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From:   Saurabh Sengar <ssengar@linux.microsoft.com>
To:     kys@microsoft.com, haiyangz@microsoft.com, sthemmin@microsoft.com,
        wei.liu@kernel.org, decui@microsoft.com,
        linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org,
        ssengar@microsoft.com, mikelley@microsoft.com
Subject: [PATCH] Drivers: hv: vmbus: Add cpu read lock
Date:   Wed,  8 Jun 2022 22:27:26 -0700
Message-Id: <1654752446-20113-1-git-send-email-ssengar@linux.microsoft.com>
Precedence: bulk

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@linux.microsoft.com>
---
 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