RE: [PATCH v3 2/4] ACPICA: Tables: Add mechanism to allow to balance late stage acpi_get_table() independently

[Zheng, Lv]

Hi, Rafael

> From: linux-acpi-owner@xxxxxxxxxxxxxxx [mailto:linux-acpi-owner@xxxxxxxxxxxxxxx] On Behalf Of Rafael J.
> Wysocki
> 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.

That's just exactly the old behavior, maybe shouldn't be called as "fix".
Should say "change to use the new behavior together" all stay unchanged.

I actually want to make the change from ACPICA side.
But it's costly to persuade ACPICA upstream to take both the "acpi_get_table_with_size()/early_acpi_os_unmap_memory() divergence reduction" change and the "table map on-demand" change.

So we just made 2 things separated, and did 1 thing once.

> 
> > 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@xxxxxxxxx>
> > Signed-off-by: Lv Zheng <lv.zheng@xxxxxxxxx>
> > ---
> >  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?

Considering this case:
A program opens a sysfs table file 65535 times: validation_count = 65535.
Load opcode is invoked by the AML interpreter, but it cannot increase the validation count, see acpi_tb_get_table(): validation_count = 65535.
Now the program closes the sysfs table file: validation_count = 0, which triggers table unmap.
But it is likely that the AML code is still accessing the namespace objects provided by this table.
A kernel crash then can be seen.

So after applying this patch, 65535 now is the threshold.
When it is reached, validation_count will remain 65535 from then on (see both acpi_tb_get_table()/acpi_tb_put_table()).
When it is reached, the 65535 validation count ensures "the old behavior" - for late stage;
When it is not reached, the 65535 validation count ensures "the new behavior" - for early stage.

Then you can see, if there's no acpi_put_table() invoked for such old behavior dependent users, the validation count can also remain 65535.
That's why I said PATCH 3 is actually breaking things.

IMO, if we really want the acpi_put_table() balance work proceeded without waiting for the ACPICA upstream to change.
We need this commit.

I actually generated this commit once.
But hesitated to send it to ACPICA upstream as it didn't look like a good idea to increase communication cost to upstream a commit that hadn't been determined to be used by ACPICA.

However if other driver maintainers want to make their acpi_get_table() invocations balanced like what Dan did here.
This commit is required.

Thanks and best regards
Lv

> 
> >  		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
> 
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