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

["Zheng, Lv" <lv.zheng@intel.com>]

Hi, Dan

> From: Dan Williams [mailto:dan.j.williams@intel.com]
> Sent: Thursday, May 4, 2017 11:45 PM
> Subject: Re: [PATCH v3 2/4] ACPICA: Tables: Add mechanism to allow to balance late stage
> acpi_get_table() independently
> 
> On Thu, May 4, 2017 at 12:18 AM, Zheng, Lv <lv.zheng@intel.com> wrote:
> > Hi, Rafael
> >
> >> From: linux-acpi-owner@vger.kernel.org [mailto:linux-acpi-owner@vger.kernel.org] 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@intel.com>
> >> > Signed-off-by: Lv Zheng <lv.zheng@intel.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?
> >
> > 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.
> >
> 
> Why do we need validation_count at all? I would think you would only
> need that if tables can be hot-removed and you need to wait for all
> active users to drain. Most tables don't have that behavior, right?
> Should we instead be reference counting the few tables that might be
> removed and leave the rest statically allocated?

For now, we need that for hot-removal in early boot stage, not late stage.
Otherwise you'll see warnings reported from check_early_ioremap_leak().
acpi_get_table()/acpi_put_table() is now just a replacement of acpi_get_table_with_size()/early_acpi_os_unmap_memory().
They are required to be paired for early stage.
But not yet enabled for late stage.

Please check https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=174cc7187
You can see a long story in this patch description.

Thanks and best regards
Lv