Re: [PATCH V3] sbitmap: order READ/WRITE freed instance and setting clear bit

[Jens Axboe <axboe@kernel.dk>]

On 3/25/19 7:33 PM, Ming Lei wrote:
> On Mon, Mar 25, 2019 at 11:52:17AM -0700, Omar Sandoval wrote:
>> On Fri, Mar 22, 2019 at 09:13:51AM +0800, Ming Lei wrote:
>>> Inside sbitmap_queue_clear(), once the clear bit is set, it will be
>>> visiable to allocation path immediately. Meantime READ/WRITE on old
>>> associated instance(such as request in case of blk-mq) may be
>>> out-of-order with the setting clear bit, so race with re-allocation
>>> may be triggered.
>>>
>>> Adds one memory barrier for ordering READ/WRITE of the freed associated
>>> instance with setting clear bit for avoiding race with re-allocation.
>>
>> Just to make sure I understand, the race is something like this?
>>
>> Thread 1			Thread 2
>> sbitmap_queue_get()
>> ...
>> write to request _____
>> sbitmap_queue_clear() \
>> 		       \	sbitmap_queue_get()
>> 		        \______ observes write
>>
>> If so, you can add
>>
>> Reviewed-by: Omar Sandoval <osandov@fb.com>
> 
> Yeah, it is one normal race between freeing and allocating.

Applied, thanks.

-- 
Jens Axboe