From: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx> Sent: Wednesday, June 8, 2022 10:27 PM
>
> Add cpus_read_lock to prevent CPUs from going offline between query and
> actual use of cpumask. cpumask_of_node is first queried, and based on it
> used later, in case any CPU goes offline between these two events, it can
> potentially cause an infinite loop of retries.
>
> Signed-off-by: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx>
> ---
> drivers/hv/channel_mgmt.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
> index 85a2142..6a88b7e 100644
> --- a/drivers/hv/channel_mgmt.c
> +++ b/drivers/hv/channel_mgmt.c
> @@ -749,6 +749,9 @@ static void init_vp_index(struct vmbus_channel *channel)
> return;
> }
>
> + /* No CPUs should come up or down during this. */
> + cpus_read_lock();
> +
> for (i = 1; i <= ncpu + 1; i++) {
> while (true) {
> numa_node = next_numa_node_id++;
> @@ -781,6 +784,7 @@ static void init_vp_index(struct vmbus_channel *channel)
> break;
> }
>
> + cpus_read_unlock();
> channel->target_cpu = target_cpu;
>
> free_cpumask_var(available_mask);
> --
> 1.8.3.1
This patch was motivated because I suggested a potential issue here during
a separate conversation with Saurabh, but it turns out I was wrong. :-(
init_vp_index() is only called from vmbus_process_offer(), and the
cpus_read_lock() is already held when init_vp_index() is called. So the
issue doesn't exist, and this patch isn't needed.
However, looking at vmbus_process_offer(), there appears to be a
different problem in that cpus_read_unlock() is not called when taking
the error return because the sub_channel_index is zero.
Michael
