[bug report v4.8] fs/locks.c: kernel oops during posix lock stress test

[Ming Lei]

Hi,

On Mon, Nov 28, 2016 at 9:40 PM, Jeff Layton <jlayton at poochiereds.net> wrote:
> On Mon, 2016-11-28 at 11:10 +0800, Ming Lei wrote:
>> Hi Guys,
>>
>> When I run stress-ng via the following steps on one ARM64 dual
>> socket system(Cavium Thunder), the kernel oops[1] can often be
>> triggered after running the stress test for several hours(sometimes
>> it may take longer):
>>
>> - git clone git://kernel.ubuntu.com/cking/stress-ng.git
>> - apply the attachment patch which just makes the posix file
>> lock stress test more aggressive
>> - run the test via '~/git/stress-ng$./stress-ng --lockf 128 --aggressive'
>>
>>
>> From the oops log, looks one garbage file_lock node is got
>> from the linked list of 'ctx->flc_posix' when the issue happens.
>>
>> BTW, the issue isn't observed on single socket Cavium Thunder yet,
>> and the same issue can be seen on Ubuntu Xenial(v4.4 based kernel)
>> too.
>>
>> Thanks,
>> Ming
>>
>
> Some questions just for clarification:
>
> - I assume this is being run on a local fs of some sort? ext4 or xfs or
> something?
>
> - have you seen this on any other arch, besides ARM?
>
> The file locking code does do some lockless checking to see whether the
> i_flctx is even present and whether the list is empty in
> locks_remove_posix. It's possible we have some barrier problems there,

I have used ebpf trace to see what is going on when 'stress-ng --lockf'
is running, and almost all exported symbols in fs/locks.c are covered.

Except for locks_alloc/locks_free/locks_copy/locks_init, the only observable
symbols are fcntl_setlk, vfs_lock_file and locks_remove_posix, but
locks_remove_posix() is just run at the begining and ending of the
test.

So seems not related with locks_remove_posix().

Then looks only fcntl_setlk() is running from different contexts
during the test,
but in this path, the 'ctx->flc_lock' is always held when operating the list.
That said it is very strange to see the list corrupted even though it is
protected by the lock.

Thanks,
Ming

> but I don't quite see how that would cause us to have a corrupt lock on
> the flc_posix list.
>