nfs41: free slot

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>
[nfs41: free slot use slotid]
Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
[nfs41: use find_first_zero_bit for nfs4_find_slot]
    While at it, obliterate lowest_free_slotid and fix-up related comments.
    As per review comment 21/85.
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
[nfs41: use __clear_bit for nfs4_free_slot]
    While at it, fix-up function comment.
    Part of review comment 22/85.
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
[nfs41: use find_last_bit in nfs4_free_slot to determine highest used slot.]
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
[nfs41: rpc_sleep_on slot_tbl_waitq must be called under slot_tbl_lock]
    Otherwise there's a race (we've hit) with nfs4_free_slot where
    nfs41_setup_sequence sees a full slot table, unlocks slot_tbl_lock,
    nfs4_free_slots happen concurrently and call rpc_wake_up_next
    where there's nobody to wake up yet, context goes back to
    nfs41_setup_sequence which goes to sleep when the slot table
    is actually empty now and there's no-one to wake it up anymore.
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4proc.c

@@ -273,6 +273,42 @@ static void renew_lease(const struct nfs_server *server, unsigned long timestamp
 
 #if defined(CONFIG_NFS_V4_1)
 
+/*
+ * nfs4_free_slot - free a slot and efficiently update slot table.
+ *
+ * freeing a slot is trivially done by clearing its respective bit
+ * in the bitmap.
+ * 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.
+ * If none found, highest_used_slotid is set to -1.
+ */
+static void
+nfs4_free_slot(struct nfs4_slot_table *tbl, u8 free_slotid)
+{
+	int slotid = free_slotid;
+
+	spin_lock(&tbl->slot_tbl_lock);
+	/* clear used bit in bitmap */
+	__clear_bit(slotid, tbl->used_slots);
+
+	/* update highest_used_slotid when it is freed */
+	if (slotid == tbl->highest_used_slotid) {
+		slotid = find_last_bit(tbl->used_slots, tbl->max_slots);
+		if (slotid >= 0 && slotid < tbl->max_slots)
+			tbl->highest_used_slotid = slotid;
+		else
+			tbl->highest_used_slotid = -1;
+	}
+	rpc_wake_up_next(&tbl->slot_tbl_waitq);
+	spin_unlock(&tbl->slot_tbl_lock);
+	dprintk("%s: free_slotid %u highest_used_slotid %d\n", __func__,
+		free_slotid, tbl->highest_used_slotid);
+}
+
 /*
  * nfs4_find_slot - efficiently look for a free slot
  *