Re: [RFC PATCH] blk-mq: Clean up references when freeing rqs

Hi Ming,

On 10/12/2020 02:07, Ming Lei wrote:

quoted

Apart from this, my concern is that we come with for a solution, but it's a
complicated solution and may not be accepted as this issue is not seen as a
problem in practice.

If that is the case, I'd suggest to consider the solution in the
following link:

https://lore.kernel.org/linux-block/20200820180335.3109216-1-ming.lei@redhat.com/ (local)

At least, the idea is simple, which can be extended to support allocate driver tags
request pool dynamically.

As I see with your approach, we may still iterate a stale request, but 
it just has not been freed, so just no use-after-free BUG, right? Rather 
it is cached until all references dropped. It may be best solution.

So I'll try an experiment today to prove your concern about 
blk_mq_queue_tag_busy_iter(). Then look at possible solution which 
builds on patch in $subject, and compare.

Thanks,
John