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From: "NeilBrown" <neilb@suse.de>
To: "Chuck Lever III" <chuck.lever@oracle.com>
Cc: "Jeff Layton" <jlayton@kernel.org>, "Dai Ngo" <dai.ngo@oracle.com>,
 "Olga Kornievskaia" <kolga@netapp.com>, "Tom Talpey" <tom@talpey.com>,
 "Linux NFS Mailing List" <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH] nfsd: fix RELEASE_LOCKOWNER
In-reply-to: <3162C5BC-8E7C-4A9A-815C-09297B56FA17@oracle.com>
References: <170589589641.23031.16356786177193106749@noble.neil.brown.name>,
 <Za57adpDbKJavMRO@tissot.1015granger.net>,
 <170596063560.23031.1725209290511630080@noble.neil.brown.name>,
 <3162C5BC-8E7C-4A9A-815C-09297B56FA17@oracle.com>
Date: Tue, 23 Jan 2024 10:31:43 +1100
Message-id: <170596630337.23031.332959396445243083@noble.neil.brown.name>

On Tue, 23 Jan 2024, Chuck Lever III wrote:
> 
> 
> > On Jan 22, 2024, at 4:57 PM, NeilBrown <neilb@suse.de> 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.
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.

Thanks,
NeilBrown


> 
> 
> > The patch should apply unchanged to stable kernels 6.2 and later.
> 
> I can add a Cc: stable #v6.2+
> 
> 
> --
> Chuck Lever
> 
> 
>