Re: [PATCH for-next v6 1/8] RDMA/rxe: Replace RB tree by xarray for indexes

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Dec 08, 2021 at 06:16:21PM -0600, Bob Pearson wrote:
> On 12/7/21 13:09, Jason Gunthorpe wrote:
> > On Mon, Dec 06, 2021 at 03:12:36PM -0600, Bob Pearson wrote:
> >>  	if (pool->flags & RXE_POOL_INDEX) {
> >> -		pool->index.tree = RB_ROOT;
> >> -		err = rxe_pool_init_index(pool, info->max_index,
> >> -					  info->min_index);
> >> -		if (err)
> >> -			goto out;
> >> +		xa_init_flags(&pool->xarray.xa, XA_FLAGS_ALLOC);
> >> +		pool->xarray.limit.max = info->max_index;
> >> +		pool->xarray.limit.min = info->min_index;
> >> +	} else {
> >> +		/* if pool not indexed just use xa spin_lock */
> >> +		spin_lock_init(&pool->xarray.xa.xa_lock);
> > 
> > xarray's don't cost anything to init, so there is no reason to do
> > something like this.
> OK
> > 
> >> +/* drop a reference to an object */
> >> +static inline bool __rxe_drop_ref(struct rxe_pool_elem *elem)
> >> +{
> >> +	bool ret;
> >> +
> >> +	rxe_pool_lock_bh(elem->pool);
> >> +	ret = kref_put(&elem->ref_cnt, rxe_elem_release);
> >> +	rxe_pool_unlock_bh(elem->pool);
> > 
> > This is a bit strange, why does something need to hold a lock around a
> > kref?
> 
> This also relates to your comment on 8/8 patch. There seems to be a race opportunity
> between the call to kref_put(&obj->elem, rxe_elem_release) and the call in
> rxe_elem_release() to xa_erase(). If a duplicate or delayed packet arrives after the the
> final kref_put() and before the xa_erase() one can still lookup the object from its index
> (qpn, rkey, etc.) and take a reference to it. The use of kref_get_unless_zero and
> locking around kref_put and __xa_erase was an attempt to fix this. Once you call kref_put
> with the ref count going to zero there is no way to prevent the object getting its
> cleanup routine called.

This is why you use kref_get_not_zero only during the xa lookup, then
during that race it returns failure

Costs nothing as the atomics are already all required.

Jason