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From:   Richard Weinberger <richard.weinberger@gmail.com>
Date:   Thu, 16 Jan 2020 23:45:10 +0100
Message-ID: <CAFLxGvzOeL_0Lq30rvbaSuxsFZSzYSvKw2V=C1gvbad9VPjiEQ@mail.gmail.com>
Subject: Re: [PATCH v2] ubifs: Fix deadlock in concurrent bulk-read and writepage
To:     Zhihao Cheng <chengzhihao1@huawei.com>
Cc:     Richard Weinberger <richard@nod.at>,
        Sascha Hauer <s.hauer@pengutronix.de>,
        "zhangyi (F)" <yi.zhang@huawei.com>, linux-mtd@lists.infradead.org,
        LKML <linux-kernel@vger.kernel.org>
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On Sat, Jan 11, 2020 at 10:43 AM Zhihao Cheng <chengzhihao1@huawei.com> wro=
te:
>
> In ubifs, concurrent execution of writepage and bulk read on the same fil=
e
> 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)                  =E2=86=91
>               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@huawei.com>
> Suggested-by: zhangyi (F) <yi.zhang@huawei.com>
> Cc: <Stable@vger.kernel.org>
> Fixes: 4793e7c5e1c ("UBIFS: add bulk-read facility")
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=3D206153
> ---
>  fs/ubifs/file.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

Applied. Thanks a lot for hunting this down!

--=20
Thanks,
//richard