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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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