Re: [PATCH for-next] RDMA/rxe: Fix "Replace red-black trees by xarrays"

[Jason Gunthorpe <jgg@nvidia.com>]

On Sun, Apr 10, 2022 at 10:13:16PM -0500, Bob Pearson wrote:
> On 4/10/22 22:06, Bart Van Assche wrote:
> > On 4/10/22 15:39, Bob Pearson wrote:
> >> Fixes: 3225717f6dfa ("RDMA/rxe: Replace red-black trees by carrays")
> >                                                              ^^^^^^^
> >                                                              xarrays?
> > 
> >> @@ -138,8 +140,10 @@ void *rxe_alloc(struct rxe_pool *pool)
> >>       elem->obj = obj;
> >>       kref_init(&elem->ref_cnt);
> >>   -    err = xa_alloc_cyclic(&pool->xa, &elem->index, elem, pool->limit,
> >> +    xa_lock_irqsave(xa, flags);
> >> +    err = __xa_alloc_cyclic(&pool->xa, &elem->index, elem, pool->limit,
> >>                     &pool->next, GFP_KERNEL);
> >> +    xa_unlock_irqrestore(xa, flags);
> > 
> > Please take a look at the xas_unlock_type() and xas_lock_type() calls in __xas_nomem(). I think that the above change will trigger a GFP_KERNEL allocation with interrupts disabled. My understanding is that GFP_KERNEL allocations may sleep and hence that the above code may cause __xas_nomem() to sleep with interrupts disabled. I don't think that is allowed.
> > 
> > Thanks,
> > 
> > Bart.
> 
> You're right. I missed that. Zhu wants to write the patch so hopefully he's on top of that.
> For now we could use GFP_ATOMIC.

Yes, you cannot use irq_save varients here. You have to know your
calling context is non-atomic already and use the irq wrapper.

Jason