linux-kernel - RE: [PATCH] Drivers: hv: vmbus: Use raw_smp_processor_id() in vmbus

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Date:   Thu, 30 Aug 2018 16:28:22 +0000
From:   KY Srinivasan <kys@...rosoft.com>
To:     Dexuan Cui <decui@...rosoft.com>,
        "'gregkh@...uxfoundation.org'" <gregkh@...uxfoundation.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>
CC:     "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
        "'driverdev-devel@...uxdriverproject.org'" 
        <driverdev-devel@...uxdriverproject.org>,
        "'olaf@...fle.de'" <olaf@...fle.de>,
        "'apw@...onical.com'" <apw@...onical.com>,
        "'jasowang@...hat.com'" <jasowang@...hat.com>,
        vkuznets <vkuznets@...hat.com>,
        "'marcelo.cerri@...onical.com'" <marcelo.cerri@...onical.com>
Subject: RE: [PATCH] Drivers: hv: vmbus: Use raw_smp_processor_id() in
 vmbus_connect()

> -----Original Message-----
> From: Dexuan Cui
> Sent: Thursday, August 23, 2018 3:31 PM
> To: 'gregkh@...uxfoundation.org' <gregkh@...uxfoundation.org>; KY
> Srinivasan <kys@...rosoft.com>; Stephen Hemminger
> <sthemmin@...rosoft.com>; Haiyang Zhang <haiyangz@...rosoft.com>
> Cc: 'linux-kernel@...r.kernel.org' <linux-kernel@...r.kernel.org>;
> 'driverdev-devel@...uxdriverproject.org' <driverdev-
> devel@...uxdriverproject.org>; 'olaf@...fle.de' <olaf@...fle.de>;
> 'apw@...onical.com' <apw@...onical.com>; 'jasowang@...hat.com'
> <jasowang@...hat.com>; vkuznets <vkuznets@...hat.com>;
> 'marcelo.cerri@...onical.com' <marcelo.cerri@...onical.com>
> Subject: [PATCH] Drivers: hv: vmbus: Use raw_smp_processor_id() in
> vmbus_connect()
> 
> 
> With CONFIG_DEBUG_PREEMPT=y, I always see this warning:
> BUG: using smp_processor_id() in preemptible [00000000]
> 
> Fix the false warning by using raw_smp_processor_id().
> 
> 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
> use raw_smp_processor_id() here.
> 
> Signed-off-by: Dexuan Cui <decui@...rosoft.com>
> Cc: stable@...r.kernel.org
> Cc: K. Y. Srinivasan <kys@...rosoft.com>
> Cc: Haiyang Zhang <haiyangz@...rosoft.com>
> Cc: Stephen Hemminger <sthemmin@...rosoft.com>
> ---
>  drivers/hv/connection.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hv/connection.c b/drivers/hv/connection.c
> index ced0418..c3b7040 100644
> --- a/drivers/hv/connection.c
> +++ b/drivers/hv/connection.c
> @@ -119,8 +119,8 @@ static int vmbus_negotiate_version(struct
> vmbus_channel_msginfo *msginfo,
>  	 */
>  	if (version >= VERSION_WIN8_1) {
>  		msg->target_vcpu =
> -
> 	hv_cpu_number_to_vp_number(smp_processor_id());
> -		vmbus_connection.connect_cpu = smp_processor_id();
> +
> 	hv_cpu_number_to_vp_number(raw_smp_processor_id());
> +		vmbus_connection.connect_cpu = raw_smp_processor_id();
Dexuan,

We want a consistent view of the CPU we are currently running on. How about doing something
like this:

diff --git a/drivers/hv/connection.c b/drivers/hv/connection.c
index ced041899456..4cc0877ba38a 100644
--- a/drivers/hv/connection.c
+++ b/drivers/hv/connection.c
@@ -119,8 +119,9 @@ static int vmbus_negotiate_version(struct vmbus_channel_msginfo *msginfo,
         */
        if (version >= VERSION_WIN8_1) {
                msg->target_vcpu =
-                       hv_cpu_number_to_vp_number(smp_processor_id());
-               vmbus_connection.connect_cpu = smp_processor_id();
+                       hv_cpu_number_to_vp_number(get_cpu());
+               vmbus_connection.connect_cpu = msg->target_vcpu;
+               put_cpu();
        } else {
                msg->target_vcpu = 0;
                vmbus_connection.connect_cpu = 0;

>  	} else {
>  		msg->target_vcpu = 0;
>  		vmbus_connection.connect_cpu = 0;
> --
> 2.7.4