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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Michael Kelley <mikelley@microsoft.com>
Cc:     stable@vger.kernel.org, kys@microsoft.com, haiyangz@microsoft.com,
        wei.liu@kernel.org, linux-kernel@vger.kernel.org,
        linux-hyperv@vger.kernel.org, decui@microsoft.com
Subject: Re: [PATCH 1/1] Drivers: hv: vmbus: Fix vmbus_wait_for_unload() to
 scan present CPUs
In-Reply-To: <1684172191-17100-1-git-send-email-mikelley@microsoft.com>
References: <1684172191-17100-1-git-send-email-mikelley@microsoft.com>
Date:   Tue, 16 May 2023 11:11:32 +0200
Message-ID: <87pm707i9n.fsf@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Michael Kelley <mikelley@microsoft.com> writes:

> vmbus_wait_for_unload() may be called in the panic path after other
> CPUs are stopped. vmbus_wait_for_unload() currently loops through
> online CPUs looking for the UNLOAD response message. But the values of
> CONFIG_KEXEC_CORE and crash_kexec_post_notifiers affect the path used
> to stop the other CPUs, and in one of the paths the stopped CPUs
> are removed from cpu_online_mask. This removal happens in both
> x86/x64 and arm64 architectures. In such a case, vmbus_wait_for_unload()
> only checks the panic'ing CPU, and misses the UNLOAD response message
> except when the panic'ing CPU is CPU 0. vmbus_wait_for_unload()
> eventually times out, but only after waiting 100 seconds.
>
> Fix this by looping through *present* CPUs in vmbus_wait_for_unload().
> The cpu_present_mask is not modified by stopping the other CPUs in the
> panic path, nor should it be.  Furthermore, the synic_message_page
> being checked in vmbus_wait_for_unload() is allocated in
> hv_synic_alloc() for all present CPUs. So looping through the
> present CPUs is more consistent.
>
> For additional safety, also add a check for the message_page being
> NULL before looking for the UNLOAD response message.
>
> Reported-by: John Starks <jostarks@microsoft.com>
> Fixes: cd95aad55793 ("Drivers: hv: vmbus: handle various crash scenarios")

I see you Cc:ed stable@ on the patch, should we also add 

Cc: stable@vger.kernel.org

here explicitly so it gets picked up by various stable backporting
scritps? I guess Wei can do it when picking the patch to the queue...

> Signed-off-by: Michael Kelley <mikelley@microsoft.com>
> ---
>  drivers/hv/channel_mgmt.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
> index 007f26d..df2ba20 100644
> --- a/drivers/hv/channel_mgmt.c
> +++ b/drivers/hv/channel_mgmt.c
> @@ -829,11 +829,14 @@ static void vmbus_wait_for_unload(void)
>  		if (completion_done(&vmbus_connection.unload_event))
>  			goto completed;
>  
> -		for_each_online_cpu(cpu) {
> +		for_each_present_cpu(cpu) {
>  			struct hv_per_cpu_context *hv_cpu
>  				= per_cpu_ptr(hv_context.cpu_context, cpu);
>  
>  			page_addr = hv_cpu->synic_message_page;
> +			if (!page_addr)
> +				continue;
> +

In theory, synic_message_page for all present CPUs is permanently
assigned in hv_synic_alloc() and we fail the whole thing if any of these
allocations fail so page_addr == NULL is likely impossible today
but there's certainly no harm in having this extra check here, this is
not a hotpath.

>  			msg = (struct hv_message *)page_addr
>  				+ VMBUS_MESSAGE_SINT;
>  
> @@ -867,11 +870,14 @@ static void vmbus_wait_for_unload(void)
>  	 * maybe-pending messages on all CPUs to be able to receive new
>  	 * messages after we reconnect.
>  	 */
> -	for_each_online_cpu(cpu) {
> +	for_each_present_cpu(cpu) {
>  		struct hv_per_cpu_context *hv_cpu
>  			= per_cpu_ptr(hv_context.cpu_context, cpu);
>  
>  		page_addr = hv_cpu->synic_message_page;
> +		if (!page_addr)
> +			continue;
> +
>  		msg = (struct hv_message *)page_addr + VMBUS_MESSAGE_SINT;
>  		msg->header.message_type = HVMSG_NONE;
>  	}

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

-- 
Vitaly