> On Jan 22, 2024, at 6:31 PM, NeilBrown <neilb@xxxxxxx> wrote:
>
> On Tue, 23 Jan 2024, Chuck Lever III wrote:
>>
>>
>>> On Jan 22, 2024, at 4:57 PM, NeilBrown <neilb@xxxxxxx> wrote:
>>>
>>> On Tue, 23 Jan 2024, Chuck Lever wrote:
>>>> On Mon, Jan 22, 2024 at 02:58:16PM +1100, NeilBrown wrote:
>>>>>
>>>>> The test on so_count in nfsd4_release_lockowner() is nonsense and
>>>>> harmful. Revert to using check_for_locks(), changing that to not sleep.
>>>>>
>>>>> First: harmful.
>>>>> As is documented in the kdoc comment for nfsd4_release_lockowner(), the
>>>>> test on so_count can transiently return a false positive resulting in a
>>>>> return of NFS4ERR_LOCKS_HELD when in fact no locks are held. This is
>>>>> clearly a protocol violation and with the Linux NFS client it can cause
>>>>> incorrect behaviour.
>>>>>
>>>>> If NFS4_RELEASE_LOCKOWNER is sent while some other thread is still
>>>>> processing a LOCK request which failed because, at the time that request
>>>>> was received, the given owner held a conflicting lock, then the nfsd
>>>>> thread processing that LOCK request can hold a reference (conflock) to
>>>>> the lock owner that causes nfsd4_release_lockowner() to return an
>>>>> incorrect error.
>>>>>
>>>>> The Linux NFS client ignores that NFS4ERR_LOCKS_HELD error because it
>>>>> never sends NFS4_RELEASE_LOCKOWNER without first releasing any locks, so
>>>>> it knows that the error is impossible. It assumes the lock owner was in
>>>>> fact released so it feels free to use the same lock owner identifier in
>>>>> some later locking request.
>>>>>
>>>>> When it does reuse a lock owner identifier for which a previous RELEASE
>>>>> failed, it will naturally use a lock_seqid of zero. However the server,
>>>>> which didn't release the lock owner, will expect a larger lock_seqid and
>>>>> so will respond with NFS4ERR_BAD_SEQID.
>>>>>
>>>>> So clearly it is harmful to allow a false positive, which testing
>>>>> so_count allows.
>>>>>
>>>>> The test is nonsense because ... well... it doesn't mean anything.
>>>>>
>>>>> so_count is the sum of three different counts.
>>>>> 1/ the set of states listed on so_stateids
>>>>> 2/ the set of active vfs locks owned by any of those states
>>>>> 3/ various transient counts such as for conflicting locks.
>>>>>
>>>>> When it is tested against '2' it is clear that one of these is the
>>>>> transient reference obtained by find_lockowner_str_locked(). It is not
>>>>> clear what the other one is expected to be.
>>>>>
>>>>> In practice, the count is often 2 because there is precisely one state
>>>>> on so_stateids. If there were more, this would fail.
>>>>>
>>>>> It my testing I see two circumstances when RELEASE_LOCKOWNER is called.
>>>>> In one case, CLOSE is called before RELEASE_LOCKOWNER. That results in
>>>>> all the lock states being removed, and so the lockowner being discarded
>>>>> (it is removed when there are no more references which usually happens
>>>>> when the lock state is discarded). When nfsd4_release_lockowner() finds
>>>>> that the lock owner doesn't exist, it returns success.
>>>>>
>>>>> The other case shows an so_count of '2' and precisely one state listed
>>>>> in so_stateid. It appears that the Linux client uses a separate lock
>>>>> owner for each file resulting in one lock state per lock owner, so this
>>>>> test on '2' is safe. For another client it might not be safe.
>>>>>
>>>>> So this patch changes check_for_locks() to use the (newish)
>>>>> find_any_file_locked() so that it doesn't take a reference on the
>>>>> nfs4_file and so never calls nfsd_file_put(), and so never sleeps.
>>>>
>>>> More to the point, find_any_file_locked() was added by commit
>>>> e0aa651068bf ("nfsd: don't call nfsd_file_put from client states
>>>> seqfile display"), which was merged several months /after/ commit
>>>> ce3c4ad7f4ce ("NFSD: Fix possible sleep during
>>>> nfsd4_release_lockowner()").
>>>
>>> Yes. To flesh out the history:
>>> nfsd_file_put() was added in v5.4. In earlier kernels check_for_locks()
>>> would never sleep. However the problem patch was backported 4.9, 4.14,
>>> and 4.19 and should be reverted.
>>
>> I don't see "NFSD: Fix possible sleep during nfsd4_release_lockowner()"
>> in any of those kernels. All but 4.19 are now EOL.
>
> I hadn't checked which were EOL. Thanks for finding the 4.19 patch and
> requesting a revert.
>
>>
>>
>>> find_any_file_locked() was added in v6.2 so when this patch is
>>> backported to 5.4, 5.10, 5.15, 5.17 - 6.1 it needs to include
>>> find_and_file_locked()
>>
>> I think I'd rather leave those unperturbed until someone hits a real
>> problem. Unless you have a distribution kernel that needs to see
>> this fix in one of the LTS kernels? The supported stable/LTS kernels
>> are 5.4, 5.10, 5.15, and 6.1.
>
> Why not fix the bug? It's a real bug that a real customer really hit.
That's what I'm asking. Was there a real customer issue? Because
ce3c4ad7f4ce was the result of a might_sleep splat, not an issue
in the field.
Since this hit a real user, is there a BugLink: or Closes: tag
I can add?
> I've fixed it in all SLE kernels - we don't depend on LTS though we do
> make use of the stable trees in various ways.
>
> But it's your call.
Well, it's not my call whether it's backported. Anyone can ask.
It /is/ my call if I do the work.
Mostly the issue is having the time to manage 5-6 stable
kernels as well as upstream development. I'd like to ensure
that any patches for which we manually request a backport have
been applied and tested by someone with a real NFS rig, and
there is a real problem getting addressed.
It's getting better with kdevops... I can cherry pick any
recent kernel and run it through a number of tests without
needing to hand-hold. Haven't tried that with the older stable
kernels, though; those require older compilers and what-not.
--
Chuck Lever
