linux-kernel - [RFT][PATCH 0/3] ACPI: ACPICA / OSL: Avoid unmapping ACPI memory inside of the AML interpreter

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Date:   Wed, 10 Jun 2020 14:17:04 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Dan Williams <dan.j.williams@...el.com>,
        Erik Kaneda <erik.kaneda@...el.com>
Cc:     rafael.j.wysocki@...el.com, Len Brown <lenb@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Ira Weiny <ira.weiny@...el.com>,
        James Morse <james.morse@....com>,
        Myron Stowe <myron.stowe@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-nvdimm@...ts.01.org, Bob Moore <robert.moore@...el.com>
Subject: [RFT][PATCH 0/3] ACPI: ACPICA / OSL: Avoid unmapping ACPI memory inside of the AML interpreter

Hi All,

This series is to address the problem with RCU synchronization occurring,
possibly relatively often, inside of acpi_ex_system_memory_space_handler(),
when the namespace and interpreter mutexes are held.

The basic idea is to avoid the actual unmapping of memory in
acpi_ex_system_memory_space_handler() by making it take the advantage of the
reference counting of memory mappings utilized by the OSL layer in Linux.

The basic assumption in patch [1/3] is that if the special
ACPI_OS_MAP_MEMORY_FAST_PATH() macro is present, it can be used to increment
the reference counter of a known-existing memory mapping in the OS layer
which then is dropped by the subsequent acpi_os_unmap_memory() without
unmapping the address range at hand.  That can be utilized by
acpi_ex_system_memory_space_handler() to prevent the reference counters of
all mappings used by it from dropping down to 0 (which also prevents the
address ranges associated with them from being unmapped) so that they can
be unmapped later (specifically, at the operation region deactivation time).

Patch [2/3] defers the unmapping even further, until the namespace and
interpreter mutexes are released, to avoid invoking the RCU synchronization
under theses mutexes.

Finally, patch [3/3] changes the OS layer in Linux to provide the
ACPI_OS_MAP_MEMORY_FAST_PATH() macro.

Note that if this macro is not defined, the code works the way it used to.

The series is available from the git branch at

 git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git \
 acpica-osl

for easier testing.

Cheers,
Rafael