linux-kernel - Re: [bug] __blk_mq_run_hw_queue suspicious rcu usage

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Date:   Tue, 17 Sep 2019 12:32:45 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     David Rientjes <rientjes@...gle.com>,
        Christoph Hellwig <hch@....de>, Keith Busch <kbusch@...nel.org>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Ming Lei <ming.lei@...hat.com>,
        Peter Gonda <pgonda@...gle.com>,
        Jianxiong Gao <jxgao@...gle.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, iommu@...ts.linux-foundation.org
Subject: Re: [bug] __blk_mq_run_hw_queue suspicious rcu usage

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