在 2023/6/30 10:19, yangerkun 写道:
在 2023/6/30 6:24, Dave Chinner 写道:
On Thu, Jun 29, 2023 at 07:55:10PM +0800, yangerkun wrote:
在 2023/6/29 7:10, Dave Chinner 写道:
On Tue, Jun 27, 2023 at 04:42:41PM +0800, yangerkun wrote:
在 2023/6/27 5:45, Dave Chinner 写道:
On Mon, Jun 26, 2023 at 09:15:42PM +0800, yangerkun wrote:
From: yangerkun <yangerkun@xxxxxxxxxx>
Combine use of xfs_trans_hold and xfs_trans_set_sync in
xfs_sync_sb_buf
can trigger a deadlock once shutdown happened concurrently.
xlog_ioend_work
will first unpin the sb(which stuck with xfs_buf_lock), then wakeup
xfs_sync_sb_buf. However, xfs_sync_sb_buf never get the chance to
unlock
sb until been wakeup by xlog_ioend_work.
xfs_sync_sb_buf
xfs_trans_getsb // lock sb buf
xfs_trans_bhold // sb buf keep lock until success commit
xfs_trans_commit
...
xfs_log_force_seq
xlog_force_lsn
xlog_wait_on_iclog
xlog_wait(&iclog->ic_force_wait... // shutdown happened
xfs_buf_relse // unlock sb buf
xlog_ioend_work
xlog_force_shutdown
xlog_state_shutdown_callbacks
xlog_cil_process_committed
xlog_cil_committed
...
xfs_buf_item_unpin
xfs_buf_lock // deadlock
wake_up_all(&iclog->ic_force_wait)
xfs_ioc_setlabel use xfs_sync_sb_buf to make sure userspace will
see the
change for sb immediately. We can simply call
xfs_ail_push_all_sync to
do this and sametime fix the deadlock.
Why is this deadlock specific to the superblock buffer?
Hi Dave,
Thanks a lot for your revirew! We find this problem when do some code
reading(which can help us to fix another growfs bug). And then
reproduce it
easily when we set label online frequently with IO error inject at the
sametime.
Right, I know how it can be triggered; that's not actually my
concern...
Can't any buffer that is held locked over a synchronous transaction
commit deadlock during a shutdown like this?
After check all place use xfs_buf_bhold, it seems xfs_sync_sb_buf
is the
only convict that combine use xfs_trans_hold and
xfs_trans_set_sync(I'm not
familiar with xfs yet, so I may have some problems with my code
check)...
Yes, I can also see that. But my concern is that this change only
addresses the symptom, but leaves the underlying deadlock unsolved.
Indeed, this isn't xfs_trans_commit() I'm worried about here; it's
the call to xfs_log_force(mp, XFS_LOG_SYNC) or
xfs_log_force_seq(XFS_LOG_SYNC) with a buffer held locked that I'm
worried about.
i.e. We have a buffer in the CIL (from a previous transaction) that
we currently hold locked while we call xfs_log_force(XFS_LOG_SYNC).
If a shutdown occurs while we are waiting for journal IO completion
to occur, then xlog_ioend_work() will attempt to lock the buffer and
deadlock, right?
e.g. I'm thinking of things like busy extent flushing (hold AGF +
AGFL + AG btree blocks locked when we call xfs_log_force()) could
also be vulnerable to the same deadlock...
You mean something like xfs_allocbt_alloc_block(call xfs_log_force to
flush busy extent which keep agf locked sametime)?
We call xfs_log_force(mp, XFS_LOG_SYNC) after lock agf and before
xfs_trans_commit. It seems ok since xfs_buf_item_unpin will not call
xfs_buf_lock because bli_refcount still keep active(once we hold locked
agf, the bli_refcount will inc in _xfs_trans_bjoin, and keep it until
xfs_trans_commit success(clean agf item) or .iop_unpin(dirty agf item,
call from xlog_ioend_work) which can be called after xfs_trans_commit
too)...
Again, I gave an example of the class of issue I'm worried about.
Again, you chased the one example given through, but haven't
mentioned a thing about all the other code paths that lead to
xfs_log_force(SYNC) that might hold buffers locked that I didn't
mention.
I don't want to have to ask every person who proposes a fix about
every possible code path the bug may manifest in -one at a time-. I
use examples to point you in the right direction for further
analysis of the rest of the code base, not because that's the only
thing I want checked. Please use your initiative to look at all the
callers of xfs_log_force(SYNC) and determine if they are all safe or
whether there are landmines lurked or even more bugs of a similar
sort.
Hi Dave,
Thank you very much for pointing this out! I'm so sorry for the lack of
awareness of a comprehensive investigation does there any other place
can trigger the bug too...
When we learn about a new issue, this is the sort of audit work that
is necessary to determine the scope of the issue. We need to perform
such audits because they direct the scope of the fix necessary. We
are not interested in slapping a band-aid fix over the symptom that
was reported - that only leads to more band-aid fixes as the same
issue appears in other places.
Yes, agree with you and thanks for your advise, it can really help me to
forbid a band-aid fix however leads to more band-aid fixes, so can
contribute better!
Now we know there is a lock ordering problem in this code, so before
we attempt to fix it we need to know how widespread it is, what the
impact is, how different code paths avoid it, etc. That requires a
code audit to determine, and that requires looking at all the paths
into xfs_log_force(XFS_LOG_SYNC) to determine if they are safe or
not and documenting that.
Yes, it's more work *right now* than slapping a quick band-aid fix
over it, but it's much less work in the long run for us and we don't
have to keep playing whack-a-mole because we fixed it the right way
the first time.
I will try to look all paths into xfs_log_force(XFS_LOG_SYNC) or
xfs_log_force_seq(XFS_LOG_SYNC) to check if it's safe or not. Thanks
again for your advise!
Thanks,
Yang Erkun.
-Dave.
Hi, Dave,
Sorry for the late reply, I was quiet busy last month and it also took
me long time to check does all the callers of
xfs_log_force(SYNC)/xfs_log_force_seq(SYNC) was safe. I'm not familiar
with xfs yet, so if there's anything wrong with the description below,
please point it out!
The logic I choose was to check will we call
xfs_log_force(SYNC)/xfs_log_force_seq(SYNC) between
xfs_buf_lock/xfs_buf_trylock and xfs_buf_unlock at the same thread
context(I have check other item's .iop_unpin, it seems only xfs_buf item
can trigger the problem since it will try to lock the buf in
xfs_buf_item_unpin; besides, different thread context call for
xfs_buf_lock&xfs_log_force(SYNC)/xfs_log_force_seq(SYNC) and
xfs_buf_unlock is safe too since this unlock will not wait until
xfs_log_force(SYNC)/xfs_log_force_seq(SYNC) success return), and once it
happend, will we trigger the bug too?
I divide the logic of calling xfs_buf_lock/xfs_buf_trylock into two
categories:
1. Later the xfs_buf will join the tp(xfs_trans_bjoin will inc
.bli_refcount)
a. xfs_trans_bjoin will inc .bli_refcount
b. xfs_buf_item_pin will inc .bli_refcount when the item of xfs_buf was
dirty
c. xfs_buf_item_committing will dec .bli_refcount no matter the item was
dirty or not, and normally it will unlock the xfs_buf, or keep the
xfs_buf locked when we see XFS_BLI_HOLD
d. xfs_buf_item_unpin will dec .bli_refcount, and it won't call
xfs_buf_lock when another .bli_refcount exist
xfs_log_force(SYNC)/xfs_log_force_seq(SYNC) can happend before we commit
the tp(like xfs_create, it will first read&lock agi buf, then call
xfs_dir_createname, which may trigger agfl fixup, the block allocation
may see the busy extent and call xfs_log_force(SYNC) to flush the busy
extent journal). It won't trigger the problem since xfs_trans_bjoin will
keep another .bli_refcount.
xfs_log_force_seq(SYNC) can happend when we commit the tp since we see
XFS_TRANS_SYNC(see __xfs_trans_commit), the only case we can trigger the
deadlock was that we have combine called xfs_trans_bhold, or we will
unlock xfs_buf in xfs_buf_item_committing. The case which this patch try
to fix was the only case combine call for xfs_trans_bhold and set
XFS_TRANS_SYNC.
After commit tp, xfs_buf will only keep locked because of
xfs_trans_bhold, and once there is a XFS_TRANS_PERM_LOG_RES tp, we may
trigger another commit, then xfs_log_force(SYNC)/xfs_log_force_seq(SYNC)
can happend too. But it is safe too since we will first rejoin xfs_buf
to tp which help protect us.
2. The xfs_buf won't join the tp
xfs_buf_readahead_map
xfs_buf_read
xfs_buf_get
xfs_buf_incore
xfs_buf_delwri_cancel
xfs_buf_delwri_submit_buffers
xfs_buf_delwri_pushbuf
xfs_buf_item_unpin
xfs_iflush_shutdown_abort
xfs_log_quiesce
xlog_do_recover
xfs_freesb
xfs_add_incompat_log_feature
xfs_clear_incompat_log_features
Most case above was io, for xfs_buf_unlock called from another thread
context, it is safe; for xfs_buf_unlock called from same thread context,
it is safe too since we won't trigger
xfs_log_force(SYNC)/xfs_log_force_seq(SYNC) between xfs_buf_lock and
xfs_buf_unlock.
From above, it seems only xfs_sync_sb_buf can trigger this deadlock...
And I prefer to add some comments to xfs_trans_bhold to notice that
there is a bug when combine use xfs_trans_bhold and
xfs_log_force(SYNC)/xfs_log_force_seq(SYNC)...
Dave, sorry again for the late reply, and look forward to your reply!
Thanks,
Yang Erkun.
