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

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Date:   Tue, 6 Jun 2017 23:41:59 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Lv Zheng <lv.zheng@...el.com>
Cc:     "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 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. Which ACPI tables
might be hot removed? If it's only a small handful of tables why not
teach the code that handles those exceptional cases to manage a
dedicated reference count mechanism? That way the other cases can be
left alone and not worry about balancing their references.