linux-kernel - Re: [PATCH v5] ACPICA: Tables: Add mechanism to allow to balance late stage acpi_get

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Date:   Wed, 7 Jun 2017 23:14:27 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Lv Zheng <lv.zheng@...el.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>, Lv Zheng <zetalog@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH v5] ACPICA: Tables: Add mechanism to allow to balance late
 stage acpi_get_table() independently

On Wed, Jun 7, 2017 at 8:41 AM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Tue, Jun 6, 2017 at 9:54 PM, Lv Zheng <lv.zheng@...el.com> wrote:
>> Considering this case:
>> 1. A program opens a sysfs table file 65535 times, it can increase
>>    validation_count and first increment cause the table to be mapped:
>>     validation_count = 65535
>> 2. AML execution causes "Load" to be executed on the same table, this time
>>    it cannot increase validation_count, so validation_count remains:
>>     validation_count = 65535
>> 3. The program closes sysfs table file 65535 times, it can decrease
>>    validation_count and the last decrement cause the table to be unmapped:
>>     validation_count = 0
>> 4. AML code still accessing the loaded table, kernel crash can be observed.
>>
>> This is because orginally ACPICA doesn't support unmapping tables during
>> OS late stage. So the current code only allows unmapping tables during OS
>> early stage, and for late stage, no acpi_put_table() clones should be
>> invoked, especially cases that can trigger frequent invocations of
>> acpi_get_table()/acpi_put_table() are forbidden:
>> 1. sysfs table accesses
>> 2. dynamic Load/Unload opcode executions
>> 3. acpi_load_table()
>> 4. etc.
>> Such frequent acpi_put_table() balance changes have to be done altogether.
>>
>> This philosophy is not convenient for Linux driver writers. Since the API
>> is just there, developers will start to use acpi_put_table() during late
>> stage. So we need to consider a better mechanism to allow them to safely
>> invoke acpi_put_table().
>>
>> This patch provides such a mechanism by adding 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.
>>
>> Note: code added in acpi_tb_put_table() is actually a no-op but changes the
>> warning message into a warning once message. Lv Zheng.
>>
>
> This still seems to be unnecessary gymnastics to keep the validation
> count around and make it work for random drivers.

Well, I'm not sure I agree here.

If we can make it work at one point, it should not be too hard to
maintain that status.

Thanks,
Rafael