linux-kernel - Re: [PATCH 2/2] Drivers: hv: vmbus: offload the handling of channels to two workqueues

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Date:   Mon, 26 Nov 2018 20:35:03 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     kys@...rosoft.com
Cc:     linux-kernel@...r.kernel.org, devel@...uxdriverproject.org,
        olaf@...fle.de, apw@...onical.com, jasowang@...hat.com,
        sthemmin@...rosoft.com, Michael.H.Kelley@...rosoft.com,
        vkuznets@...hat.com, Haiyang Zhang <haiyangz@...rosoft.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH 2/2] Drivers: hv: vmbus: offload the handling of channels
 to two workqueues

On Mon, Nov 26, 2018 at 02:29:57AM +0000, kys@...uxonhyperv.com wrote:
> From: Dexuan Cui <decui@...rosoft.com>
> 
> vmbus_process_offer() mustn't call channel->sc_creation_callback()
> directly for sub-channels, because sc_creation_callback() ->
> vmbus_open() may never get the host's response to the
> OPEN_CHANNEL message (the host may rescind a channel at any time,
> e.g. in the case of hot removing a NIC), and vmbus_onoffer_rescind()
> may not wake up the vmbus_open() as it's blocked due to a non-zero
> vmbus_connection.offer_in_progress, and finally we have a deadlock.
> 
> The above is also true for primary channels, if the related device
> drivers use sync probing mode by default.
> 
> And, usually the handling of primary channels and sub-channels can
> depend on each other, so we should offload them to different
> workqueues to avoid possible deadlock, e.g. in sync-probing mode,
> NIC1's netvsc_subchan_work() can race with NIC2's netvsc_probe() ->
> rtnl_lock(), and causes deadlock: the former gets the rtnl_lock
> and waits for all the sub-channels to appear, but the latter
> can't get the rtnl_lock and this blocks the handling of sub-channels.
> 
> The patch can fix the multiple-NIC deadlock described above for
> v3.x kernels (e.g. RHEL 7.x) which don't support async-probing
> of devices, and v4.4, v4.9, v4.14 and v4.18 which support async-probing
> but don't enable async-probing for Hyper-V drivers (yet).
> 
> The patch can also fix the hang issue in sub-channel's handling described
> above for all versions of kernels, including v4.19 and v4.20-rc3.
> 
> So the patch should be applied to all the existing kernels.
> 
> Fixes: 8195b1396ec8 ("hv_netvsc: fix deadlock on hotplug")
> Cc: stable@...r.kernel.org
> Cc: Stephen Hemminger <sthemmin@...rosoft.com>
> Cc: K. Y. Srinivasan <kys@...rosoft.com>
> Cc: Haiyang Zhang <haiyangz@...rosoft.com>
> Signed-off-by: Dexuan Cui <decui@...rosoft.com>
> Signed-off-by: K. Y. Srinivasan <kys@...rosoft.com>
> ---
>  drivers/hv/channel_mgmt.c | 188 +++++++++++++++++++++++++-------------
>  drivers/hv/connection.c   |  24 ++++-
>  drivers/hv/hyperv_vmbus.h |   7 ++
>  include/linux/hyperv.h    |   7 ++
>  4 files changed, 161 insertions(+), 65 deletions(-)

As Sasha pointed out, this patch does not even apply :(