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 > > -- > 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 -- 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
