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

Sasha Levin sashal at kernel.org
Mon Nov 26 05:23:31 UTC 2018

On Mon, Nov 26, 2018 at 02:29:57AM +0000, kys at linuxonhyperv.com wrote:
>From: Dexuan Cui <decui at microsoft.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 at vger.kernel.org
>Cc: Stephen Hemminger <sthemmin at microsoft.com>
>Cc: K. Y. Srinivasan <kys at microsoft.com>
>Cc: Haiyang Zhang <haiyangz at microsoft.com>
>Signed-off-by: Dexuan Cui <decui at microsoft.com>
>Signed-off-by: K. Y. Srinivasan <kys at microsoft.com>

This patch doesn't apply on next/linus/char-misc; there seems to be a
missing patch that touches vmbus_process_offer() which isn't a part of
this series.


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