[PATCH v2 1/2] fuse: Fix possible deadlock when writing back dirty pages

Baolin Wang
2021-03-27T06:37:05Z

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() separately, 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@linux.alibaba.com>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
Changes from v1:
- Use fuse_wait_on_page_writeback() instead to wait for page stable.
---
fs/fuse/file.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 8cccecb..9a30093 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1101,9 +1101,6 @@ static ssize_t fuse_send_write_pages(struct fuse_io_args *ia,
unsigned int offset, i;
int err;

- for (i = 0; i < ap->num_pages; i++)
- fuse_wait_on_page_writeback(inode, ap->pages[i]->index);
-
fuse_write_args_fill(ia, ff, pos, count);
ia->write.in.flags = fuse_write_flags(iocb);
if (fm->fc->handle_killpriv_v2 && !capable(CAP_FSETID))
@@ -1140,6 +1137,7 @@ static ssize_t fuse_fill_write_pages(struct fuse_args_pages *ap,
unsigned int max_pages)
{
struct fuse_conn *fc = get_fuse_conn(mapping->host);
+ struct inode *inode = mapping->host;
unsigned offset = pos & (PAGE_SIZE - 1);
size_t count = 0;
int err;
@@ -1166,6 +1164,8 @@ static ssize_t fuse_fill_write_pages(struct fuse_args_pages *ap,
if (!page)
break;

+ fuse_wait_on_page_writeback(inode, page->index);
+
if (mapping_writably_mapped(mapping))
flush_dcache_page(page);

--
1.8.3.1