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Subject[RFC PATCH 02/11] Drivers: hv: vmbus: Don't bind the offer&rescind works to a specific CPU
Date
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@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
---
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
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