On Thu, Oct 27, 2016 at 10:04 AM, Bart Van Assche <bvanassche@xxxxxxx> wrote:
> On 10/26/16 18:30, Ming Lei wrote:
>>
>> On Thu, Oct 27, 2016 at 6:53 AM, Bart Van Assche
>> <bart.vanassche@xxxxxxxxxxx> 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@xxxxxxxxxxx>
>>> Cc: Christoph Hellwig <hch@xxxxxx>
>>> Cc: Ming Lei <tom.leiming@xxxxxxxxx>
>>> Cc: Hannes Reinecke <hare@xxxxxxxx>
>>> Cc: Johannes Thumshirn <jthumshirn@xxxxxxx>
>>> ---
>>> 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
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