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    SubjectRe: [PATCH v3 2/4] ACPICA: Tables: Add mechanism to allow to balance late stage acpi_get_table() independently
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    On Thursday, May 04, 2017 07:18:28 AM Zheng, Lv 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.

    OK, so this is overflow detection in disguise. :-)

    It is quite confusing, IMO. It would be better to define a limit symbol like
    ACPI_TABLE_VCOUNT_MAX below the natural maximum of the data type
    (say, make it equal to 65534 if the data type is unsigned short int) and then
    make *both* acpi_tb_get_table() and acpi_tb_put_table() refuse to update
    validation_count *and* print a "validation count overflow" message once it
    has become greater than ACPI_TABLE_VCOUNT_MAX (in which case it will
    natrually stay at ACPI_TABLE_VCOUNT_MAX+1).

    Thanks,
    Rafael

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