[PATCH 2/3] Staging: hv: Re-implement the synchronization for util channels

[K. Y. Srinivasan]

The util module expects that the util channels are fully initialized
when the module loads. To deal with the race condition which can result
in a NULL pointer dereferencing if the util module were to load before
all the util channels are fully initialized, in commit:

	commit: 8b5d6d3bd3e34e4cc67d875c8c88007c1c9aa96
	Author: Haiyang Zhang <haiyangz at microsoft.com>
	Date:   Fri May 28 23:22:44 2010 +000

code was introduced in the vmbus driver to ensure that all the
util channels were fully initialized before returning from the load
of the vmbus driver. This solution has several problems: if for whatever
reason, any util channel were to fail to initialize, vmbus driver would
wait indefinitely. We deal with this synchronization issue very differently
in this patch. 

Signed-off-by: K. Y. Srinivasan <kys at microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz at microsoft.com>
Signed-off-by: Abhishek Kane <v-abkane at microsoft.com>
Signed-off-by: Hank Janssen <hjanssen at microsoft.com>
---
 drivers/staging/hv/channel_mgmt.c |   24 ++++++++++++++++++----
 drivers/staging/hv/hv_util.c      |   39 +++++++++++++++++-------------------
 2 files changed, 37 insertions(+), 26 deletions(-)

diff --git a/drivers/staging/hv/channel_mgmt.c b/drivers/staging/hv/channel_mgmt.c
index 1021713..b7343bc 100644
--- a/drivers/staging/hv/channel_mgmt.c
+++ b/drivers/staging/hv/channel_mgmt.c
@@ -181,6 +181,24 @@ void chn_cb_negotiate(void *context)
 	struct icmsg_hdr *icmsghdrp;
 	struct icmsg_negotiate *negop = NULL;
 
+	if (channel->util_index >= 0) {
+		/*
+		 * This is a properly initialized util channel.
+		 * Route this callback appropriately and setup state
+		 * so that we don't need to reroute again.
+		 */
+		if (hv_cb_utils[channel->util_index].callback != NULL) {
+			/*
+			 * The util driver has established a handler for
+			 * this service; do the magic.
+			 */
+			channel->onchannel_callback =
+			hv_cb_utils[channel->util_index].callback;
+			(hv_cb_utils[channel->util_index].callback)(channel);
+			return;
+		}
+	}
+
 	buflen = PAGE_SIZE;
 	buf = kmalloc(buflen, GFP_ATOMIC);
 
@@ -217,7 +235,6 @@ struct hyperv_service_callback hv_cb_utils[MAX_MSG_TYPES] = {
 			0x31, 0x60, 0x0B, 0X0E, 0x13, 0x52, 0x34, 0x49,
 			0x81, 0x8B, 0x38, 0XD9, 0x0C, 0xED, 0x39, 0xDB
 		},
-		.callback = chn_cb_negotiate,
 		.log_msg = "Shutdown channel functionality initialized"
 	},
 
@@ -229,7 +246,6 @@ struct hyperv_service_callback hv_cb_utils[MAX_MSG_TYPES] = {
 			0x30, 0xe6, 0x27, 0x95, 0xae, 0xd0, 0x7b, 0x49,
 			0xad, 0xce, 0xe8, 0x0a, 0xb0, 0x17, 0x5c, 0xaf
 		},
-		.callback = chn_cb_negotiate,
 		.log_msg = "Timesync channel functionality initialized"
 	},
 	/* {57164f39-9115-4e78-ab55-382f3bd5422d} */
@@ -240,7 +256,6 @@ struct hyperv_service_callback hv_cb_utils[MAX_MSG_TYPES] = {
 			0x39, 0x4f, 0x16, 0x57, 0x15, 0x91, 0x78, 0x4e,
 			0xab, 0x55, 0x38, 0x2f, 0x3b, 0xd5, 0x42, 0x2d
 		},
-		.callback = chn_cb_negotiate,
 		.log_msg = "Heartbeat channel functionality initialized"
 	},
 	/* {A9A0F4E7-5A45-4d96-B827-8A841E8C03E6} */
@@ -250,7 +265,6 @@ struct hyperv_service_callback hv_cb_utils[MAX_MSG_TYPES] = {
 			0xe7, 0xf4, 0xa0, 0xa9, 0x45, 0x5a, 0x96, 0x4d,
 			0xb8, 0x27, 0x8a, 0x84, 0x1e, 0x8c, 0x3,  0xe6
 		},
-		.callback = chn_cb_negotiate,
 		.log_msg = "KVP channel functionality initialized"
 	},
 };
@@ -428,7 +442,7 @@ static void vmbus_process_offer(struct work_struct *work)
 				   sizeof(struct hv_guid)) == 0 &&
 				vmbus_open(newchannel, 2 * PAGE_SIZE,
 						 2 * PAGE_SIZE, NULL, 0,
-						 hv_cb_utils[cnt].callback,
+						 chn_cb_negotiate,
 						 newchannel) == 0) {
 				hv_cb_utils[cnt].channel = newchannel;
 				newchannel->util_index = cnt;
diff --git a/drivers/staging/hv/hv_util.c b/drivers/staging/hv/hv_util.c
index 5605374..c164b54 100644
--- a/drivers/staging/hv/hv_util.c
+++ b/drivers/staging/hv/hv_util.c
@@ -256,22 +256,13 @@ static int __init init_hyperv_utils(void)
 		return -ENOMEM;
 	}
 
-	hv_cb_utils[HV_SHUTDOWN_MSG].channel->onchannel_callback =
-		&shutdown_onchannelcallback;
 	hv_cb_utils[HV_SHUTDOWN_MSG].callback = &shutdown_onchannelcallback;
 
-	hv_cb_utils[HV_TIMESYNC_MSG].channel->onchannel_callback =
-		&timesync_onchannelcallback;
 	hv_cb_utils[HV_TIMESYNC_MSG].callback = &timesync_onchannelcallback;
 
-	hv_cb_utils[HV_HEARTBEAT_MSG].channel->onchannel_callback =
-		&heartbeat_onchannelcallback;
 	hv_cb_utils[HV_HEARTBEAT_MSG].callback = &heartbeat_onchannelcallback;
 
-	hv_cb_utils[HV_KVP_MSG].channel->onchannel_callback =
-		&hv_kvp_onchannelcallback;
-
-
+	hv_cb_utils[HV_KVP_MSG].callback = &hv_kvp_onchannelcallback;
 
 	return 0;
 }
@@ -280,20 +271,26 @@ static void exit_hyperv_utils(void)
 {
 	pr_info("De-Registered HyperV Utility Driver\n");
 
-	hv_cb_utils[HV_SHUTDOWN_MSG].channel->onchannel_callback =
-		&chn_cb_negotiate;
-	hv_cb_utils[HV_SHUTDOWN_MSG].callback = &chn_cb_negotiate;
+	if (hv_cb_utils[HV_SHUTDOWN_MSG].channel != NULL)
+		hv_cb_utils[HV_SHUTDOWN_MSG].channel->onchannel_callback =
+			&chn_cb_negotiate;
+	hv_cb_utils[HV_SHUTDOWN_MSG].callback = NULL;
+
+	if (hv_cb_utils[HV_TIMESYNC_MSG].channel != NULL)
+		hv_cb_utils[HV_TIMESYNC_MSG].channel->onchannel_callback =
+			&chn_cb_negotiate;
+	hv_cb_utils[HV_TIMESYNC_MSG].callback = NULL;
 
-	hv_cb_utils[HV_TIMESYNC_MSG].channel->onchannel_callback =
-		&chn_cb_negotiate;
-	hv_cb_utils[HV_TIMESYNC_MSG].callback = &chn_cb_negotiate;
+	if (hv_cb_utils[HV_HEARTBEAT_MSG].channel != NULL)
+		hv_cb_utils[HV_HEARTBEAT_MSG].channel->onchannel_callback =
+			&chn_cb_negotiate;
+	hv_cb_utils[HV_HEARTBEAT_MSG].callback = NULL;
 
-	hv_cb_utils[HV_HEARTBEAT_MSG].channel->onchannel_callback =
-		&chn_cb_negotiate;
-	hv_cb_utils[HV_HEARTBEAT_MSG].callback = &chn_cb_negotiate;
+	if (hv_cb_utils[HV_KVP_MSG].channel != NULL)
+		hv_cb_utils[HV_KVP_MSG].channel->onchannel_callback =
+			&chn_cb_negotiate;
+	hv_cb_utils[HV_KVP_MSG].callback = NULL;
 
-	hv_cb_utils[HV_KVP_MSG].channel->onchannel_callback =
-		&chn_cb_negotiate;
 	hv_kvp_deinit();
 
 	kfree(shut_txf_buf);
-- 
1.7.4.1