Re: [bug report] kernel 5.18.0-rc4 oops with invalid wait context

[Chuck Lever III <chuck.lever@oracle.com>]

> On May 11, 2022, at 2:57 PM, Chuck Lever III <chuck.lever@oracle.com> wrote:
> 
> 
> 
>> On May 11, 2022, at 1:31 PM, Trond Myklebust <trondmy@hammerspace.com> wrote:
>> 
>> On Wed, 2022-05-11 at 16:27 +0000, Chuck Lever III wrote:
>>> 
>>> 
>>>> On May 11, 2022, at 12:09 PM, Trond Myklebust
>>>> <trondmy@hammerspace.com> wrote:
>>>> 
>>>> On Wed, 2022-05-11 at 14:57 +0000, Chuck Lever III wrote:
>>>>> 
>>>>> 
>>>>>> On May 10, 2022, at 10:24 AM, Chuck Lever III
>>>>>> <chuck.lever@oracle.com> wrote:
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>>> On May 3, 2022, at 3:11 PM, dai.ngo@oracle.com wrote:
>>>>>>> 
>>>>>>> Hi,
>>>>>>> 
>>>>>>> I just noticed there were couple of oops in my Oracle6 in
>>>>>>> nfs4.dev network.
>>>>>>> I'm not sure who ran which tests (be useful to know) that
>>>>>>> caused
>>>>>>> these oops.
>>>>>>> 
>>>>>>> Here is the stack traces:
>>>>>>> 
>>>>>>> [286123.154006] BUG: sleeping function called from invalid
>>>>>>> context at kernel/locking/rwsem.c:1585
>>>>>>> [286123.155126] in_atomic(): 1, irqs_disabled(): 0,
>>>>>>> non_block: 0,
>>>>>>> pid: 3983, name: nfsd
>>>>>>> [286123.155872] preempt_count: 1, expected: 0
>>>>>>> [286123.156443] RCU nest depth: 0, expected: 0
>>>>>>> [286123.156771] 1 lock held by nfsd/3983:
>>>>>>> [286123.156786]  #0: ffff888006762520 (&clp->cl_lock){+.+.}-
>>>>>>> {2:2}, at: nfsd4_release_lockowner+0x306/0x850 [nfsd]
>>>>>>> [286123.156949] Preemption disabled at:
>>>>>>> [286123.156961] [<0000000000000000>] 0x0
>>>>>>> [286123.157520] CPU: 1 PID: 3983 Comm: nfsd Kdump: loaded Not
>>>>>>> tainted 5.18.0-rc4+ #1
>>>>>>> [286123.157539] Hardware name: innotek GmbH
>>>>>>> VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
>>>>>>> [286123.157552] Call Trace:
>>>>>>> [286123.157565]  <TASK>
>>>>>>> [286123.157581]  dump_stack_lvl+0x57/0x7d
>>>>>>> [286123.157609]  __might_resched.cold+0x222/0x26b
>>>>>>> [286123.157644]  down_read_nested+0x68/0x420
>>>>>>> [286123.157671]  ? down_write_nested+0x130/0x130
>>>>>>> [286123.157686]  ? rwlock_bug.part.0+0x90/0x90
>>>>>>> [286123.157705]  ? rcu_read_lock_sched_held+0x81/0xb0
>>>>>>> [286123.157749]  xfs_file_fsync+0x3b9/0x820
>>>>>>> [286123.157776]  ? lock_downgrade+0x680/0x680
>>>>>>> [286123.157798]  ? xfs_filemap_pfn_mkwrite+0x10/0x10
>>>>>>> [286123.157823]  ? nfsd_file_put+0x100/0x100 [nfsd]
>>>>>>> [286123.157921]  nfsd_file_flush.isra.0+0x1b/0x220 [nfsd]
>>>>>>> [286123.158007]  nfsd_file_put+0x79/0x100 [nfsd]
>>>>>>> [286123.158088]  check_for_locks+0x152/0x200 [nfsd]
>>>>>>> [286123.158191]  nfsd4_release_lockowner+0x4cf/0x850 [nfsd]
>>>>>>> [286123.158393]  ? nfsd4_locku+0xd10/0xd10 [nfsd]
>>>>>>> [286123.158488]  ? rcu_read_lock_bh_held+0x90/0x90
>>>>>>> [286123.158525]  nfsd4_proc_compound+0xd15/0x25a0 [nfsd]
>>>>>>> [286123.158699]  nfsd_dispatch+0x4ed/0xc30 [nfsd]
>>>>>>> [286123.158974]  ? rcu_read_lock_bh_held+0x90/0x90
>>>>>>> [286123.159010]  svc_process_common+0xd8e/0x1b20 [sunrpc]
>>>>>>> [286123.159043]  ? svc_generic_rpcbind_set+0x450/0x450
>>>>>>> [sunrpc]
>>>>>>> [286123.159043]  ? nfsd_svc+0xc50/0xc50 [nfsd]
>>>>>>> [286123.159043]  ? svc_sock_secure_port+0x27/0x40 [sunrpc]
>>>>>>> [286123.159043]  ? svc_recv+0x1100/0x2390 [sunrpc]
>>>>>>> [286123.159043]  svc_process+0x361/0x4f0 [sunrpc]
>>>>>>> [286123.159043]  nfsd+0x2d6/0x570 [nfsd]
>>>>>>> [286123.159043]  ? nfsd_shutdown_threads+0x2a0/0x2a0 [nfsd]
>>>>>>> [286123.159043]  kthread+0x29f/0x340
>>>>>>> [286123.159043]  ? kthread_complete_and_exit+0x20/0x20
>>>>>>> [286123.159043]  ret_from_fork+0x22/0x30
>>>>>>> [286123.159043]  </TASK>
>>>>>>> [286123.187052] BUG: scheduling while atomic:
>>>>>>> nfsd/3983/0x00000002
>>>>>>> [286123.187551] INFO: lockdep is turned off.
>>>>>>> [286123.187918] Modules linked in: nfsd auth_rpcgss nfs_acl
>>>>>>> lockd
>>>>>>> grace sunrpc
>>>>>>> [286123.188527] Preemption disabled at:
>>>>>>> [286123.188535] [<0000000000000000>] 0x0
>>>>>>> [286123.189255] CPU: 1 PID: 3983 Comm: nfsd Kdump: loaded
>>>>>>> Tainted: G        W         5.18.0-rc4+ #1
>>>>>>> [286123.190233] Hardware name: innotek GmbH
>>>>>>> VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
>>>>>>> [286123.190910] Call Trace:
>>>>>>> [286123.190910]  <TASK>
>>>>>>> [286123.190910]  dump_stack_lvl+0x57/0x7d
>>>>>>> [286123.190910]  __schedule_bug.cold+0x133/0x143
>>>>>>> [286123.190910]  __schedule+0x16c9/0x20a0
>>>>>>> [286123.190910]  ? schedule_timeout+0x314/0x510
>>>>>>> [286123.190910]  ? __sched_text_start+0x8/0x8
>>>>>>> [286123.190910]  ? internal_add_timer+0xa4/0xe0
>>>>>>> [286123.190910]  schedule+0xd7/0x1f0
>>>>>>> [286123.190910]  schedule_timeout+0x319/0x510
>>>>>>> [286123.190910]  ? rcu_read_lock_held_common+0xe/0xa0
>>>>>>> [286123.190910]  ? usleep_range_state+0x150/0x150
>>>>>>> [286123.190910]  ? lock_acquire+0x331/0x490
>>>>>>> [286123.190910]  ? init_timer_on_stack_key+0x50/0x50
>>>>>>> [286123.190910]  ? do_raw_spin_lock+0x116/0x260
>>>>>>> [286123.190910]  ? rwlock_bug.part.0+0x90/0x90
>>>>>>> [286123.190910]  io_schedule_timeout+0x26/0x80
>>>>>>> [286123.190910]  wait_for_completion_io_timeout+0x16a/0x340
>>>>>>> [286123.190910]  ? rcu_read_lock_bh_held+0x90/0x90
>>>>>>> [286123.190910]  ? wait_for_completion+0x330/0x330
>>>>>>> [286123.190910]  submit_bio_wait+0x135/0x1d0
>>>>>>> [286123.190910]  ? submit_bio_wait_endio+0x40/0x40
>>>>>>> [286123.190910]  ? xfs_iunlock+0xd5/0x300
>>>>>>> [286123.190910]  ? bio_init+0x295/0x470
>>>>>>> [286123.190910]  blkdev_issue_flush+0x69/0x80
>>>>>>> [286123.190910]  ? blk_unregister_queue+0x1e0/0x1e0
>>>>>>> [286123.190910]  ? bio_kmalloc+0x90/0x90
>>>>>>> [286123.190910]  ? xfs_iunlock+0x1b4/0x300
>>>>>>> [286123.190910]  xfs_file_fsync+0x354/0x820
>>>>>>> [286123.190910]  ? lock_downgrade+0x680/0x680
>>>>>>> [286123.190910]  ? xfs_filemap_pfn_mkwrite+0x10/0x10
>>>>>>> [286123.190910]  ? nfsd_file_put+0x100/0x100 [nfsd]
>>>>>>> [286123.190910]  nfsd_file_flush.isra.0+0x1b/0x220 [nfsd]
>>>>>>> [286123.190910]  nfsd_file_put+0x79/0x100 [nfsd]
>>>>>>> [286123.190910]  check_for_locks+0x152/0x200 [nfsd]
>>>>>>> [286123.190910]  nfsd4_release_lockowner+0x4cf/0x850 [nfsd]
>>>>>>> [286123.190910]  ? nfsd4_locku+0xd10/0xd10 [nfsd]
>>>>>>> [286123.190910]  ? rcu_read_lock_bh_held+0x90/0x90
>>>>>>> [286123.190910]  nfsd4_proc_compound+0xd15/0x25a0 [nfsd]
>>>>>>> [286123.190910]  nfsd_dispatch+0x4ed/0xc30 [nfsd]
>>>>>>> [286123.190910]  ? rcu_read_lock_bh_held+0x90/0x90
>>>>>>> [286123.190910]  svc_process_common+0xd8e/0x1b20 [sunrpc]
>>>>>>> [286123.190910]  ? svc_generic_rpcbind_set+0x450/0x450
>>>>>>> [sunrpc]
>>>>>>> [286123.190910]  ? nfsd_svc+0xc50/0xc50 [nfsd]
>>>>>>> [286123.190910]  ? svc_sock_secure_port+0x27/0x40 [sunrpc]
>>>>>>> [286123.190910]  ? svc_recv+0x1100/0x2390 [sunrpc]
>>>>>>> [286123.190910]  svc_process+0x361/0x4f0 [sunrpc]
>>>>>>> [286123.190910]  nfsd+0x2d6/0x570 [nfsd]
>>>>>>> [286123.190910]  ? nfsd_shutdown_threads+0x2a0/0x2a0 [nfsd]
>>>>>>> [286123.190910]  kthread+0x29f/0x340
>>>>>>> [286123.190910]  ? kthread_complete_and_exit+0x20/0x20
>>>>>>> [286123.190910]  ret_from_fork+0x22/0x30
>>>>>>> [286123.190910]  </TASK>
>>>>>>> 
>>>>>>> The problem is the process tries to sleep while holding the
>>>>>>> cl_lock by nfsd4_release_lockowner. I think the problem was
>>>>>>> introduced with the filemap_flush in nfsd_file_put since
>>>>>>> 'b6669305d35a nfsd: Reduce the number of calls to
>>>>>>> nfsd_file_gc()'.
>>>>>>> The filemap_flush is later replaced by nfsd_file_flush by
>>>>>>> '6b8a94332ee4f nfsd: Fix a write performance regression'.
>>>>>> 
>>>>>> That seems plausible, given the traces above.
>>>>>> 
>>>>>> But it begs the question: why was a vfs_fsync() needed by
>>>>>> RELEASE_LOCKOWNER in this case? I've tried to reproduce the
>>>>>> problem, and even added a might_sleep() call in
>>>>>> nfsd_file_flush()
>>>>>> but haven't been able to reproduce.
>>>>> 
>>>>> Trond, I'm assuming you switched to a synchronous flush here to
>>>>> capture writeback errors. There's no other requirement for
>>>>> waiting for the flush to complete, right?
>>>> 
>>>> It's because the file is unhashed, so it is about to be closed and
>>>> garbage collected as soon as the refcount goes to zero.
>>>> 
>>>>> 
>>>>> To enable nfsd_file_put() to be invoked in atomic contexts again,
>>>>> would the following be a reasonable short term fix:
>>>>> 
>>>>> diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
>>>>> index 2c1b027774d4..96c8d07788f4 100644
>>>>> --- a/fs/nfsd/filecache.c
>>>>> +++ b/fs/nfsd/filecache.c
>>>>> @@ -304,7 +304,7 @@ nfsd_file_put(struct nfsd_file *nf)
>>>>> {
>>>>>        set_bit(NFSD_FILE_REFERENCED, &nf->nf_flags);
>>>>>        if (test_bit(NFSD_FILE_HASHED, &nf->nf_flags) == 0) {
>>>>> -               nfsd_file_flush(nf);
>>>>> +               filemap_flush(nf->nf_file->f_mapping);
>>>>>                nfsd_file_put_noref(nf);
>>>>>        } else {
>>>>>                nfsd_file_put_noref(nf);
>>>>> 
>>>>> 
>>>> 
>>>> filemap_flush() sleeps, and so does nfsd_file_put_noref() (it can
>>>> call
>>>> filp_close() + fput()), so this kind of change isn't going to work
>>>> no
>>>> matter how you massage it.
>>> 
>>> Er. Wouldn't that mean we would have seen "sleep while spinlock is
>>> held" BUGs since nfsd4_release_lockowner() was added? It's done
>>> at least an fput() while holding clp->cl_lock since it was added,
>>> I think.
>> 
>> 
>> There is nothing magical about using WB_SYNC_NONE as far as the
>> writeback code is concerned. write_cache_pages() will still happily
>> call lock_page() and sleep on that lock if it is contended. The
>> writepage/writepages code will happily try to allocate memory if
>> necessary.
>> 
>> The only difference is that it won't sleep waiting for the PG_writeback
>> bit.
>> 
>> So, no, you can't safely call filemap_flush() from a spin locked
>> context, and
>> yes, the bug has been there from day 1. It was not introduced by me.
>> 
>> Also, as I said, nfsd_file_put_noref() is not safe to call from a spin
>> locked context. Again, this was not introduced any time recently.
> 
> OK. I'm trying to figure out how bad the problem is and how
> far back to apply the fix.
> 
> I agree that the arrangement of the code path means
> nfsd4_release_lockowner() has always called fput() or
> filemap_flush() while cl_lock was held.
> 
> But again, I'm not aware of recent instances of this particular
> splat. So I'm wondering if a recent change has made this issue
> easier to hit. We might not be able to answer that until we
> find out how the bake-a-thon testers managed to trigger the
> issue on Dai's server.
> 
> 
>>>> Is there any reason why you need a reference to the nfs_file there?
>>>> Wouldn't holding the fp->fi_lock be sufficient, since you're
>>>> already in
>>>> an atomic context? As long as one of the entries in fp->fi_fds[] is
>>>> non-zero then you should be safe.
>>> 
>>> Sure, check_for_locks() seems to be the only problematic caller.
>>> Other callers appear to be careful to call nfsd_file_put() only
>>> after releasing spinlocks.
>>> 
>>> I would like a fix that can be backported without fuss. I was
>>> thinking changing check_for_locks() might get a little too
>>> hairy for that, but I'll check it out.
> 
> Notably: check_for_locks() needs to drop fi_lock before taking
> flc_lock because the OPEN path can take flc_lock first, then

Bzzt. Not OPEN, but rather nfsd_file_acquire().


> fi_lock, via nfsd_break_deleg_cb(). Holding a reference to the
> nfsd_file guarantees that the inode won't go away while
> check_for_locks() examines the flc_posix list without holding
> fi_lock.
> 
> So my first take on this was we need nfsd4_release_lockowner()
> to drop cl_lock before check_for_locks() is called.
> 
> 
> --
> Chuck Lever

--
Chuck Lever