Date:   Fri, 26 Mar 2021 15:18:15 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     miklos@...redi.hu
Cc:     tao.peng@...ux.alibaba.com, baolin.wang@...ux.alibaba.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] fuse: Fix possible deadlock when writing back dirty pages

We can meet below deadlock scenario when writing back dirty pages, and
writing files at the same time. The deadlock scenario can be reproduced
by:

- A writeback worker thread A is trying to write a bunch of dirty pages by
fuse_writepages(), and the fuse_writepages() will lock one page (named page 1),
add it into rb_tree with setting writeback flag, and unlock this page 1,
then try to lock next page (named page 2).

- But at the same time a file writing can be triggered by another process B,
to write several pages by fuse_perform_write(), the fuse_perform_write()
will lock all required pages firstly, then wait for all writeback pages
are completed by fuse_wait_on_page_writeback().

- Now the process B can already lock page 1 and page 2, and wait for page 1
waritehack is completed (page 1 is under writeback set by process A). But
process A can not complete the writeback of page 1, since it is still
waiting for locking page 2, which was locked by process B already.

A deadlock is occurred.

To fix this issue, we should make sure each page writeback is completed after
lock the page in fuse_fill_write_pages(), and then write them together when
all pages are stable.

[1450578.772896] INFO: task kworker/u259:6:119885 blocked for more than 120 seconds.
[1450578.796179] kworker/u259:6  D    0 119885      2 0x00000028
[1450578.796185] Workqueue: writeback wb_workfn (flush-0:78)
[1450578.796188] Call trace:
[1450578.798804]  __switch_to+0xd8/0x148
[1450578.802458]  __schedule+0x280/0x6a0
[1450578.806112]  schedule+0x34/0xe8
[1450578.809413]  io_schedule+0x20/0x40
[1450578.812977]  __lock_page+0x164/0x278
[1450578.816718]  write_cache_pages+0x2b0/0x4a8
[1450578.820986]  fuse_writepages+0x84/0x100 [fuse]
[1450578.825592]  do_writepages+0x58/0x108
[1450578.829412]  __writeback_single_inode+0x48/0x448
[1450578.834217]  writeback_sb_inodes+0x220/0x520
[1450578.838647]  __writeback_inodes_wb+0x50/0xe8
[1450578.843080]  wb_writeback+0x294/0x3b8
[1450578.846906]  wb_do_writeback+0x2ec/0x388
[1450578.850992]  wb_workfn+0x80/0x1e0
[1450578.854472]  process_one_work+0x1bc/0x3f0
[1450578.858645]  worker_thread+0x164/0x468
[1450578.862559]  kthread+0x108/0x138
[1450578.865960] INFO: task doio:207752 blocked for more than 120 seconds.
[1450578.888321] doio            D    0 207752 207740 0x00000000
[1450578.888329] Call trace:
[1450578.890945]  __switch_to+0xd8/0x148
[1450578.894599]  __schedule+0x280/0x6a0
[1450578.898255]  schedule+0x34/0xe8
[1450578.901568]  fuse_wait_on_page_writeback+0x8c/0xc8 [fuse]
[1450578.907128]  fuse_perform_write+0x240/0x4e0 [fuse]
[1450578.912082]  fuse_file_write_iter+0x1dc/0x290 [fuse]
[1450578.917207]  do_iter_readv_writev+0x110/0x188
[1450578.921724]  do_iter_write+0x90/0x1c8
[1450578.925598]  vfs_writev+0x84/0xf8
[1450578.929071]  do_writev+0x70/0x110
[1450578.932552]  __arm64_sys_writev+0x24/0x30
[1450578.936727]  el0_svc_common.constprop.0+0x80/0x1f8
[1450578.941694]  el0_svc_handler+0x30/0x80
[1450578.945606]  el0_svc+0x10/0x14

Suggested-by: Peng Tao <tao.peng@...ux.alibaba.com>
Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
---
 fs/fuse/file.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 8cccecb..af082b6 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1166,6 +1166,8 @@ static ssize_t fuse_fill_write_pages(struct fuse_args_pages *ap,
 		if (!page)
 			break;
 
+		wait_on_page_writeback(page);
+
 		if (mapping_writably_mapped(mapping))
 			flush_dcache_page(page);
 
-- 
1.8.3.1

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