Re: [PATCH] Btrfs: fix assertion failure during fsync and use of stale transaction

[Josef Bacik <josef@toxicpanda.com>]

On Tue, Sep 10, 2019 at 03:26:49PM +0100, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> Sometimes when fsync'ing a file we need to log that other inodes exist and
> when we need to do that we acquire a reference on the inodes and then drop
> that reference using iput() after logging them.
> 
> That generally is not a problem except if we end up doing the final iput()
> (dropping the last reference) on the inode and that inode has a link count
> of 0, which can happen in a very short time window if the logging path
> gets a reference on the inode while it's being unlinked.
> 
> In that case we end up getting the eviction callback, btrfs_evict_inode(),
> invoked through the iput() call chain which needs to drop all of the
> inode's items from its subvolume btree, and in order to do that, it needs
> to join a transaction at the helper function evict_refill_and_join().
> However because the task previously started a transaction at the fsync
> handler, btrfs_sync_file(), it has current->journal_info already pointing
> to a transaction handle and therefore evict_refill_and_join() will get
> that transaction handle from btrfs_join_transaction(). From this point on,
> two different problems can happen:
> 
> 1) evict_refill_and_join() will often change the transaction handle's
>    block reserve (->block_rsv) and set its ->bytes_reserved field to a
>    value greater than 0. If evict_refill_and_join() never commits the
>    transaction, the eviction handler ends up decreasing the reference
>    count (->use_count) of the transaction handle through the call to
>    btrfs_end_transaction(), and after that point we have a transaction
>    handle with a NULL ->block_rsv (which is the value prior to the
>    transaction join from evict_refill_and_join()) and a ->bytes_reserved
>    value greater than 0. If after the eviction/iput completes the inode
>    logging path hits an error or it decides that it must fallback to a
>    transaction commit, the btrfs fsync handle, btrfs_sync_file(), gets a
>    non-zero value from btrfs_log_dentry_safe(), and because of that
>    non-zero value it tries to commit the transaction using a handle with
>    a NULL ->block_rsv and a non-zero ->bytes_reserved value. This makes
>    the transaction commit hit an assertion failure at
>    btrfs_trans_release_metadata() because ->bytes_reserved is not zero but
>    the ->block_rsv is NULL. The produced stack trace for that is like the
>    following:
> 
>    [192922.917158] assertion failed: !trans->bytes_reserved, file: fs/btrfs/transaction.c, line: 816
>    [192922.917553] ------------[ cut here ]------------
>    [192922.917922] kernel BUG at fs/btrfs/ctree.h:3532!
>    [192922.918310] invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC PTI
>    [192922.918666] CPU: 2 PID: 883 Comm: fsstress Tainted: G        W         5.1.4-btrfs-next-47 #1
>    [192922.919035] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.2-0-gf9626ccb91-prebuilt.qemu-project.org 04/01/2014
>    [192922.919801] RIP: 0010:assfail.constprop.25+0x18/0x1a [btrfs]
>    (...)
>    [192922.920925] RSP: 0018:ffffaebdc8a27da8 EFLAGS: 00010286
>    [192922.921315] RAX: 0000000000000051 RBX: ffff95c9c16a41c0 RCX: 0000000000000000
>    [192922.921692] RDX: 0000000000000000 RSI: ffff95cab6b16838 RDI: ffff95cab6b16838
>    [192922.922066] RBP: ffff95c9c16a41c0 R08: 0000000000000000 R09: 0000000000000000
>    [192922.922442] R10: ffffaebdc8a27e70 R11: 0000000000000000 R12: ffff95ca731a0980
>    [192922.922820] R13: 0000000000000000 R14: ffff95ca84c73338 R15: ffff95ca731a0ea8
>    [192922.923200] FS:  00007f337eda4e80(0000) GS:ffff95cab6b00000(0000) knlGS:0000000000000000
>    [192922.923579] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>    [192922.923948] CR2: 00007f337edad000 CR3: 00000001e00f6002 CR4: 00000000003606e0
>    [192922.924329] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>    [192922.924711] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>    [192922.925105] Call Trace:
>    [192922.925505]  btrfs_trans_release_metadata+0x10c/0x170 [btrfs]
>    [192922.925911]  btrfs_commit_transaction+0x3e/0xaf0 [btrfs]
>    [192922.926324]  btrfs_sync_file+0x44c/0x490 [btrfs]
>    [192922.926731]  do_fsync+0x38/0x60
>    [192922.927138]  __x64_sys_fdatasync+0x13/0x20
>    [192922.927543]  do_syscall_64+0x60/0x1c0
>    [192922.927939]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
>    (...)
>    [192922.934077] ---[ end trace f00808b12068168f ]---
> 
> 2) If evict_refill_and_join() decides to commit the transaction, it will
>    be able to do it, since the nested transaction join only increments the
>    transaction handle's ->use_count reference counter and it does not
>    prevent the transaction from getting committed. This means that after

This brings up a good point, we should probably not allow the commit in this
case, or add an ASSERT(use_count == 1) or something, cause this would be bad.
Thanks,

Josef