Re: [PATCH v2] btrfs: fix data race when accessing the inode's disk_i_size at btrfs_drop_extents()

[haoran zheng]

Thanks for the advice, I will explain in detail why it's a harmless race.
And submit patch v3.

On Mon, Dec 2, 2024 at 10:40 PM Filipe Manana <fdmanana@xxxxxxxxxx> wrote:
>
> On Mon, Dec 2, 2024 at 1:35 PM Hao-ran Zheng <zhenghaoran154@xxxxxxxxx> wrote:
> >
> > A data race occurs when the function `insert_ordered_extent_file_extent()`
> > and the function `btrfs_inode_safe_disk_i_size_write()` are executed
> > concurrently. The function `insert_ordered_extent_file_extent()` is not
> > locked when reading inode->disk_i_size, causing
> > `btrfs_inode_safe_disk_i_size_write()` to cause data competition when
> > writing inode->disk_i_size, thus affecting the value of `modify_tree`.
> >
> > Since the impact of data race is limited, it is recommended to use the
> > `data_race` mark judgment.
>
> This should explain why it's a harmless race.
> Also it's better to explicitly say harmless race rather than say it
> has limited impact, because the latter gives the idea of rare problems
> when we don't have any.
>
> >
> > The specific call stack that appears during testing is as follows:
> > ============DATA_RACE============
> >  btrfs_drop_extents+0x89a/0xa060 [btrfs]
> >  insert_reserved_file_extent+0xb54/0x2960 [btrfs]
> >  insert_ordered_extent_file_extent+0xff5/0x1760 [btrfs]
> >  btrfs_finish_one_ordered+0x1b85/0x36a0 [btrfs]
> >  btrfs_finish_ordered_io+0x37/0x60 [btrfs]
> >  finish_ordered_fn+0x3e/0x50 [btrfs]
> >  btrfs_work_helper+0x9c9/0x27a0 [btrfs]
> >  process_scheduled_works+0x716/0xf10
> >  worker_thread+0xb6a/0x1190
> >  kthread+0x292/0x330
> >  ret_from_fork+0x4d/0x80
> >  ret_from_fork_asm+0x1a/0x30
> > ============OTHER_INFO============
> >  btrfs_inode_safe_disk_i_size_write+0x4ec/0x600 [btrfs]
> >  btrfs_finish_one_ordered+0x24c7/0x36a0 [btrfs]
> >  btrfs_finish_ordered_io+0x37/0x60 [btrfs]
> >  finish_ordered_fn+0x3e/0x50 [btrfs]
> >  btrfs_work_helper+0x9c9/0x27a0 [btrfs]
> >  process_scheduled_works+0x716/0xf10
> >  worker_thread+0xb6a/0x1190
> >  kthread+0x292/0x330
> >  ret_from_fork+0x4d/0x80
> >  ret_from_fork_asm+0x1a/0x30
> > =================================
> >
> > Signed-off-by: Hao-ran Zheng <zhenghaoran154@xxxxxxxxx>
> > ---
> > V1->V2: Modify patch description and format
> > ---
> >  fs/btrfs/file.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> > index 4fb521d91b06..f9631713f67d 100644
> > --- a/fs/btrfs/file.c
> > +++ b/fs/btrfs/file.c
> > @@ -242,7 +242,7 @@ int btrfs_drop_extents(struct btrfs_trans_handle *trans,
> >         if (args->drop_cache)
> >                 btrfs_drop_extent_map_range(inode, args->start, args->end - 1, false);
> >
> > -       if (args->start >= inode->disk_i_size && !args->replace_extent)
> > +       if (data_race(args->start >= inode->disk_i_size && !args->replace_extent))
>
> Don't surround the whole condition with data_race().
> Just put it around the disk_i_size check:
>
> if (data_race(args->start >= inode->disk_i_size) && !args->replace_extent))
>
> This makes it more clear it's about disk_i_size and nothing else.
>
> Thanks.
>
> >                 modify_tree = 0;
> >
> >         update_refs = (btrfs_root_id(root) != BTRFS_TREE_LOG_OBJECTID);
> > --
> > 2.34.1
> >
> >