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

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

[Zhihao Cheng]

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@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=206153
---
 fs/ubifs/file.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/ubifs/file.c b/fs/ubifs/file.c
index cd52585..c649048 100644
--- a/fs/ubifs/file.c
+++ b/fs/ubifs/file.c
@@ -786,7 +786,9 @@ static int ubifs_do_bulk_read(struct ubifs_info *c, struct bu_info *bu,
 
 		if (page_offset > end_index)
 			break;
-		page = find_or_create_page(mapping, page_offset, ra_gfp_mask);
+		page = pagecache_get_page(mapping, page_offset,
+				 FGP_LOCK|FGP_ACCESSED|FGP_CREAT|FGP_NOWAIT,
+				 ra_gfp_mask);
 		if (!page)
 			break;
 		if (!PageUptodate(page))
-- 
2.7.4


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

[Richard Weinberger]

On Sat, Jan 11, 2020 at 10:43 AM Zhihao Cheng <chengzhihao1@huawei.com> 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@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=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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