Re: [PATCH] aio: Convert ioctx_table to XArray

[Jens Axboe]

On 12/11/18 11:53 AM, Matthew Wilcox wrote:
> On Tue, Dec 11, 2018 at 11:46:53AM -0700, Jens Axboe wrote:
>> On 12/11/18 11:45 AM, Matthew Wilcox wrote:
>>> I think we need the rcu read lock here to prevent ctx from being freed
>>> under us by free_ioctx().
>>
>> Then that begs the question, how about __xa_load() that is already called
>> under RCU read lock?
> 
> I've been considering adding it to the API, yes.  I was under the
> impression that nested rcu_read_lock() calls were not expensive, even
> with CONFIG_PREEMPT.

They are not expensive, but they are not free either. And if we know we
are already under a rcu read lock, it seems pretty pointless. For the
two cases (memremap and aio), the rcu read lock is right there, before
the call. Easy to verify that it's safe.

-- 
Jens Axboe