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

[Yanjun Zhu <yanjun.zhu@linux.dev>]

在 2022/4/12 0:02, Jason Gunthorpe 写道:
> 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.
Got it.

__xa_alloc_cyclic(..., GFP_NOWAIT) will unlock xa lock internally. But 
it does not handle flags. So the irq is still disabled. Because 
GFP_NOWAIT is used in __xa_alloc_cyclic, it will not sleep.

__xa_alloc_cyclic will lock xa lock and return to xa_unlock_irqrestore.

So the following code should be OK?

         xa_lock_irqsave(&pool->xa, flags);
         err = __xa_alloc_cyclic(&pool->xa, &elem->index, elem, pool->limit,
                                 &pool->next, GFP_NOWAIT);
         xa_unlock_irqrestore(&pool->xa, flags);


Zhu Yanjun

> 
> Jason