linux-kernel - Re: [PATCH 2/2] NFS: Limit the number of retries

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Date:   Wed, 4 Nov 2020 08:22:20 -0500
From:   Olga Kornievskaia <aglo@...ch.edu>
To:     Wenle Chen <solomonchenclever@...il.com>
Cc:     Trond Myklebust <trondmy@...merspace.com>,
        "anna.schumaker@...app.com" <anna.schumaker@...app.com>,
        "chenwenle@...wei.com" <chenwenle@...wei.com>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "nixiaoming@...wei.com" <nixiaoming@...wei.com>
Subject: Re: [PATCH 2/2] NFS: Limit the number of retries

On Wed, Nov 4, 2020 at 6:36 AM Wenle Chen <solomonchenclever@...il.com> wrote:
>
>
>
> Trond Myklebust 於 2020/11/3 上午1:45 寫道:
> > On Tue, 2020-11-03 at 00:24 +0800, Wenle Chen wrote:
> >>    We can't wait forever, even if the state
> >> is always delayed.
> >>
> >> Signed-off-by: Wenle Chen <chenwenle@...wei.com>
> >> ---
> >>   fs/nfs/nfs4proc.c | 4 +++-
> >>   1 file changed, 3 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
> >> index f6b5dc792b33..bb2316bf13f6 100644
> >> --- a/fs/nfs/nfs4proc.c
> >> +++ b/fs/nfs/nfs4proc.c
> >> @@ -7390,15 +7390,17 @@ int nfs4_lock_delegation_recall(struct
> >> file_lock *fl, struct nfs4_state *state,
> >>   {
> >>          struct nfs_server *server = NFS_SERVER(state->inode);
> >>          int err;
> >> +       int retry = 3;
> >>
> >>          err = nfs4_set_lock_state(state, fl);
> >>          if (err != 0)
> >>                  return err;
> >>          do {
> >>                  err = _nfs4_do_setlk(state, F_SETLK, fl,
> >> NFS_LOCK_NEW);
> >> -               if (err != -NFS4ERR_DELAY)
> >> +               if (err != -NFS4ERR_DELAY || retry == 0)
> >>                          break;
> >>                  ssleep(1);
> >> +               --retry;
> >>          } while (1);
> >>          return nfs4_handle_delegation_recall_error(server, state,
> >> stateid, fl, err);
> >>   }
> >
> > This patch will just cause the locks to be silently lost, no?
> >
> This loop was introduced in commit 3d7a9520f0c3e to simplify the delay
> retry loop. Before this, the function nfs4_lock_delegation_recall would
> return a -EAGAIN to do a whole retry loop.

This commit was not simplifying retry but actually handling the error.
Without it the error isn't handled and client falsely thinks it holds
the lock. Limiting the number of retries as Trond points out would
lead to the same problem which in the end is data corruption.
Alternative would be to fail the application. However ERR_DELAY is a
transient error and the server would, when ready, return something
else. If server is broken and continues to do so then the server needs
to be fix (client isn't coded to the broken server). I don't see a
good argument for limiting the number of re-tries.

> When we retried three times and waited three seconds, it was still in
> delay. I think we can get a whole loop and check the other points if it
> was changed or not. It is just a proposal.