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From:   Michael Kelley <mikelley@microsoft.com>
To:     "Andrea Parri (Microsoft)" <parri.andrea@gmail.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>
Subject: RE: [PATCH 1/2] Drivers: hv: vmbus: Resolve race between
 init_vp_index() and CPU hotplug
Thread-Topic: [PATCH 1/2] Drivers: hv: vmbus: Resolve race between
 init_vp_index() and CPU hotplug
Thread-Index: AQHWMF02y64NPbAdtUulklFpuaSRq6i0bhnw
Date:   Fri, 22 May 2020 18:34:14 +0000
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References: <20200522171901.204127-1-parri.andrea@gmail.com>
 <20200522171901.204127-2-parri.andrea@gmail.com>
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From: Andrea Parri (Microsoft) <parri.andrea@gmail.com> Sent: Friday, May 2=
2, 2020 10:19 AM
>=20
> vmbus_process_offer() does two things (among others):
>=20
>  1) first, it sets the channel's target CPU with cpu_hotplug_lock;
>  2) it then adds the channel to the channel list(s) with channel_mutex.
>=20
> Since cpu_hotplug_lock is released before (2), the channel's target CPU
> (as designated in (1)) can be deemed "free" by hv_synic_cleanup() and go
> offline before the channel is added to the list.
>=20
> Fix the race condition by "extending" the cpu_hotplug_lock critical
> section to include (2) (and (1)), nesting the channel_mutex critical
> section within the cpu_hotplug_lock critical section as done elsewhere
> (hv_synic_cleanup(), target_cpu_store()) in the hyperv drivers code.
>=20
> Move even further by extending the channel_mutex critical section to
> include (1) (and (2)): this change allows to remove (the now redundant)
> bind_channel_to_cpu_lock, and generally simplifies the handling of the
> target CPUs (that are now always modified with channel_mutex held).
>=20
> Fixes: d570aec0f2154e ("Drivers: hv: vmbus: Synchronize init_vp_index() v=
s. CPU hotplug")
> Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
> ---
>  drivers/hv/channel_mgmt.c | 46 +++++++++++++++------------------------
>  1 file changed, 18 insertions(+), 28 deletions(-)

Reviewed-by: Michael Kelley <mikelley@microsoft.com>