[PATCH 05/12] blk-mq: Introduce blk_mq_quiesce_queue()

[Ming Lei]

On Thu, Oct 27, 2016 at 10:04 AM, Bart Van Assche <bvanassche at acm.org> wrote:
> On 10/26/16 18:30, Ming Lei wrote:
>>
>> On Thu, Oct 27, 2016 at 6:53 AM, Bart Van Assche
>> <bart.vanassche at sandisk.com> wrote:
>>>
>>> blk_mq_quiesce_queue() waits until ongoing .queue_rq() invocations
>>> have finished. This function does *not* wait until all outstanding
>>> requests have finished (this means invocation of request.end_io()).
>>> The algorithm used by blk_mq_quiesce_queue() is as follows:
>>> * Hold either an RCU read lock or an SRCU read lock around
>>>   .queue_rq() calls. The former is used if .queue_rq() does not
>>>   block and the latter if .queue_rq() may block.
>>> * blk_mq_quiesce_queue() calls synchronize_srcu() or
>>>   synchronize_rcu() to wait for .queue_rq() invocations that
>>>   started before blk_mq_quiesce_queue() was called.
>>> * The blk_mq_hctx_stopped() calls that control whether or not
>>>   .queue_rq() will be called are called with the (S)RCU read lock
>>>   held. This is necessary to avoid race conditions against
>>>   the "blk_mq_stop_hw_queues(q); blk_mq_quiesce_queue(q);"
>>>   sequence from another thread.
>>>
>>> Signed-off-by: Bart Van Assche <bart.vanassche at sandisk.com>
>>> Cc: Christoph Hellwig <hch at lst.de>
>>> Cc: Ming Lei <tom.leiming at gmail.com>
>>> Cc: Hannes Reinecke <hare at suse.com>
>>> Cc: Johannes Thumshirn <jthumshirn at suse.de>
>>> ---
>>>  block/Kconfig          |  1 +
>>>  block/blk-mq.c         | 69
>>> +++++++++++++++++++++++++++++++++++++++++++++-----
>>>  include/linux/blk-mq.h |  3 +++
>>>  include/linux/blkdev.h |  1 +
>>>  4 files changed, 67 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/block/Kconfig b/block/Kconfig
>>> index 1d4d624..0562ef9 100644
>>> --- a/block/Kconfig
>>> +++ b/block/Kconfig
>>> @@ -5,6 +5,7 @@ menuconfig BLOCK
>>>         bool "Enable the block layer" if EXPERT
>>>         default y
>>>         select SBITMAP
>>> +       select SRCU
>>>         help
>>>          Provide block layer support for the kernel.
>>>
>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>> index 0cf21c2..4945437 100644
>>> --- a/block/blk-mq.c
>>> +++ b/block/blk-mq.c
>>> @@ -115,6 +115,31 @@ void blk_mq_unfreeze_queue(struct request_queue *q)
>>>  }
>>>  EXPORT_SYMBOL_GPL(blk_mq_unfreeze_queue);
>>>
>>> +/**
>>> + * blk_mq_quiesce_queue() - wait until all ongoing queue_rq calls have
>>> finished
>>> + * @q: request queue.
>>> + *
>>> + * Note: this function does not prevent that the struct request end_io()
>>> + * callback function is invoked. Additionally, it is not prevented that
>>> + * new queue_rq() calls occur unless the queue has been stopped first.
>>> + */
>>> +void blk_mq_quiesce_queue(struct request_queue *q)
>>> +{
>>> +       struct blk_mq_hw_ctx *hctx;
>>> +       unsigned int i;
>>> +       bool rcu = false;
>>
>>
>> Before synchronizing SRCU/RCU, we have to set a per-hctx flag
>> or per-queue flag to block comming .queue_rq(), seems I mentioned
>> that before:
>>
>>    https://www.spinics.net/lists/linux-rdma/msg41389.html
>
>
> Hello Ming,
>
> Thanks for having included an URL to an archived version of that discussion.
> What I remember about that discussion is that I proposed to use the existing
> flag BLK_MQ_S_STOPPED instead of introducing a
> new QUEUE_FLAG_QUIESCING flag and that you agreed with that proposal. See
> also https://www.spinics.net/lists/linux-rdma/msg41430.html.

Yes, I am fine with either one, but the flag need to set in
blk_mq_quiesce_queue(), doesnt't it?


Thanks,
Ming Lei