Re: [PATCH v2] btrfs: fix warning when putting transaction with qgroups enabled after abort

[Josef Bacik <josef@toxicpanda.com>]

On Fri, Jul 14, 2023 at 01:42:06PM +0100, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> If we have a transaction abort with qgroups enabled we get a warning
> triggered when doing the final put on the transaction, like this:
> 
>   [161552.678901] ------------[ cut here ]------------
>   [161552.681530] WARNING: CPU: 4 PID: 81745 at fs/btrfs/transaction.c:144 btrfs_put_transaction+0x123/0x130 [btrfs]
>   [161552.681759] Modules linked in: btrfs blake2b_generic xor (...)
>   [161552.681934] CPU: 4 PID: 81745 Comm: btrfs-transacti Tainted: G        W          6.4.0-rc6-btrfs-next-134+ #1
>   [161552.681945] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014
>   [161552.681951] RIP: 0010:btrfs_put_transaction+0x123/0x130 [btrfs]
>   [161552.682139] Code: bd a0 01 00 (...)
>   [161552.682146] RSP: 0018:ffffa168c0527e28 EFLAGS: 00010286
>   [161552.682155] RAX: ffff936042caed00 RBX: ffff93604a3eb448 RCX: 0000000000000000
>   [161552.682161] RDX: ffff93606421b028 RSI: ffffffff92ff0878 RDI: ffff93606421b010
>   [161552.682166] RBP: ffff93606421b000 R08: 0000000000000000 R09: ffffa168c0d07c20
>   [161552.682171] R10: 0000000000000000 R11: ffff93608dc52950 R12: ffffa168c0527e70
>   [161552.682175] R13: ffff93606421b000 R14: ffff93604a3eb420 R15: ffff93606421b028
>   [161552.682181] FS:  0000000000000000(0000) GS:ffff93675fb00000(0000) knlGS:0000000000000000
>   [161552.682187] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   [161552.682193] CR2: 0000558ad262b000 CR3: 000000014feda005 CR4: 0000000000370ee0
>   [161552.682211] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>   [161552.682216] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>   [161552.682221] Call Trace:
>   [161552.682229]  <TASK>
>   [161552.682236]  ? __warn+0x80/0x130
>   [161552.682250]  ? btrfs_put_transaction+0x123/0x130 [btrfs]
>   [161552.682430]  ? report_bug+0x1f4/0x200
>   [161552.682444]  ? handle_bug+0x42/0x70
>   [161552.682456]  ? exc_invalid_op+0x14/0x70
>   [161552.682467]  ? asm_exc_invalid_op+0x16/0x20
>   [161552.682483]  ? btrfs_put_transaction+0x123/0x130 [btrfs]
>   [161552.682661]  btrfs_cleanup_transaction+0xe7/0x5e0 [btrfs]
>   [161552.682838]  ? _raw_spin_unlock_irqrestore+0x23/0x40
>   [161552.682847]  ? try_to_wake_up+0x94/0x5e0
>   [161552.682856]  ? __pfx_process_timeout+0x10/0x10
>   [161552.682872]  transaction_kthread+0x103/0x1d0 [btrfs]
>   [161552.683047]  ? __pfx_transaction_kthread+0x10/0x10 [btrfs]
>   [161552.683217]  kthread+0xee/0x120
>   [161552.683227]  ? __pfx_kthread+0x10/0x10
>   [161552.683237]  ret_from_fork+0x29/0x50
>   [161552.683259]  </TASK>
>   [161552.683262] ---[ end trace 0000000000000000 ]---
> 
> This corresponds to this line of code:
> 
>   void btrfs_put_transaction(struct btrfs_transaction *transaction)
>   {
>       (...)
>           WARN_ON(!RB_EMPTY_ROOT(
>                           &transaction->delayed_refs.dirty_extent_root));
>       (...)
>   }
> 
> The warning happens because btrfs_qgroup_destroy_extent_records(), called
> in the transaction abort path, we free all entries from the rbtree
> "dirty_extent_root" with rbtree_postorder_for_each_entry_safe(), but we
> don't actually empty the rbtree - it's still pointing to nodes that were
> freed.
> 
> So set the rbtree's root node to NULL to avoid this warning (assign
> RB_ROOT).
> 
> Fixes: 81f7eb00ff5b ("btrfs: destroy qgroup extent records on transaction abort")
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef