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From:   Ben Hutchings <ben@decadent.org.uk>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
CC:     akpm@linux-foundation.org, "Dexuan Cui" <decui@microsoft.com>,
        "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
        "Haiyang Zhang" <haiyangz@microsoft.com>,
        "Stephen Hemminger" <sthemmin@microsoft.com>,
        "K. Y. Srinivasan" <kys@microsoft.com>
Date:   Sun, 09 Dec 2018 21:50:33 +0000
Message-ID: <lsq.1544392233.23981975@decadent.org.uk>
Subject: [PATCH 3.16 279/328] Drivers: hv: vmbus: Use get/put_cpu() in
 vmbus_connect()
In-Reply-To: <lsq.1544392232.713909046@decadent.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Dexuan Cui <decui@microsoft.com>

commit 41e270f6898e7502be9fd6920ee0a108ca259d36 upstream.

With CONFIG_DEBUG_PREEMPT=y, I always see this warning:
BUG: using smp_processor_id() in preemptible [00000000]

Fix the false warning by using get/put_cpu().

Here vmbus_connect() sends a message to the host and waits for the
host's response. The host will deliver the response message and an
interrupt on CPU msg->target_vcpu, and later the interrupt handler
will wake up vmbus_connect(). vmbus_connect() doesn't really have
to run on the same cpu as CPU msg->target_vcpu, so it's safe to
call put_cpu() just here.

Signed-off-by: Dexuan Cui <decui@microsoft.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.16:
 - smp_processor_id() is only used once here
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 drivers/hv/connection.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/drivers/hv/connection.c
+++ b/drivers/hv/connection.c
@@ -68,6 +68,7 @@ static int vmbus_negotiate_version(struc
 					__u32 version)
 {
 	int ret = 0;
+	unsigned int cur_cpu;
 	struct vmbus_channel_initiate_contact *msg;
 	unsigned long flags;
 
@@ -80,8 +81,11 @@ static int vmbus_negotiate_version(struc
 	msg->interrupt_page = virt_to_phys(vmbus_connection.int_page);
 	msg->monitor_page1 = virt_to_phys(vmbus_connection.monitor_pages[0]);
 	msg->monitor_page2 = virt_to_phys(vmbus_connection.monitor_pages[1]);
-	if (version == VERSION_WIN8_1)
-		msg->target_vcpu = hv_context.vp_index[smp_processor_id()];
+	if (version == VERSION_WIN8_1) {
+		cur_cpu = get_cpu();
+		msg->target_vcpu = hv_context.vp_index[cur_cpu];
+		put_cpu();
+	}
 
 	/*
 	 * Add to list before we send the request since we may