Re: [PATCH] NFSv4: fix stateid refreshing when CLOSE racing with OPEN

[Murphy Zhou <jencce.kernel@gmail.com>]

On Thu, Oct 10, 2019 at 02:46:40PM +0000, Trond Myklebust wrote:
> On Thu, 2019-10-10 at 15:40 +0800, Murphy Zhou wrote:
> > Since commit:
> >   [0e0cb35] NFSv4: Handle NFS4ERR_OLD_STATEID in CLOSE/OPEN_DOWNGRADE
> > 
> > xfstests generic/168 on v4.2 starts to fail because reflink call
> > gets:
> >   +XFS_IOC_CLONE_RANGE: Resource temporarily unavailable
> > 
> > In tshark output, NFS4ERR_OLD_STATEID stands out when comparing with
> > good ones:
> > 
> >  5210   NFS 406 V4 Reply (Call In 5209) OPEN StateID: 0xadb5
> >  5211   NFS 314 V4 Call GETATTR FH: 0x8d44a6b1
> >  5212   NFS 250 V4 Reply (Call In 5211) GETATTR
> >  5213   NFS 314 V4 Call GETATTR FH: 0x8d44a6b1
> >  5214   NFS 250 V4 Reply (Call In 5213) GETATTR
> >  5216   NFS 422 V4 Call WRITE StateID: 0xa818 Offset: 851968 Len:
> > 65536
> >  5218   NFS 266 V4 Reply (Call In 5216) WRITE
> >  5219   NFS 382 V4 Call OPEN DH: 0x8d44a6b1/
> >  5220   NFS 338 V4 Call CLOSE StateID: 0xadb5
> >  5222   NFS 406 V4 Reply (Call In 5219) OPEN StateID: 0xa342
> >  5223   NFS 250 V4 Reply (Call In 5220) CLOSE Status:
> > NFS4ERR_OLD_STATEID
> >  5225   NFS 338 V4 Call CLOSE StateID: 0xa342
> >  5226   NFS 314 V4 Call GETATTR FH: 0x8d44a6b1
> >  5227   NFS 266 V4 Reply (Call In 5225) CLOSE
> >  5228   NFS 250 V4 Reply (Call In 5226) GETATTR
> > 
> > It's easy to reproduce. By printing some logs, found that we are
> > making
> > CLOSE seqid larger then OPEN seqid when racing.
> > 
> > Fix this by not bumping seqid when it's equal to OPEN seqid. Also
> > put the whole changing process into seqlock read protection in case
> > really bad luck, and this is the same locking behavior with the
> > old deleted function.
> > 
> > Fixes: 0e0cb35b417f ("NFSv4: Handle NFS4ERR_OLD_STATEID in
> > CLOSE/OPEN_DOWNGRADE")
> > Signed-off-by: Murphy Zhou <jencce.kernel@gmail.com>
> > ---
> >  fs/nfs/nfs4proc.c | 13 ++++++++-----
> >  1 file changed, 8 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
> > index 11eafcf..6db5a09 100644
> > --- a/fs/nfs/nfs4proc.c
> > +++ b/fs/nfs/nfs4proc.c
> > @@ -3334,12 +3334,13 @@ static void
> > nfs4_sync_open_stateid(nfs4_stateid *dst,
> >  			break;
> >  		}
> >  		seqid_open = state->open_stateid.seqid;
> > -		if (read_seqretry(&state->seqlock, seq))
> > -			continue;
> >  
> >  		dst_seqid = be32_to_cpu(dst->seqid);
> >  		if ((s32)(dst_seqid - be32_to_cpu(seqid_open)) < 0)
> >  			dst->seqid = seqid_open;
> > +
> > +		if (read_seqretry(&state->seqlock, seq))
> > +			continue;
> 
> What's the intention of this change? Neither dst_seqid nor dst->seqid
> are protected by the state->seqlock so why move this code under the
> lock.

Because seqid_open could have changed when writing to dst->seqid ?

The old nfs4_refresh_open_stateid function put the writing under the lock.

> 
> >  		break;
> >  	}
> >  }
> > @@ -3367,14 +3368,16 @@ static bool
> > nfs4_refresh_open_old_stateid(nfs4_stateid *dst,
> >  			break;
> >  		}
> >  		seqid_open = state->open_stateid.seqid;
> > -		if (read_seqretry(&state->seqlock, seq))
> > -			continue;
> >  
> >  		dst_seqid = be32_to_cpu(dst->seqid);
> > -		if ((s32)(dst_seqid - be32_to_cpu(seqid_open)) >= 0)
> > +		if ((s32)(dst_seqid - be32_to_cpu(seqid_open)) > 0)
> >  			dst->seqid = cpu_to_be32(dst_seqid + 1);
> 
> This negates the whole intention of the patch you reference in the
> 'Fixes:', which was to allow us to CLOSE files even if seqid bumps have
> been lost due to interrupted RPC calls e.g. when using 'soft' or
> 'softerr' mounts.
> With the above change, the check could just be tossed out altogether,
> because dst_seqid will never become larger than seqid_open.

Hmm.. I got it wrong. Thanks for the explanation.

> 
> >  		else
> >  			dst->seqid = seqid_open;
> > +
> > +		if (read_seqretry(&state->seqlock, seq))
> > +			continue;
> > +
> 
> Again, why this change to code that doesn't appear to need protection?
> If the bump does succeed above, then looping back will actually cause
> unpredictable behaviour.

Same as above. This change have nothing to do with the stateid race. I
just found it when reading the patch.

Thanks,
M
> 
> >  		ret = true;
> >  		break;
> >  	}
> -- 
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com
> 
>