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

[Baolin Wang]

Hi,

在 2021/4/13 16:57, Miklos Szeredi 写道:
> On Mon, Apr 12, 2021 at 3:23 PM Baolin Wang
> <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
> > Hi Miklos,
> >
> > 在 2021/3/27 14:36, Baolin Wang 写道:
> > > 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@xxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> >
> > Do you have any comments for this patch set? Thanks.
>
> Hi,
>
> I guess this is related:
>
> https://lore.kernel.org/linux-fsdevel/20210209100115.GB1208880@xxxxxxxxxxxxxxxxxxxxxxxxx/
>
> Can you verify that the patch at the above link fixes your issue?

Sorry I missed this patch before, and I've tested this patch, it seems 
can solve the deadlock issue I met before.

But look at this patch in detail, I think this patch only reduced the 
deadlock window, but did not remove the possible deadlock scenario 
completely like I explained in the commit log.

Since the fuse_fill_write_pages() can still lock the partitail page in 
your patch, and will be wait for the partitail page waritehack is 
completed if writeback is set in fuse_send_write_pages().

But at the same time, a writeback worker thread may be waiting for 
trying to lock the partitail page to write a bunch of dirty pages by 
fuse_writepages().

Then the deadlock issue can still be occurred. And I think the deadlock 
issue I met is not same with the deadlock issue solved by your patch.