On Thursday, May 04, 2017 07:18:28 AM Zheng, Lv wrote: > 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. 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 -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
