Re: [PATCH 1/2] block: generic request_queue reference counting

[Dan Williams]

On Tue, Oct 6, 2015 at 3:46 AM, Ming Lei <tom.leiming@xxxxxxxxx> wrote:
> On Tue, Oct 6, 2015 at 7:23 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>> On Sun, Oct 4, 2015 at 12:52 AM, Ming Lei <tom.leiming@xxxxxxxxx> wrote:
>>> On Wed, Sep 30, 2015 at 8:41 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>>>> Allow pmem, and other synchronous/bio-based block drivers, to fallback
>>>
>>> Just a bit curious, why not extend it for all(both synchronous and
>>> asynchrounous) bio-based drivers? As you mentioned in introductory
>>> message, all bio based drivers may have this kind of problem.
>>>
>>> One idea I thought of is to hold the usage counter in bio life time,
>>> instead of request's life time like in blk-mq.
>>>
>>>> on a per-cpu reference count managed by the core for tracking queue
>>>> live/dead state.
>>>>
>>>> The existing per-cpu reference count for the blk_mq case is promoted to
>>>> be used in all block i/o scenarios.  This involves initializing it by
>>>> default, waiting for it to drop to zero at exit, and holding a live
>>>> reference over the invocation of q->make_request_fn() in
>>>
>>> It isn't enough for asynchrounous bio drivers.
>>
>> True, but I think that support is a follow on extension of the
>> mechanism.  It seems to me not as straightforward as holding a
>> per-request reference or a reference over the submission path.
>
> Given it is a general problem for all bio based drivers, it seems
> better to figure out one general solution instead of the partial fix
> only for synchrounous bio based drivers.
>
> IMO, it should work by holding the usage counter in BIO's lifetime,
> for example, getting it before calling q->make_request_fn(q, bio), and
> putting it after bio's completion(in bio_endio()).  Even though there is
> still a small window between all bio's completion and all request's
> completion, and it should have been addressed easily for both
> !mq_ops and mq_ops.

I'm not convinced it is that simple, i.e. that all bio_endio()
invocations are guaranteed to have a 1:1 matching invocation through
generic_make_request().  If you want to help with that audit I'd be
appreciative and open to putting together a follow-on patch.  In the
meantime this starting point already makes things better for pmem,
nd_blk, brd, etc...
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