linux-kernel - RE: [PATCH] hv: hv_balloon: Fixed an issue in the earor handling code if probe failed

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Message-ID: <PH0PR21MB3025EB93E33F29C913CA4193D7C49@PH0PR21MB3025.namprd21.prod.outlook.com>
Date:   Sat, 7 May 2022 15:13:24 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Shradha Gupta <shradhagupta@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] hv: hv_balloon: Fixed an issue in the earor handling code
 if probe failed

From: Shradha Gupta <shradhagupta@...rosoft.com>
> Subject: [PATCH] hv: hv_balloon: Fixed an issue in the earor handling code if probe failed

Typo in the "Subject" line --  'earor' should be 'error'.   Also the prefix
to use has varied historically, but is either "hv_balloon:" or the full
"Drivers: hv: balloon:".   Use one of those two prefixes rather than
creating yet another variant.

And let me suggest simpler wording.  For example:

hv_balloon: Fix balloon_probe() and balloon_remove() error handling

> 
> If the balloon_probe() function fails, we do some cleanup and similar
> functions are called again when after this a balloon_remove()
> call is made. This fix makes sure that the cleanup is not called
> twice. Also made sure if dm_state is DM_INIT_ERROR, the clean up is
> already done properly.

I don't think this commit message is quite correct.  There is indeed
some missing cleanup in balloon_probe() if balloon_connect_vsp() fails.
But if balloon_probe() fails, balloon_remove() will never be called, so
there's no duplicate cleanup in that scenario.

But if balloon_resume() fails, then balloon_remove() could be
called at some point and mistakenly do duplicate cleanup.  That's
how balloon_remove() could be called with dm_state set to
DM_INIT_ERROR. 

Also commit messages should generally not use language like "this
patch" or "this fix".  Just state the change in the imperative
voice.

I would suggest wording such as the following:

Add missing cleanup in balloon_probe() if the call to
balloon_connect_vsp() fails.  Also correctly handle cleanup in
balloon_remove() when dm_state is DM_INIT_ERROR because
balloon_resume() failed.

> 
> Signed-off-by: Shradha Gupta <shradhagupta@...rosoft.com>
> ---
>  drivers/hv/hv_balloon.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> index eee7402cfc02..7c62c1c3ffc7 100644
> --- a/drivers/hv/hv_balloon.c
> +++ b/drivers/hv/hv_balloon.c
> @@ -1841,8 +1841,13 @@ static int balloon_probe(struct hv_device *dev,
>  	hv_set_drvdata(dev, &dm_device);
> 
>  	ret = balloon_connect_vsp(dev);
> -	if (ret != 0)
> +	if (ret != 0) {
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +		unregister_memory_notifier(&hv_memory_nb);
> +		restore_online_page_callback(&hv_online_page);
> +#endif
>  		return ret;
> +	}

Rather than coding the cleanup steps directly under the "if"
statement, I would suggest using "goto connect_error", and adding
a "connect_error" label immediately after the vmbus_close() in
the "probe_error" sequence.  This approach avoids some
duplicate code.

> 
>  	enable_page_reporting();
>  	dm_device.state = DM_INITIALIZED;
> @@ -1882,12 +1887,14 @@ static int balloon_remove(struct hv_device *dev)
>  	cancel_work_sync(&dm->ha_wrk.wrk);
> 
>  	kthread_stop(dm->thread);
> -	disable_page_reporting();
> -	vmbus_close(dev->channel);
> +	if (dm_device.state != DM_INIT_ERROR) {
> +		disable_page_reporting();
> +		vmbus_close(dev->channel);
>  #ifdef CONFIG_MEMORY_HOTPLUG
> -	unregister_memory_notifier(&hv_memory_nb);
> -	restore_online_page_callback(&hv_online_page);
> +		unregister_memory_notifier(&hv_memory_nb);
> +		restore_online_page_callback(&hv_online_page);
>  #endif
> +	}

This case where dm_device.state != DM_INIT_ERROR can only
occur when balloon_resume() fails.   Looking at balloon_resume(),
its error handling code has already done the cleanup except for
disable_page_reporting().   A call to disable_page_reporting()
needs to be done somewhere for this case, and for consistency
balloon_resume() is probably the best place.

I would also suggest adding a comment just above the new "if"
statement you've added, indicating that the case being handled
is when balloon_resume() fails.  Again, balloon_remove() won't
be called if balloon_probe() fails, so it is a little bit surprising
to end up in balloon_remove() with the state being DM_INIT_ERROR.

Michael

>  	spin_lock_irqsave(&dm_device.ha_lock, flags);
>  	list_for_each_entry_safe(has, tmp, &dm->ha_region_list, list) {
>  		list_for_each_entry_safe(gap, tmp_gap, &has->gap_list, list) {
> --
> 2.17.1
>