[RFC PATCH 02/11] Drivers: hv: vmbus: Don't bind the offer&rescind works to a specific CPU

[Andrea Parri (Microsoft)]

The offer and rescind works are currently scheduled on the so called
"connect CPU".  However, this is not really needed: we can synchronize
the works by relying on the usage of the offer_in_progress counter and
of the channel_mutex mutex.  This synchronization is already in place.
So, remove this unnecessary "bind to the connect CPU" constraint and
update the inline comments accordingly.

Suggested-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@xxxxxxxxx>
---
 drivers/hv/channel_mgmt.c | 21 ++++++++++++++++-----
 drivers/hv/vmbus_drv.c    | 39 ++++++++++++++++++++++++++++-----------
 2 files changed, 44 insertions(+), 16 deletions(-)

diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index 0370364169c4e..1191f3d76d111 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -1025,11 +1025,22 @@ static void vmbus_onoffer_rescind(struct vmbus_channel_message_header *hdr)
 	 * offer comes in first and then the rescind.
 	 * Since we process these events in work elements,
 	 * and with preemption, we may end up processing
-	 * the events out of order. Given that we handle these
-	 * work elements on the same CPU, this is possible only
-	 * in the case of preemption. In any case wait here
-	 * until the offer processing has moved beyond the
-	 * point where the channel is discoverable.
+	 * the events out of order.  We rely on the synchronization
+	 * provided by offer_in_progress and by channel_mutex for
+	 * ordering these events:
+	 *
+	 * { Initially: offer_in_progress = 1 }
+	 *
+	 * CPU1				CPU2
+	 *
+	 * [vmbus_process_offer()]	[vmbus_onoffer_rescind()]
+	 *
+	 * LOCK channel_mutex		WAIT_ON offer_in_progress == 0
+	 * DECREMENT offer_in_progress	LOCK channel_mutex
+	 * INSERT chn_list		SEARCH chn_list
+	 * UNLOCK channel_mutex		UNLOCK channel_mutex
+	 *
+	 * Forbids: CPU2's SEARCH from *not* seeing CPU1's INSERT
 	 */
 
 	while (atomic_read(&vmbus_connection.offer_in_progress) != 0) {
diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 7600615e13754..903b1ec6a259e 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1048,8 +1048,9 @@ void vmbus_on_msg_dpc(unsigned long data)
 		/*
 		 * The host can generate a rescind message while we
 		 * may still be handling the original offer. We deal with
-		 * this condition by ensuring the processing is done on the
-		 * same CPU.
+		 * this condition by relying on the synchronization provided
+		 * by offer_in_progress and by channel_mutex.  See also the
+		 * inline comments in vmbus_onoffer_rescind().
 		 */
 		switch (hdr->msgtype) {
 		case CHANNELMSG_RESCIND_CHANNELOFFER:
@@ -1071,16 +1072,34 @@ void vmbus_on_msg_dpc(unsigned long data)
 			 * work queue: the RESCIND handler can not start to
 			 * run before the OFFER handler finishes.
 			 */
-			schedule_work_on(VMBUS_CONNECT_CPU,
-					 &ctx->work);
+			schedule_work(&ctx->work);
 			break;
 
 		case CHANNELMSG_OFFERCHANNEL:
+			/*
+			 * The host sends the offer message of a given channel
+			 * before sending the rescind message of the same
+			 * channel.  These messages are sent to the guest's
+			 * connect CPU; the guest then starts processing them
+			 * in the tasklet handler on this CPU:
+			 *
+			 * VMBUS_CONNECT_CPU
+			 *
+			 * [vmbus_on_msg_dpc()]
+			 * atomic_inc()  // CHANNELMSG_OFFERCHANNEL
+			 * queue_work()
+			 * ...
+			 * [vmbus_on_msg_dpc()]
+			 * schedule_work()  // CHANNELMSG_RESCIND_CHANNELOFFER
+			 *
+			 * We rely on the memory-ordering properties of the
+			 * queue_work() and schedule_work() primitives, which
+			 * guarantee that the atomic increment will be visible
+			 * to the CPUs which will execute the offer & rescind
+			 * works by the time these works will start execution.
+			 */
 			atomic_inc(&vmbus_connection.offer_in_progress);
-			queue_work_on(VMBUS_CONNECT_CPU,
-				      vmbus_connection.work_queue,
-				      &ctx->work);
-			break;
+			fallthrough;
 
 		default:
 			queue_work(vmbus_connection.work_queue, &ctx->work);
@@ -1124,9 +1143,7 @@ static void vmbus_force_channel_rescinded(struct vmbus_channel *channel)
 
 	INIT_WORK(&ctx->work, vmbus_onmessage_work);
 
-	queue_work_on(VMBUS_CONNECT_CPU,
-		      vmbus_connection.work_queue,
-		      &ctx->work);
+	queue_work(vmbus_connection.work_queue, &ctx->work);
 }
 #endif /* CONFIG_PM_SLEEP */
 
-- 
2.24.0