Re: [PATCH 2/4] pnfs_submit: Only update stateid if it is more recent than current

[Benny Halevy <bhalevy@panasas.com>]

On 2010-10-06 16:35, Fred Isaman wrote:
> Right now, when we set the stateid, we blindly overwrite the current
> one, allowing the seqid to incorrectly roll backward.
> 
> Signed-off-by: Fred Isaman <iisaman@netapp.com>
> ---
>  fs/nfs/pnfs.c |   38 ++++++++++++++++++++++++++++++++------
>  1 files changed, 32 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/nfs/pnfs.c b/fs/nfs/pnfs.c
> index 39bce9b..555955b 100644
> --- a/fs/nfs/pnfs.c
> +++ b/fs/nfs/pnfs.c
> @@ -459,16 +459,42 @@ pnfs_destroy_all_layouts(struct nfs_client *clp)
>  	}
>  }
>  
> +/* update lo->stateid with new if is more recent
> + *
> + * lo->stateid could be the open stateid, in which case we just use what given.
> + */
>  static void
>  pnfs_set_layout_stateid(struct pnfs_layout_hdr *lo,
> -			const nfs4_stateid *stateid)
> +			const nfs4_stateid *new)
>  {
> -	/* TODO - should enforce that embedded seqid, in the case
> -	 * that the two stateid.others are equal,  only increases.
> -	 * Complicated by wrap-around.
> -	 */
> +	nfs4_stateid *old = &lo->stateid;
> +	bool overwrite = false;
> +
>  	write_seqlock(&lo->seqlock);
> -	memcpy(lo->stateid.data, stateid->data, sizeof(lo->stateid.data));
> +	if (!test_bit(NFS_LAYOUT_STATEID_SET, &lo->state) ||
> +	    memcmp(old->stateid.other, new->stateid.other, sizeof(new->stateid.other)))
> +		overwrite = true;
> +	else {
> +		u32 oldseq, newseq, limit;
> +
> +		oldseq = be32_to_cpu(old->stateid.seqid);
> +		newseq = be32_to_cpu(new->stateid.seqid);
> +		/* There are no good bounds on window size, so just
> +		 * use a ridiculously large window of 2^31.
> +		 */
> +		limit = oldseq + (1 << 31);
> +		if (oldseq < limit) {
> +			/* The easy, non-wraparound case */
> +			if (oldseq < newseq && newseq < limit)
> +				overwrite = true;
> +		} else {
> +			/* Near wraparound edge */
> +			if (oldseq < newseq || newseq < limit)
> +				overwrite = true;
> +		}

Wouldn't it be simpler to just look at (int32_t)(newseq - oldseq)?

Benny

> +	}
> +	if (overwrite)
> +		memcpy(&old->stateid, &new->stateid, sizeof(new->stateid));
>  	write_sequnlock(&lo->seqlock);
>  }
>