vmbus: remove hv_event_tasklet_disable/enable

With the recent introduction of per-channel tasklet, we need to update
the way we handle the 3 concurrency issues:

1. hv_process_channel_removal -> percpu_channel_deq vs.
vmbus_chan_sched -> list_for_each_entry(..., percpu_list);

2. vmbus_process_offer -> percpu_channel_enq/deq vs. vmbus_chan_sched.

3. vmbus_close_internal vs. the per-channel tasklet vmbus_on_event;

The first 2 issues can be handled by Stephen's recent patch
"vmbus: use rcu for per-cpu channel list", and the third issue
can be handled by calling tasklet_disable in vmbus_close_internal here.

We don't need the original hv_event_tasklet_disable/enable since we
now use per-channel tasklet instead of the previous per-CPU tasklet,
and actually we must remove them due to the side effect now:
vmbus_process_offer -> hv_event_tasklet_enable -> tasklet_schedule will
start the per-channel callback prematurely, cauing NULL dereferencing
(the channel may haven't been properly configured to run the callback yet).

Fixes: 631e63a9f346 ("vmbus: change to per channel tasklet")

Signed-off-by: Dexuan Cui <[e-mail]>
Cc: "K. Y. Srinivasan" <[e-mail]>
Cc: Haiyang Zhang <[e-mail]>
Cc: Stephen Hemminger <[e-mail]>
Tested-by: Vitaly Kuznetsov <[e-mail]>
Signed-off-by: K. Y. Srinivasan <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/hv/channel.c

···
 	int ret;
 
 	/*
-	 * vmbus_on_event(), running in the tasklet, can race
+	 * vmbus_on_event(), running in the per-channel tasklet, can race
 	 * with vmbus_close_internal() in the case of SMP guest, e.g., when
 	 * the former is accessing channel->inbound.ring_buffer, the latter
-	 * could be freeing the ring_buffer pages.
-	 *
-	 * To resolve the race, we can serialize them by disabling the
-	 * tasklet when the latter is running here.
+	 * could be freeing the ring_buffer pages, so here we must stop it
+	 * first.
 	 */
-	hv_event_tasklet_disable(channel);
+	tasklet_disable(&channel->callback_event);
 
 	/*
 	 * In case a device driver's probe() fails (e.g.,
···
 		get_order(channel->ringbuffer_pagecount * PAGE_SIZE));
 
 out:
-	hv_event_tasklet_enable(channel);
-
 	return ret;
 }
 

drivers/hv/channel_mgmt.c

···
 		       true);
 }
 
-void hv_event_tasklet_disable(struct vmbus_channel *channel)
-{
-	tasklet_disable(&channel->callback_event);
-}
-
-void hv_event_tasklet_enable(struct vmbus_channel *channel)
-{
-	tasklet_enable(&channel->callback_event);
-
-	/* In case there is any pending event */
-	tasklet_schedule(&channel->callback_event);
-}
-
 void hv_process_channel_removal(struct vmbus_channel *channel, u32 relid)
 {
 	unsigned long flags;
···
 	BUG_ON(!channel->rescind);
 	BUG_ON(!mutex_is_locked(&vmbus_connection.channel_mutex));
 
-	hv_event_tasklet_disable(channel);
 	if (channel->target_cpu != get_cpu()) {
 		put_cpu();
 		smp_call_function_single(channel->target_cpu,
···
 		percpu_channel_deq(channel);
 		put_cpu();
 	}
-	hv_event_tasklet_enable(channel);
 
 	if (channel->primary_channel == NULL) {
 		list_del(&channel->listentry);
···
 
 	init_vp_index(newchannel, dev_type);
 
-	hv_event_tasklet_disable(newchannel);
 	if (newchannel->target_cpu != get_cpu()) {
 		put_cpu();
 		smp_call_function_single(newchannel->target_cpu,
···
 		percpu_channel_enq(newchannel);
 		put_cpu();
 	}
-	hv_event_tasklet_enable(newchannel);
 
 	/*
 	 * This state is used to indicate a successful open
···
 	list_del(&newchannel->listentry);
 	mutex_unlock(&vmbus_connection.channel_mutex);
 
-	hv_event_tasklet_disable(newchannel);
 	if (newchannel->target_cpu != get_cpu()) {
 		put_cpu();
 		smp_call_function_single(newchannel->target_cpu,
···
 		percpu_channel_deq(newchannel);
 		put_cpu();
 	}
-	hv_event_tasklet_enable(newchannel);
 
 	vmbus_release_relid(newchannel->offermsg.child_relid);
 

include/linux/hyperv.h

···
 				const int *srv_version, int srv_vercnt,
 				int *nego_fw_version, int *nego_srv_version);
 
-void hv_event_tasklet_disable(struct vmbus_channel *channel);
-void hv_event_tasklet_enable(struct vmbus_channel *channel);
-
 void hv_process_channel_removal(struct vmbus_channel *channel, u32 relid);
 
 void vmbus_setevent(struct vmbus_channel *channel);