Re: [bug] __blk_mq_run_hw_queue suspicious rcu usage

[Jens Axboe]

On 9/17/19 12:23 PM, David Rientjes wrote:
> On Mon, 16 Sep 2019, David Rientjes wrote:
> 
>> Brijesh and Tom, we currently hit this any time we boot an SEV enabled
>> Ubuntu 18.04 guest; I assume that guest kernels, especially those of such
>> major distributions, are expected to work with warnings and BUGs when
>> certain drivers are enabled.
>>
>> If the vmap purge lock is to remain a mutex (any other reason that
>> unmapping aliases can block?) then it appears that allocating a dmapool
>> is the only alternative.  Is this something that you'll be addressing
>> generically or do we need to get buy-in from the maintainers of this
>> specific driver?
>>
> 
> We've found that the following applied on top of 5.2.14 suppresses the
> warnings.
> 
> Christoph, Keith, Jens, is this something that we could do for the nvme
> driver?  I'll happily propose it formally if it would be acceptable.

No, this is not going to be acceptable, I'm afraid. This tells blk-mq
that the driver always needs blocking context for queueing IO, which
will increase latencies for the cases where we'd otherwise issue IO
directly from the context that queues it.

-- 
Jens Axboe