linux-kernel - Re: [PATCH v3 2/4] ACPICA: Tables: Add mechanism to allow to balance late stage acpi_get

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Date:   Fri, 28 Apr 2017 22:56 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Lv Zheng <lv.zheng@...el.com>
Cc:     "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Len Brown <len.brown@...el.com>, Lv Zheng <zetalog@...il.com>,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v3 2/4] ACPICA: Tables: Add mechanism to allow to balance late stage acpi_get_table() independently

On Friday, April 28, 2017 01:30:20 PM Lv Zheng wrote:
> For all frequent late stage acpi_get_table() clone invocations, we should
> only fix them altogether, otherwise, excessive acpi_put_table() could
> unexpectedly unmap the table used by the other users. Thus the current plan
> is to fix all acpi_get_table() clones together or to fix none of them.

I honestly don't think that fixing none of them is a valid option here.

> This prevents kernel developers from improving the late stage code quality
> without waiting for the ACPICA upstream to improve first.
> 
> This patch adds a mechanism to stop decrementing validation count to
> prevent the table unmapping operations so that acpi_put_table() balance
> fixes can be done independently to each others.
> 
> Cc: Dan Williams <dan.j.williams@...el.com>
> Signed-off-by: Lv Zheng <lv.zheng@...el.com>
> ---
>  drivers/acpi/acpica/tbutils.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/acpi/acpica/tbutils.c b/drivers/acpi/acpica/tbutils.c
> index 7abe665..b517bd0 100644
> --- a/drivers/acpi/acpica/tbutils.c
> +++ b/drivers/acpi/acpica/tbutils.c
> @@ -445,12 +445,18 @@ void acpi_tb_put_table(struct acpi_table_desc *table_desc)
>  
>  	ACPI_FUNCTION_TRACE(acpi_tb_put_table);
>  
> -	if (table_desc->validation_count == 0) {
> +	if ((table_desc->validation_count + 1) == 0) {

This means that validation_count has reached the maximum value, right?

>  		ACPI_WARNING((AE_INFO,
> -			      "Table %p, Validation count is zero before decrement\n",
> +			      "Table %p, Validation count is about to expire, decrement is unsafe\n",
>  			      table_desc));

So why is it unsafe to decrement it?

>  		return_VOID;
>  	}
> +	if (table_desc->validation_count == 0) {
> +		ACPI_ERROR((AE_INFO,
> +			   "Table %p, Validation count is zero before decrement\n",
> +			   table_desc));
> +		return_VOID;
> +	}
>  	table_desc->validation_count--;
>  
>  	if (table_desc->validation_count == 0) {
> 

Thanks,
Rafael