RE: [PATCH 1/2] Drivers: hv: vmbus: Resolve race between init_vp_index() and CPU hotplug

[Michael Kelley]

From: Andrea Parri (Microsoft) <parri.andrea@xxxxxxxxx> Sent: Friday, May 22, 2020 10:19 AM
> 
> vmbus_process_offer() does two things (among others):
> 
>  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.
> 
> 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.
> 
> 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.
> 
> 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).
> 
> Fixes: d570aec0f2154e ("Drivers: hv: vmbus: Synchronize init_vp_index() vs. CPU hotplug")
> Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@xxxxxxxxx>
> ---
>  drivers/hv/channel_mgmt.c | 46 +++++++++++++++------------------------
>  1 file changed, 18 insertions(+), 28 deletions(-)

Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>