Re: [pnfs] [RFC 22/85] nfs41: free slot

[Benny Halevy <bhalevy@panasas.com>]

On Nov. 10, 2008, 22:21 +0200, Benny Halevy <bhalevy@panasas.com> wrote:
> Free a slot in the slot table.
> 
> Mark the slot as free in the bitmap-based allocation table
> by clearing a bit corresponding to the slotid.
> 
> Update lowest_free_slotid if freed slotid is lower than that.
> Update highest_used_slotid.  In the case the freed slotid
> equals the highest_used_slotid, scan downwards for the next
> highest used slotid using the optimized fls* functions.
> 
> Finally, wake up thread waiting on slot_tbl_waitq for a free slot
> to become available.
> 
> Signed-off-by: Benny Halevy <bhalevy@panasas.com>
> ---
>  fs/nfs/nfs4proc.c |   54 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 54 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
> index 4ded3a5..d26f61d 100644
> --- a/fs/nfs/nfs4proc.c
> +++ b/fs/nfs/nfs4proc.c
> @@ -243,6 +243,60 @@ static inline int bmp2idx(unsigned long *base, unsigned long *bmp,
>  }
>  
>  /*
> + * nfs4_find_slot - free a slot and efficiently update slot table.
> + *
> + * freeing a slot is trivially done by clearing its respective bit
> + * in the bitmap.
> + * The lowest_free_slotid is updated if the freed slotid is lower than that.
> + * If the freed slotid equals highest_used_slotid we want to update it
> + * so that the server would be able to size down the slot table if needed,
> + * otherwise we know that the highest_used_slotid is still in use.
> + * When updating highest_used_slotid there may be "holes" in the bitmap
> + * so we need to scan down from highest_used_slotid to 0 looking for the now
> + * highest slotid in use.
> + * We use the fls cpu-optimized function to look for the last set bit in each
> + * word in the bitmap.
> + * If none found, highest_used_slotid is set to -1.
> + */
> +static void
> +nfs41_free_slot(struct nfs4_slot_table *tbl, struct nfs4_slot *slot)
> +{
> +	int slotid = slot_idx(tbl, slot);
> +	unsigned long *used, u;
> +	int bit;
> +
> +	spin_lock(&tbl->slot_tbl_lock);
> +	/* clear used bit in bitmap */
> +	used = idx2bmp(tbl->used_slots, slotid, &bit);
> +	*used &= ~(1 << bit);
> +
> +	if (slotid < tbl->lowest_free_slotid)
> +		tbl->lowest_free_slotid = slotid;
> +
> +	/* update highest_used_slotid when it is freed */
> +	if (slotid == tbl->highest_used_slotid) {
> +		tbl->highest_used_slotid = -1;
> +		slotid--;
> +		/* scan down for the highest used slotid */
> +		while (slotid >= 0) {
> +			used = idx2bmp(tbl->used_slots, slotid, NULL);

review 11-14: look into using a bitmap_ function, add one if needed

> +			u = *used;
> +			if (!u) {
> +				/* all slots marked free in this word */
> +				slotid -= BITS_PER_LONG;
> +				continue;
> +			}
> +			bit = __fls(u);
> +			tbl->highest_used_slotid = bmp2idx(tbl->used_slots,
> +							   used, bit);
> +			break;
> +		}
> +	}
> +	spin_unlock(&tbl->slot_tbl_lock);
> +	rpc_wake_up_next(&tbl->slot_tbl_waitq);
> +}
> +
> +/*
>   * nfs4_find_slot - efficiently look for a free slot
>   *
>   * nfs4_find_slot uses the ffz cpu-optimized function to look for an unset