linux-kernel - Re: [RFC PATCH 03/11] Drivers: hv: vmbus: Replace the per-CPU channel lists with a global array of channels

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Date:   Fri, 03 Apr 2020 16:56:43 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Andrea Parri <parri.andrea@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        "K . Y . Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, linux-hyperv@...r.kernel.org,
        Michael Kelley <mikelley@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [RFC PATCH 03/11] Drivers: hv: vmbus: Replace the per-CPU channel lists with a global array of channels

Andrea Parri <parri.andrea@...il.com> writes:

> On Mon, Mar 30, 2020 at 02:45:54PM +0200, Vitaly Kuznetsov wrote:
>> Andrea Parri <parri.andrea@...il.com> writes:
>> 
>> >> Correct me if I'm wrong, but currently vmbus_chan_sched() accesses
>> >> per-cpu list of channels on the same CPU so we don't need a spinlock to
>> >> guarantee that during an interrupt we'll be able to see the update if it
>> >> happened before the interrupt (in chronological order). With a global
>> >> list of relids, who guarantees that an interrupt handler on another CPU
>> >> will actually see the modified list? 
>> >
>> > Thanks for pointing this out!
>> >
>> > The offer/resume path presents implicit full memory barriers, program
>> > -order after the array store which should guarantee the visibility of
>> > the store to *all* CPUs before the offer/resume can complete (c.f.,
>> >
>> >   tools/memory-model/Documentation/explanation.txt, Sect. #13
>> >
>> > and assuming that the offer/resume for a channel must complete before
>> > the corresponding handler, which seems to be the case considered that
>> > some essential channel fields are initialized only later...)
>> >
>> > IIUC, the spin lock approach you suggested will work and be "simpler";
>> > an obvious side effect would be, well, a global synchronization point
>> > in vmbus_chan_sched()...
>> >
>> > Thoughts?
>> 
>> This is, of course, very theoretical as if we're seeing an interrupt for
>> a channel at the same time we're writing its relid we're already in
>> trouble. I can, however, try to suggest one tiny improvement:
>
> Indeed.  I think the idea (still quite informal) is that:
>
>   1) the mapping of the channel relid is propagated to (visible from)
>      all CPUs before add_channel_work is queued (full barrier in
>      queue_work()),
>
>   2) add_channel_work is queued before the channel is opened (aka,
>      before the channel ring buffer is allocate/initalized and the
>      OPENCHANNEL msg is sent and acked from Hyper-V, cf. OPEN_STATE),
>
>   3) the channel is opened before Hyper-V can start sending interrupts
>      for the channel, and hence before vmbus_chan_sched() can find the
>      channel relid in recv_int_page set,
>
>   4) vmbus_chan_sched() finds the channel's relid in recv_int_page
>      set before it search/load from the channel array (full barrier
>      in sync_test_and_clear_bit()).
>
> This is for the "normal"/not resuming from hibernation case; for the
> latter, notice that:
>
>   a) vmbus_isr() (and vmbus_chan_sched()) can not run until when
>      vmbus_bus_resume() has finished (@resume_noirq callback),
>
>   b) vmbus_bus_resume() can not complete before nr_chan_fixup_on_resume
>      equals 0 in check_ready_for_resume_event().
>      
> (and check_ready_for_resume_event() does also provides a full barrier).
>
> If makes sense to you, I'll try to add some of the above in comments.
>

It does, thank you!

-- 
Vitaly