Re: [PATCH 2/2] NFSv4: Fix dropped lock for racing OPEN and delegation return

[Trond Myklebust <trondmy@hammerspace.com>]

On Tue, 2023-06-27 at 14:31 -0400, Benjamin Coddington wrote:
> Commmit f5ea16137a3f ("NFSv4: Retry LOCK on OLD_STATEID during
> delegation
> return") attempted to solve this problem by using nfs4's generic
> async error
> handling, but introduced a regression where v4.0 lock recovery would
> hang.
> The additional complexity introduced by overloading that error
> handling is
> not necessary for this case.
> 
> The problem as originally explained in the above commit is:
> 
>     There's a small window where a LOCK sent during a delegation
> return can
>     race with another OPEN on client, but the open stateid has not
> yet been
>     updated.  In this case, the client doesn't handle the OLD_STATEID
> error
>     from the server and will lose this lock, emitting:
>     "NFS: nfs4_handle_delegation_recall_error: unhandled error -
> 10024".
> 
> We want a fix that is much more focused to the original problem.  Fix
> this
> issue by returning -EAGAIN from the
> nfs4_handle_delegation_recall_error() on
> OLD_STATEID, and use that as a signal for the delegation return code
> to
> retry the LOCK operation.  We should at this point be able to send
> along
> the updated stateid.
> 
> Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
> ---
>  fs/nfs/delegation.c | 4 +++-
>  fs/nfs/nfs4proc.c   | 1 +
>  2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/nfs/delegation.c b/fs/nfs/delegation.c
> index cf7365581031..23aeb02319a5 100644
> --- a/fs/nfs/delegation.c
> +++ b/fs/nfs/delegation.c
> @@ -160,7 +160,9 @@ static int nfs_delegation_claim_locks(struct
> nfs4_state *state, const nfs4_state
>                 if (nfs_file_open_context(fl->fl_file)->state !=
> state)
>                         continue;
>                 spin_unlock(&flctx->flc_lock);
> -               status = nfs4_lock_delegation_recall(fl, state,
> stateid);
> +               do {
> +                       status = nfs4_lock_delegation_recall(fl,
> state, stateid);
> +               } while (status == -EAGAIN);
>                 if (status < 0)
>                         goto out;
>                 spin_lock(&flctx->flc_lock);
> diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
> index 6bb14f6cfbc0..399db73a57f4 100644
> --- a/fs/nfs/nfs4proc.c
> +++ b/fs/nfs/nfs4proc.c
> @@ -2262,6 +2262,7 @@ static int
> nfs4_handle_delegation_recall_error(struct nfs_server *server, struct
>                 case -NFS4ERR_BAD_HIGH_SLOT:
>                 case -NFS4ERR_CONN_NOT_BOUND_TO_SESSION:
>                 case -NFS4ERR_DEADSESSION:
> +               case -NFS4ERR_OLD_STATEID:
>                         return -EAGAIN;

Hmm... Rather than issuing a blanket EAGAIN, we really should be
looking at using either nfs4_refresh_lock_old_stateid() or
nfs4_refresh_open_old_stateid(), depending on whether the stateid that
saw the NFS4ERR_OLD_STATEID was a lock stateid or an open stateid.

>                 case -NFS4ERR_STALE_CLIENTID:
>                 case -NFS4ERR_STALE_STATEID:

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com