Hi, Dan > From: Dan Williams [mailto:dan.j.williams@xxxxxxxxx] > 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@xxxxxxxxx> 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. > > 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 ��.n��������+%������w��{.n�����{�����ܨ}���Ơz�j:+v�����w����ޙ��&�)ߡ�a����z�ޗ���ݢj��w�f
