e/0xb0 [btrfs]
> [36700.671698] [<ffffffffa05e3e54>] __btrfs_write_out_cache+0x8c4/0xa80 [btrfs]
> [36700.671704] [<ffffffffa05e4362>] btrfs_write_out_cache+0xb2/0xf0 [btrfs]
> [36700.671710] [<ffffffffa05c4441>] ? free_extent_buffer+0x61/0xc0 [btrfs]
> [36700.671716] [<ffffffffa0594c82>] btrfs_write_dirty_block_groups+0x562/0x650 [btrfs]
> [36700.671723] [<ffffffffa0610092>] commit_cowonly_roots+0x171/0x24b [btrfs]
> [36700.671729] [<ffffffffa05a4dde>] btrfs_commit_transaction+0x4fe/0xa10 [btrfs]
> [36700.671735] [<ffffffffa0610af3>] create_subvol+0x5c0/0x636 [btrfs]
> [36700.671742] [<ffffffffa05d49ff>] btrfs_mksubvol.isra.60+0x33f/0x3f0 [btrfs]
> [36700.671747] [<ffffffffa05d4bf2>] btrfs_ioctl_snap_create_transid+0x142/0x190 [btrfs]
> [36700.671752] [<ffffffffa05d4c6c>] ? btrfs_ioctl_snap_create+0x2c/0x80 [btrfs]
> [36700.671757] [<ffffffffa05d4c9e>] btrfs_ioctl_snap_create+0x5e/0x80 [btrfs]
> [36700.671759] [<ffffffff8113a764>] ? handle_pte_fault+0x84/0x920
> [36700.671764] [<ffffffffa05d87eb>] btrfs_ioctl+0xf0b/0x1d00 [btrfs]
> [36700.671766] [<ffffffff8113c120>] ? handle_mm_fault+0x210/0x310
> [36700.671768] [<ffffffff816f83a4>] ? __do_page_fault+0x284/0x4e0
> [36700.671770] [<ffffffff81180aa6>] do_vfs_ioctl+0x96/0x550
> [36700.671772] [<ffffffff81170fe3>] ? __sb_end_write+0x33/0x70
> [36700.671774] [<ffffffff81180ff1>] SyS_ioctl+0x91/0xb0
> [36700.671775] [<ffffffff816fcdc2>] system_call_fastpath+0x16/0x1b
>
> Signed-off-by: Filipe David Borba Manana <fdmanana@xxxxxxxxx>
> ---
> fs/btrfs/volumes.c | 25 ++++++++++++++-----------
> 1 file changed, 14 insertions(+), 11 deletions(-)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index f42e412..44cd21b 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -3465,7 +3465,7 @@ static int btrfs_uuid_scan_kthread(void *data)
> int slot;
> struct btrfs_root_item root_item;
> u32 item_size;
> - struct btrfs_trans_handle *trans;
> + struct btrfs_trans_handle *trans = NULL;
>
> path = btrfs_alloc_path();
> if (!path) {
> @@ -3509,7 +3509,13 @@ static int btrfs_uuid_scan_kthread(void *data)
> (int)sizeof(root_item));
> if (btrfs_root_refs(&root_item) == 0)
> goto skip;
> - if (!btrfs_is_empty_uuid(root_item.uuid)) {
> +
> + if (!btrfs_is_empty_uuid(root_item.uuid) ||
> + !btrfs_is_empty_uuid(root_item.received_uuid)) {
> + if (trans)
> + goto update_tree;
> +
> + btrfs_release_path(path);
> /*
> * 1 - subvol uuid item
> * 1 - received_subvol uuid item
> @@ -3519,6 +3525,11 @@ static int btrfs_uuid_scan_kthread(void *data)
> ret = PTR_ERR(trans);
> break;
> }
> + continue;
> + } else
> + goto skip;
Just a formatting nit, you still need to do
} else {
goto skip
}
> +update_tree:
> + if (!btrfs_is_empty_uuid(root_item.uuid)) {
> ret = btrfs_uuid_tree_add(trans, fs_info->uuid_root,
> root_item.uuid,
> BTRFS_UUID_KEY_SUBVOL,
> @@ -3533,15 +3544,6 @@ static int btrfs_uuid_scan_kthread(void *data)
> }
>
> if (!btrfs_is_empty_uuid(root_item.received_uuid)) {
> - if (!trans) {
> - /* 1 - received_subvol uuid item */
> - trans = btrfs_start_transaction(
> - fs_info->uuid_root, 1);
> - if (IS_ERR(trans)) {
> - ret = PTR_ERR(trans);
> - break;
> - }
> - }
> ret = btrfs_uuid_tree_add(trans, fs_info->uuid_root,
> root_item.received_uuid,
> BTRFS_UUID_KEY_RECEIVED_SUBVOL,
> @@ -3557,6 +3559,7 @@ static int btrfs_uuid_scan_kthread(void *data)
>
> if (trans) {
> ret = btrfs_end_transaction(trans, fs_info->uuid_root);
> + trans = NULL;
We set trans = NULL before we read the root item so we don't need this extra bit
here. Great find, thanks,
Josef
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