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

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, Apr 11, 2022 at 03:52:23PM +0000, Pearson, Robert B wrote:
> 
> > Yes, you cannot use irq_save variants here. You have to know your calling context is non-atomic already and use the irq wrapper.
> 
> Not sure why. If you call irqsave in a non-atomic context doesn't it
> behave the same as irq? I.e. flags = 0.  xarray provides
> xa_lock_xxx() for people to use. Are you saying I have to call
> xa_alloc_cyclic_xxx() instead which is the same thing?

The xa_lock is a magic thing, when you call a __xa_*(.., GFP_KERNEL)
type function then it will unlock and relock the xa_lock internally.

Thus you cannot wrapper an irqsave lock across the GFP_KERNEL call
sites because XA doesn't know the 'flags' to be able to properly
unlock it.

> The problem is I've gotten gun shy about people calling into the
> verbs APIs in strange contexts. The rules don't seem to be written
> down. If I could be certain that no one ever is allowed to call a
> verbs API in an interrupt then this is correct but how do I know
> that?

rxe used GFP_KERNEL so it already has assumed it is a process context.

Jason