linux-kernel - Re: [PATCH v4 2/4] ACPICA: Tables: Add mechanism to allow to balance late stage acpi_get

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Date:   Fri, 12 May 2017 23:03:52 +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 v4 2/4] ACPICA: Tables: Add mechanism to allow to balance late stage acpi_get_table() independently

On Tuesday, May 09, 2017 01:57:41 PM Lv Zheng wrote:
> For all frequent late stage acpi_get_table() clone invocations, we should
> only change them altogether, otherwise, excessive acpi_put_table() could
> unexpectedly unmap the table used by the other users. Thus the current plan
> is to change all acpi_get_table() clones together or to change none of
> them. However in practical, this is not convenient as this can prevent
> kernel developers' efforts of improving the late stage code quality before
> waiting for the ACPICA upstream to improve first.
> 
> This patch adds a validation count threashold, when it is reached, the
> validation count can no longer be incremented/decremented to invalidate the
> table descriptor (means preventing table unmappings) so that acpi_put_table()
> balance changes can be done independently to each others. Lv Zheng.
> 
> Cc: Dan Williams <dan.j.williams@...el.com>
> Signed-off-by: Lv Zheng <lv.zheng@...el.com>
> ---
>  drivers/acpi/acpica/tbutils.c | 24 +++++++++++++++---------
>  include/acpi/actbl.h          |  9 +++++++++
>  2 files changed, 24 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/acpi/acpica/tbutils.c b/drivers/acpi/acpica/tbutils.c
> index 7abe665..04beafc 100644
> --- a/drivers/acpi/acpica/tbutils.c
> +++ b/drivers/acpi/acpica/tbutils.c
> @@ -416,9 +416,13 @@ acpi_tb_get_table(struct acpi_table_desc *table_desc,
>  		}
>  	}
>  
> -	table_desc->validation_count++;
> -	if (table_desc->validation_count == 0) {
> -		table_desc->validation_count--;
> +	if (table_desc->validation_count < ACPI_MAX_TABLE_VALIDATIONS) {
> +		table_desc->validation_count++;
> +		if (table_desc->validation_count >= ACPI_MAX_TABLE_VALIDATIONS) {
> +			ACPI_WARNING((AE_INFO,
> +				      "Table %p, Validation count overflows\n",
> +				      table_desc));
> +		}
>  	}
>  
>  	*out_table = table_desc->pointer;
> @@ -445,13 +449,15 @@ void acpi_tb_put_table(struct acpi_table_desc *table_desc)
>  
>  	ACPI_FUNCTION_TRACE(acpi_tb_put_table);
>  
> -	if (table_desc->validation_count == 0) {
> -		ACPI_WARNING((AE_INFO,
> -			      "Table %p, Validation count is zero before decrement\n",
> -			      table_desc));
> -		return_VOID;
> +	if (table_desc->validation_count < ACPI_MAX_TABLE_VALIDATIONS) {
> +		table_desc->validation_count--;
> +		if (table_desc->validation_count >= ACPI_MAX_TABLE_VALIDATIONS) {

Is this going to ever trigger?

We've already verified that validation_count is not 0 and that it is less than
ACPI_MAX_TABLE_VALIDATIONS and we have decremented it, so how can it be
greater than or equal to ACPI_MAX_TABLE_VALIDATIONS here?

> +			ACPI_WARNING((AE_INFO,
> +				      "Table %p, Validation count underflows\n",
> +				      table_desc));
> +			return_VOID;
> +		}
>  	}
> -	table_desc->validation_count--;
>  
>  	if (table_desc->validation_count == 0) {
>

Thanks,
Rafael