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

[Wenle Chen]

Olga Kornievskaia 於 2020/11/4 下午9:22 寫道:
> On Wed, Nov 4, 2020 at 6:36 AM Wenle Chen <solomonchenclever@xxxxxxxxx> 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@xxxxxxxxxx>
> > > > ---
> > > >    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.
In the function nfs_end_delegation_return, it would get the return 
err=-EAGAIN and check the client is active and get a retry. I has so 
thought. Maybe I think wrong. I will understand more carefully. Thinks.