[PATCH v3] vmbus: fix missed ring events on boot

[Stephen Hemminger]

During initialization, the channel initialization code schedules the
tasklet to scan the VMBUS receive event page (i.e. simulates an
interrupt). The problem was that it invokes the tasklet on a different
CPU from where it normally runs and therefore if an event is present,
it will clear the bit but not find the associated channel.

This can lead to missed events, typically stuck tasks, during bootup
when sub channels are being initialized. Typically seen as stuck
boot with 8 or more CPU's.

This patch is for 4.10 and 4.9; but was introduced by fix to solve
a race on updates. It is not necessary for 4.11 and later since
commit 631e63a9f346 ("vmbus: change to per channel tasklet").
which eliminated the common tasklet which caused the problem.

Cc: stable at vger.kernel.org
Fixes: 638fea33aee8 ("Drivers: hv: vmbus: fix the race when querying & updating the percpu list")
Signed-off-by: Stephen Hemminger <sthemmin at microsoft.com>
---
Patch is against linux-4.10.y branch

v3 - avoid possible preempt race
v2 - simplify only change hv_event_tasklet_enable

 drivers/hv/channel_mgmt.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index 0af7e39006c8..74afef6271f0 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -361,8 +361,17 @@ void hv_event_tasklet_enable(struct vmbus_channel *channel)
 	tasklet = hv_context.event_dpc[channel->target_cpu];
 	tasklet_enable(tasklet);
 
-	/* In case there is any pending event */
-	tasklet_schedule(tasklet);
+	/*
+	 * In case there is any pending event schedule a rescan
+	 * but must be on the correct CPU for the channel.
+	 */
+	if (channel->target_cpu == get_cpu())
+		tasklet_schedule(tasklet);
+	else
+		smp_call_function_single(channel->target_cpu,
+					 (smp_call_func_t)tasklet_schedule,
+					 tasklet, false);
+	put_cpu();
 }
 
 void hv_process_channel_removal(struct vmbus_channel *channel, u32 relid)
-- 
2.11.0