Re: [PATCH v2] ubifs: Fix deadlock in concurrent bulk-read and writepage

[Richard Weinberger <richard.weinberger@xxxxxxxxx>]

On Sat, Jan 11, 2020 at 10:43 AM Zhihao Cheng <chengzhihao1@xxxxxxxxxx> wrote:
>
> In ubifs, concurrent execution of writepage and bulk read on the same file
> may cause ABBA deadlock, for example (Reproduce method see Link):
>
> Process A(Bulk-read starts from page4)         Process B(write page4 back)
>   vfs_read                                       wb_workfn or fsync
>   ...                                            ...
>   generic_file_buffered_read                     write_cache_pages
>     ubifs_readpage                                 LOCK(page4)
>
>       ubifs_bulk_read                              ubifs_writepage
>         LOCK(ui->ui_mutex)                           ubifs_write_inode
>
>           ubifs_do_bulk_read                           LOCK(ui->ui_mutex)
>             find_or_create_page(alloc page4)                  ↑
>               LOCK(page4)                   <--     ABBA deadlock occurs!
>
> In order to ensure the serialization execution of bulk read, we can't
> remove the big lock 'ui->ui_mutex' in ubifs_bulk_read(). Instead, we
> allow ubifs_do_bulk_read() to lock page failed by replacing
> find_or_create_page(FGP_LOCK) with
> pagecache_get_page(FGP_LOCK | FGP_NOWAIT).
>
> Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
> Suggested-by: zhangyi (F) <yi.zhang@xxxxxxxxxx>
> Cc: <Stable@xxxxxxxxxxxxxxx>
> Fixes: 4793e7c5e1c ("UBIFS: add bulk-read facility")
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206153
> ---
>  fs/ubifs/file.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

Applied. Thanks a lot for hunting this down!

-- 
Thanks,
//richard

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