From: Stephen Brennan <stephen.s.brennan@oracle.com>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Jan Kara <jack@suse.cz>, Alexander Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/2] fsnotify: fix softlockups iterating over d_subdirs
Date: Thu, 27 Oct 2022 15:06:48 -0700 [thread overview]
Message-ID: <87czachqfb.fsf@oracle.com> (raw)
In-Reply-To: <CAOQ4uxhwFGddgJP5xPYDysoa4GFPYu6Bj7rgHVXTEuZk+QKYQQ@mail.gmail.com>
Amir Goldstein <amir73il@gmail.com> writes:
> On Wed, Oct 19, 2022 at 2:52 AM Stephen Brennan
> <stephen.s.brennan@oracle.com> wrote:
>>
>> Amir Goldstein <amir73il@gmail.com> writes:
>> > On Tue, Oct 18, 2022 at 7:12 AM Stephen Brennan
>> > <stephen.s.brennan@oracle.com> wrote:
>> >>
>> >> Hi Jan, Amir, Al,
>> >>
>> >> Here's my first shot at implementing what we discussed. I tested it using the
>> >> negative dentry creation tool I mentioned in my previous message, with a similar
>> >> workflow. Rather than having a bunch of threads accessing the directory to
>> >> create that "thundering herd" of CPUs in __fsnotify_update_child_dentry_flags, I
>> >> just started a lot of inotifywait tasks:
>> >>
>> >> 1. Create 100 million negative dentries in a dir
>> >> 2. Use trace-cmd to watch __fsnotify_update_child_dentry_flags:
>> >> trace-cmd start -p function_graph -l __fsnotify_update_child_dentry_flags
>> >> sudo cat /sys/kernel/debug/tracing/trace_pipe
>> >> 3. Run a lot of inotifywait tasks: for i in {1..10} inotifywait $dir & done
>> >>
>> >> With step #3, I see only one execution of __fsnotify_update_child_dentry_flags.
>> >> Once that completes, all the inotifywait tasks say "Watches established".
>> >> Similarly, once an access occurs in the directory, a single
>> >> __fsnotify_update_child_dentry_flags execution occurs, and all the tasks exit.
>> >> In short: it works great!
>> >>
>> >> However, while testing this, I've observed a dentry still in use warning during
>> >> unmount of rpc_pipefs on the "nfs" dentry during shutdown. NFS is of course in
>> >> use, and I assume that fsnotify must have been used to trigger this. The error
>> >> is not there on mainline without my patch so it's definitely caused by this
>> >> code. I'll continue debugging it but I wanted to share my first take on this so
>> >> you could take a look.
>> >>
>> >> [ 1595.197339] BUG: Dentry 000000005f5e7197{i=67,n=nfs} still in use (2) [unmount of rpc_pipefs rpc_pipefs]
>> >>
>> >
>> > Hmm, the assumption we made about partial stability of d_subdirs
>> > under dir inode lock looks incorrect for rpc_pipefs.
>> > None of the functions that update the rpc_pipefs dcache take the parent
>> > inode lock.
>>
>> That may be, but I'm confused how that would trigger this issue. If I'm
>> understanding correctly, this warning indicates a reference counting
>> bug.
>
> Yes.
> On generic_shutdown_super() there should be no more
> references to dentries.
>
>>
>> If __fsnotify_update_child_dentry_flags() had gone to sleep and the list
>> were edited, then it seems like there could be only two possibilities
>> that could cause bugs:
>>
>> 1. The dentry we slept holding a reference to was removed from the list,
>> and maybe moved to a different one, or just removed. If that were the
>> case, we're quite unlucky, because we'll start looping indefinitely as
>> we'll never get back to the beginning of the list, or worse.
>>
>> 2. A dentry adjacent to the one we held a reference to was removed. In
>> that case, our dentry's d_child pointers should get rearranged, and when
>> we wake, we should see those updates and continue.
>>
>> In neither of those cases do I understand where we could have done a
>> dget() unpaired with a dput(), which is what seemingly would trigger
>> this issue.
>>
>
> I got the same impression.
Well I feel stupid. The reason behind this seems to be... that
d_find_any_alias() returns a reference to the dentry, and I promptly
leaked that. I'll have it fixed in v3 which I'm going through testing
now.
Stephen
next prev parent reply other threads:[~2022-10-27 22:07 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-13 22:27 [RFC] fsnotify: allow sleepable child dentry flag update Stephen Brennan
2022-10-13 23:51 ` Al Viro
2022-11-01 21:47 ` Stephen Brennan
2022-10-14 8:01 ` Amir Goldstein
2022-10-17 7:59 ` Stephen Brennan
2022-10-17 11:44 ` Amir Goldstein
2022-10-17 16:59 ` Stephen Brennan
2022-10-17 17:42 ` Amir Goldstein
2022-10-17 9:09 ` Jan Kara
2022-10-18 4:12 ` [PATCH 0/2] fsnotify: fix softlockups iterating over d_subdirs Stephen Brennan
2022-10-18 4:12 ` [PATCH 1/2] fsnotify: Protect i_fsnotify_mask and child flags with inode rwsem Stephen Brennan
2022-10-18 7:39 ` Amir Goldstein
2022-10-21 0:33 ` Stephen Brennan
2022-10-21 7:22 ` Amir Goldstein
2022-10-18 4:12 ` [PATCH 2/2] fsnotify: allow sleepable child flag update Stephen Brennan
2022-10-18 5:36 ` Amir Goldstein
2022-10-27 7:50 ` kernel test robot
2022-10-27 8:44 ` Yujie Liu
2022-10-27 22:12 ` Stephen Brennan
2022-10-18 8:07 ` [PATCH 0/2] fsnotify: fix softlockups iterating over d_subdirs Amir Goldstein
2022-10-18 23:52 ` Stephen Brennan
2022-10-19 5:33 ` Amir Goldstein
2022-10-27 22:06 ` Stephen Brennan [this message]
2022-10-28 8:58 ` Amir Goldstein
2022-10-21 1:03 ` [PATCH v2 0/3] " Stephen Brennan
2022-10-21 1:03 ` [PATCH v2 1/3] fsnotify: Use d_find_any_alias to get dentry associated with inode Stephen Brennan
2022-10-21 9:25 ` Amir Goldstein
2022-10-21 1:03 ` [PATCH v2 2/3] fsnotify: Protect i_fsnotify_mask and child flags with inode rwsem Stephen Brennan
2022-10-21 4:01 ` kernel test robot
2022-10-21 8:22 ` Amir Goldstein
2022-10-21 9:18 ` Amir Goldstein
2022-10-25 18:02 ` Stephen Brennan
2022-10-26 5:41 ` Amir Goldstein
2022-10-21 9:17 ` Christian Brauner
2022-10-21 9:21 ` Amir Goldstein
2022-10-21 1:03 ` [PATCH v2 3/3] fsnotify: allow sleepable child flag update Stephen Brennan
2022-10-28 0:10 ` [PATCH v3 0/3] fsnotify: fix softlockups iterating over d_subdirs Stephen Brennan
2022-10-28 0:10 ` [PATCH v3 1/3] fsnotify: Use d_find_any_alias to get dentry associated with inode Stephen Brennan
2022-11-10 1:12 ` Stephen Brennan
2022-10-28 0:10 ` [PATCH v3 2/3] fsnotify: Protect i_fsnotify_mask and child flags with inode rwsem Stephen Brennan
2022-10-28 9:11 ` Amir Goldstein
2022-11-10 0:03 ` kernel test robot
2022-11-10 1:06 ` Stephen Brennan
2022-10-28 0:10 ` [PATCH v3 3/3] fsnotify: allow sleepable child flag update Stephen Brennan
2022-10-28 9:32 ` Amir Goldstein
2022-11-01 21:25 ` Stephen Brennan
2022-11-01 17:51 ` [PATCH v3 0/3] fsnotify: fix softlockups iterating over d_subdirs Jan Kara
2022-11-01 20:48 ` Stephen Brennan
2022-11-02 8:55 ` Amir Goldstein
2022-11-10 20:04 ` Stephen Brennan
2022-11-02 17:52 ` Jan Kara
2022-11-04 23:33 ` Stephen Brennan
2022-11-07 11:56 ` Jan Kara
2022-11-11 22:06 ` [PATCH v4 0/5] " Stephen Brennan
2022-11-11 22:06 ` [PATCH v4 1/5] fsnotify: clear PARENT_WATCHED flags lazily Stephen Brennan
2022-11-11 22:06 ` [PATCH v4 2/5] fsnotify: Use d_find_any_alias to get dentry associated with inode Stephen Brennan
2022-11-12 8:53 ` Amir Goldstein
2022-11-11 22:06 ` [PATCH v4 3/5] dnotify: move fsnotify_recalc_mask() outside spinlock Stephen Brennan
2022-11-12 9:06 ` Amir Goldstein
2022-11-11 22:06 ` [PATCH v4 4/5] fsnotify: allow sleepable child flag update Stephen Brennan
2022-11-12 10:00 ` Amir Goldstein
2022-11-15 7:10 ` kernel test robot
2022-11-11 22:06 ` [PATCH v4 5/5] fsnotify: require inode lock held during " Stephen Brennan
2022-11-12 9:42 ` Amir Goldstein
2022-11-11 22:08 ` [PATCH v4 0/5] fsnotify: fix softlockups iterating over d_subdirs Stephen Brennan
2022-11-22 11:50 ` Jan Kara
2022-11-22 14:03 ` Amir Goldstein
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