This patch series is a result of discussion at the refcount_t BOF the Linux Plumbers Conference. In this discussion, we identified a need for looking closely and investigating atomic_t usages in the kernel when it is used strictly as a counter without it controlling object lifetimes and state changes.
There are a number of atomic_t usages in the kernel where atomic_t api is used strictly for counting and not for managing object lifetime. In some cases, atomic_t might not even be needed.
The purpose of these counters is twofold: 1. clearly differentiate atomic_t counters from atomic_t usages that guard object lifetimes, hence prone to overflow and underflow errors. It allows tools that scan for underflow and overflow on atomic_t usages to detect overflow and underflows to scan just the cases that are prone to errors. 2. provides non-atomic counters for cases where atomic isn't necessary.
Simple atomic and non-atomic counters api provides interfaces for simple atomic and non-atomic counters that just count, and don't guard resource lifetimes. Counters will wrap around to 0 when it overflows and should not be used to guard resource lifetimes, device usage and open counts that control state changes, and pm states.
Using counter_atomic to guard lifetimes could lead to use-after free when it overflows and undefined behavior when used to manage state changes and device usage/open states.
This patch series introduces Simple atomic and non-atomic counters. Counter atomic ops leverage atomic_t and provide a sub-set of atomic_t ops.
In addition this patch series converts a few drivers to use the new api. The following criteria is used for select variables for conversion:
1. Variable doesn't guard object lifetimes, manage state changes e.g: device usage counts, device open counts, and pm states. 2. Variable is used for stats and counters. 3. The conversion doesn't change the overflow behavior.
Changes since RFC: -- Thanks for reviews and reviewed-by, and Acked-by tags. Updated the patches with the tags. -- Addressed Kees's comments: 1. Non-atomic counters renamed to counter_simple32 and counter_simple64 to clearly indicate size. 2. Added warning for counter_simple* usage and it should be used only when there is no need for atomicity. 3. Renamed counter_atomic to counter_atomic32 to clearly indicate size. 4. Renamed counter_atomic_long to counter_atomic64 and it now uses atomic64_t ops and indicates size. 5. Test updated for the API renames. 6. Added helper functions for test results printing 7. Verified that the test module compiles in kunit env. and test module can be loaded to run the test. 8. Updated Documentation to reflect the intent to make the API restricted so it can never be used to guard object lifetimes and state management. I left _return ops for now, inc_return is necessary for now as per the discussion we had on this topic. -- Updated driver patches with API name changes. -- We discussed if binder counters can be non-atomic. For now I left them the same as the RFC patch - using counter_atomic32 -- Unrelated to this patch series: The patch series review uncovered improvements could be made to test_async_driver_probe and vmw_vmci/vmci_guest. I will track these for fixing later.
Shuah Khan (11): counters: Introduce counter_simple* and counter_atomic* counters selftests:lib:test_counters: add new test for counters drivers/base: convert deferred_trigger_count and probe_count to counter_atomic32 drivers/base/devcoredump: convert devcd_count to counter_atomic32 drivers/acpi: convert seqno counter_atomic32 drivers/acpi/apei: convert seqno counter_atomic32 drivers/android/binder: convert stats, transaction_log to counter_atomic32 drivers/base/test/test_async_driver_probe: convert to use counter_atomic32 drivers/char/ipmi: convert stats to use counter_atomic32 drivers/misc/vmw_vmci: convert num guest devices counter to counter_atomic32 drivers/edac: convert pci counters to counter_atomic32
Documentation/core-api/counters.rst | 174 +++++++++ MAINTAINERS | 8 + drivers/acpi/acpi_extlog.c | 5 +- drivers/acpi/apei/ghes.c | 5 +- drivers/android/binder.c | 41 +-- drivers/android/binder_internal.h | 3 +- drivers/base/dd.c | 19 +- drivers/base/devcoredump.c | 5 +- drivers/base/test/test_async_driver_probe.c | 23 +- drivers/char/ipmi/ipmi_msghandler.c | 9 +- drivers/char/ipmi/ipmi_si_intf.c | 9 +- drivers/edac/edac_pci.h | 5 +- drivers/edac/edac_pci_sysfs.c | 28 +- drivers/misc/vmw_vmci/vmci_guest.c | 9 +- include/linux/counters.h | 350 +++++++++++++++++++ lib/Kconfig | 10 + lib/Makefile | 1 + lib/test_counters.c | 276 +++++++++++++++ tools/testing/selftests/lib/Makefile | 1 + tools/testing/selftests/lib/config | 1 + tools/testing/selftests/lib/test_counters.sh | 5 + 21 files changed, 913 insertions(+), 74 deletions(-) create mode 100644 Documentation/core-api/counters.rst create mode 100644 include/linux/counters.h create mode 100644 lib/test_counters.c create mode 100755 tools/testing/selftests/lib/test_counters.sh
Add a new selftest for testing counter_simple* and counter_atomic* Counters API. This test load test_counters test modules and unloads.
The test module runs tests and prints results in dmesg.
There are a number of atomic_t usages in the kernel where atomic_t api is used strictly for counting and not for managing object lifetime. In some cases, atomic_t might not even be needed.
The purpose of these counters is twofold: 1. clearly differentiate atomic_t counters from atomic_t usages that guard object lifetimes, hence prone to overflow and underflow errors. It allows tools that scan for underflow and overflow on atomic_t usages to detect overflow and underflows to scan just the cases that are prone to errors. 2. provides non-atomic counters for cases where atomic isn't necessary.
Simple atomic and non-atomic counters api provides interfaces for simple atomic and non-atomic counters that just count, and don't guard resource lifetimes. Counters will wrap around to 0 when it overflows and should not be used to guard resource lifetimes, device usage and open counts that control state changes, and pm states.
Using counter_atomic* to guard lifetimes could lead to use-after free when it overflows and undefined behavior when used to manage state changes and device usage/open states.
Signed-off-by: Shuah Khan skhan@linuxfoundation.org --- MAINTAINERS | 1 + tools/testing/selftests/lib/Makefile | 1 + tools/testing/selftests/lib/config | 1 + tools/testing/selftests/lib/test_counters.sh | 5 +++++ 4 files changed, 8 insertions(+) create mode 100755 tools/testing/selftests/lib/test_counters.sh
diff --git a/MAINTAINERS b/MAINTAINERS index 886ee9b5f164..b308e80a1391 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -15847,6 +15847,7 @@ L: linux-kernel@vger.kernel.org S: Maintained F: include/linux/counters.h F: lib/test_counters.c +F: tools/testing/selftests/lib/test_counters.sh
SIMPLE FIRMWARE INTERFACE (SFI) S: Obsolete diff --git a/tools/testing/selftests/lib/Makefile b/tools/testing/selftests/lib/Makefile index a105f094676e..e8960d7934e2 100644 --- a/tools/testing/selftests/lib/Makefile +++ b/tools/testing/selftests/lib/Makefile @@ -5,5 +5,6 @@ all:
TEST_PROGS := printf.sh bitmap.sh prime_numbers.sh strscpy.sh +TEST_PROGS += test_counters.sh
include ../lib.mk diff --git a/tools/testing/selftests/lib/config b/tools/testing/selftests/lib/config index b80ee3f6e265..6ed25024d371 100644 --- a/tools/testing/selftests/lib/config +++ b/tools/testing/selftests/lib/config @@ -3,3 +3,4 @@ CONFIG_TEST_BITMAP=m CONFIG_PRIME_NUMBERS=m CONFIG_TEST_STRSCPY=m CONFIG_TEST_BITOPS=m +CONFIG_TEST_COUNTERS=m diff --git a/tools/testing/selftests/lib/test_counters.sh b/tools/testing/selftests/lib/test_counters.sh new file mode 100755 index 000000000000..d1a130190e3f --- /dev/null +++ b/tools/testing/selftests/lib/test_counters.sh @@ -0,0 +1,5 @@ +#!/bin/sh +# SPDX-License-Identifier: GPL-2.0 +# Tests the Simple Atomic and Non-atomic Counters interfaces using +# test_counters kernel module +$(dirname $0)/../kselftest/module.sh "test_counters" test_counters
On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
-- Addressed Kees's comments:
- Non-atomic counters renamed to counter_simple32 and counter_simple64 to clearly indicate size.
- Added warning for counter_simple* usage and it should be used only when there is no need for atomicity.
- Renamed counter_atomic to counter_atomic32 to clearly indicate size.
- Renamed counter_atomic_long to counter_atomic64 and it now uses atomic64_t ops and indicates size.
- Test updated for the API renames.
- Added helper functions for test results printing
- Verified that the test module compiles in kunit env. and test module can be loaded to run the test.
Thanks for all of this!
- Updated Documentation to reflect the intent to make the API restricted so it can never be used to guard object lifetimes and state management. I left _return ops for now, inc_return is necessary for now as per the discussion we had on this topic.
I still *really* do not want dec_return() to exist. That is asking for trouble. I'd prefer inc_return() not exist either, but I can live with it. ;)
On 9/25/20 5:52 PM, Kees Cook wrote:
On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
-- Addressed Kees's comments: 1. Non-atomic counters renamed to counter_simple32 and counter_simple64 to clearly indicate size. 2. Added warning for counter_simple* usage and it should be used only when there is no need for atomicity. 3. Renamed counter_atomic to counter_atomic32 to clearly indicate size. 4. Renamed counter_atomic_long to counter_atomic64 and it now uses atomic64_t ops and indicates size. 5. Test updated for the API renames. 6. Added helper functions for test results printing 7. Verified that the test module compiles in kunit env. and test module can be loaded to run the test.
Thanks for all of this!
8. Updated Documentation to reflect the intent to make the API restricted so it can never be used to guard object lifetimes and state management. I left _return ops for now, inc_return is necessary for now as per the discussion we had on this topic.I still *really* do not want dec_return() to exist. That is asking for trouble. I'd prefer inc_return() not exist either, but I can live with it. ;)
Thanks. I am equally concerned about adding anything that can be used to guard object lifetimes. So I will make sure this set won't expand and plan to remove dec_return() if we don't find any usages.
thanks, -- Shuah
On Fri, Sep 25, 2020 at 06:13:37PM -0600, Shuah Khan wrote:
On 9/25/20 5:52 PM, Kees Cook wrote:
On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
-- Addressed Kees's comments: 1. Non-atomic counters renamed to counter_simple32 and counter_simple64 to clearly indicate size. 2. Added warning for counter_simple* usage and it should be used only when there is no need for atomicity. 3. Renamed counter_atomic to counter_atomic32 to clearly indicate size. 4. Renamed counter_atomic_long to counter_atomic64 and it now uses atomic64_t ops and indicates size. 5. Test updated for the API renames. 6. Added helper functions for test results printing 7. Verified that the test module compiles in kunit env. and test module can be loaded to run the test.
Thanks for all of this!
8. Updated Documentation to reflect the intent to make the API restricted so it can never be used to guard object lifetimes and state management. I left _return ops for now, inc_return is necessary for now as per the discussion we had on this topic.I still *really* do not want dec_return() to exist. That is asking for trouble. I'd prefer inc_return() not exist either, but I can live with it. ;)
Thanks. I am equally concerned about adding anything that can be used to guard object lifetimes. So I will make sure this set won't expand and plan to remove dec_return() if we don't find any usages.
I would like it much stronger than "if". dec_return() needs to be just dec() and read(). It will not be less efficient (since they're both inlines), but it _will_ create a case where the atomicity cannot be used for ref counting. My point is that anything that _requires_ dec_return() (or, frankly, inc_return()) is _not_ "just" a statistical counter. It may not be a refcounter, but it relies on the inc/dec atomicity for some reason beyond counting in once place and reporting it in another.
On 9/26/20 10:33 AM, Kees Cook wrote:
On Fri, Sep 25, 2020 at 06:13:37PM -0600, Shuah Khan wrote:
On 9/25/20 5:52 PM, Kees Cook wrote:
On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
-- Addressed Kees's comments: 1. Non-atomic counters renamed to counter_simple32 and counter_simple64 to clearly indicate size. 2. Added warning for counter_simple* usage and it should be used only when there is no need for atomicity. 3. Renamed counter_atomic to counter_atomic32 to clearly indicate size. 4. Renamed counter_atomic_long to counter_atomic64 and it now uses atomic64_t ops and indicates size. 5. Test updated for the API renames. 6. Added helper functions for test results printing 7. Verified that the test module compiles in kunit env. and test module can be loaded to run the test.
Thanks for all of this!
8. Updated Documentation to reflect the intent to make the API restricted so it can never be used to guard object lifetimes and state management. I left _return ops for now, inc_return is necessary for now as per the discussion we had on this topic.I still *really* do not want dec_return() to exist. That is asking for trouble. I'd prefer inc_return() not exist either, but I can live with it. ;)
I didn't read this correctly the first time around.
Thanks. I am equally concerned about adding anything that can be used to guard object lifetimes. So I will make sure this set won't expand and plan to remove dec_return() if we don't find any usages.
I would like it much stronger than "if". dec_return() needs to be just dec() and read(). It will not be less efficient (since they're both inlines), but it _will_ create a case where the atomicity cannot be used for ref counting. My point is that anything that _requires_ dec_return() (or, frankly, inc_return()) is _not_ "just" a statistical counter. It may not be a refcounter, but it relies on the inc/dec atomicity for some reason beyond counting in once place and reporting it in another.
I am not thinking about efficiency rather two calls instead of one if an decrement needs to followed by return. In any case, I agree with you that there is no need to add dec_return now without any use-cases.
I will update the patch series to remove it.
thanks, -- Shuah
On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
This patch series is a result of discussion at the refcount_t BOF the Linux Plumbers Conference. In this discussion, we identified a need for looking closely and investigating atomic_t usages in the kernel when it is used strictly as a counter without it controlling object lifetimes and state changes.
BTW, I realized the KSPP issue tracker hadn't broken this task out of the refcount_t conversion issue[1] into a separate issue, so I've created it now: https://github.com/KSPP/linux/issues/106
-Kees
[1] https://github.com/KSPP/linux/issues/104
On 9/26/20 10:22 AM, Kees Cook wrote:
On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
This patch series is a result of discussion at the refcount_t BOF the Linux Plumbers Conference. In this discussion, we identified a need for looking closely and investigating atomic_t usages in the kernel when it is used strictly as a counter without it controlling object lifetimes and state changes.
BTW, I realized the KSPP issue tracker hadn't broken this task out of the refcount_t conversion issue[1] into a separate issue, so I've created it now: https://github.com/KSPP/linux/issues/106
Cool. Thanks.
-- Shuah
On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
- Verified that the test module compiles in kunit env. and test module can be loaded to run the test.
I meant write it using KUnit interfaces (e.g. KUNIT_EXPECT*(), kunit_test_suite(), etc): https://www.kernel.org/doc/html/latest/dev-tools/kunit/
Though I see the docs are still not updated[1] to reflect the Kconfig (CONFIG_foo_KUNIT_TEST) and file naming conventions (foo_kunit.c).
-Kees
[1] https://lore.kernel.org/lkml/20200911042404.3598910-1-davidgow@google.com/
On 9/26/20 10:29 AM, Kees Cook wrote:
On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
7. Verified that the test module compiles in kunit env. and test module can be loaded to run the test.I meant write it using KUnit interfaces (e.g. KUNIT_EXPECT*(), kunit_test_suite(), etc): https://www.kernel.org/doc/html/latest/dev-tools/kunit/
Though I see the docs are still not updated[1] to reflect the Kconfig (CONFIG_foo_KUNIT_TEST) and file naming conventions (foo_kunit.c).
I would like to be able to run this test outside Kunit env., hence the choice to go with a module and kselftest script. It makes it easier to test as part of my workflow as opposed to doing a kunit and build and running it that way.
I don't mind adding TEST_COUNTERS to kunit default configs though.
thanks, -- Shuah
On Mon, Sep 28, 2020 at 04:41:47PM -0600, Shuah Khan wrote:
On 9/26/20 10:29 AM, Kees Cook wrote:
On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
7. Verified that the test module compiles in kunit env. and test module can be loaded to run the test.I meant write it using KUnit interfaces (e.g. KUNIT_EXPECT*(), kunit_test_suite(), etc): https://www.kernel.org/doc/html/latest/dev-tools/kunit/
Though I see the docs are still not updated[1] to reflect the Kconfig (CONFIG_foo_KUNIT_TEST) and file naming conventions (foo_kunit.c).
I would like to be able to run this test outside Kunit env., hence the choice to go with a module and kselftest script. It makes it easier to test as part of my workflow as opposed to doing a kunit and build and running it that way.
It does -- you just load it normally like before and it prints out everything just fine. This is how I use the lib/test_user_copy.c and lib/test_overflow.c before/after their conversions.
On 9/28/20 5:13 PM, Kees Cook wrote:
On Mon, Sep 28, 2020 at 04:41:47PM -0600, Shuah Khan wrote:
On 9/26/20 10:29 AM, Kees Cook wrote:
On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
7. Verified that the test module compiles in kunit env. and test module can be loaded to run the test.I meant write it using KUnit interfaces (e.g. KUNIT_EXPECT*(), kunit_test_suite(), etc): https://www.kernel.org/doc/html/latest/dev-tools/kunit/
Though I see the docs are still not updated[1] to reflect the Kconfig (CONFIG_foo_KUNIT_TEST) and file naming conventions (foo_kunit.c).
I would like to be able to run this test outside Kunit env., hence the choice to go with a module and kselftest script. It makes it easier to test as part of my workflow as opposed to doing a kunit and build and running it that way.
It does -- you just load it normally like before and it prints out everything just fine. This is how I use the lib/test_user_copy.c and lib/test_overflow.c before/after their conversions.
I am not seeing any kunit links to either of these tests. I find the lib/test_overflow.c very hard to read.
I am going to stick with what I have for now and handle conversion later.
I think it might be a good idea to add tests for atomic_t and refcount_t APIS as well at some point.
thanks, -- Shuah
On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
This patch series is a result of discussion at the refcount_t BOF the Linux Plumbers Conference. In this discussion, we identified a need for looking closely and investigating atomic_t usages in the kernel when it is used strictly as a counter without it controlling object lifetimes and state changes.
There are a number of atomic_t usages in the kernel where atomic_t api is used strictly for counting and not for managing object lifetime. In some cases, atomic_t might not even be needed. The purpose of these counters is twofold: 1. clearly differentiate atomic_t counters from atomic_t usages that guard object lifetimes, hence prone to overflow and underflow errors. It allows tools that scan for underflow and overflow on atomic_t usages to detect overflow and underflows to scan just the cases that are prone to errors. 2. provides non-atomic counters for cases where atomic isn't necessary.
Nice series :)
It appears there is no user of counter_simple in this series other than the selftest. Would you be planning to add any conversions in the series itself, for illustration of use? Sorry if I missed a usage.
Also how do we guard against atomicity of counter_simple RMW operations? Is the implication that it should be guarded using other synchronization to prevent lost-update problem?
Some more comments:
1. atomic RMW operations that have a return value are fully ordered. Would you be adding support to counter_simple for such ordering as well, for consistency?
2. I felt counter_atomic and counter_atomic64 would be nice equivalents to the atomic and atomic64 naming currently used (i.e. dropping the '32'). However that is just my opinion and I am ok with either naming.
thanks!
- Joel
Simple atomic and non-atomic counters api provides interfaces for simple atomic and non-atomic counters that just count, and don't guard resource lifetimes. Counters will wrap around to 0 when it overflows and should not be used to guard resource lifetimes, device usage and open counts that control state changes, and pm states. Using counter_atomic to guard lifetimes could lead to use-after free when it overflows and undefined behavior when used to manage state changes and device usage/open states.
This patch series introduces Simple atomic and non-atomic counters. Counter atomic ops leverage atomic_t and provide a sub-set of atomic_t ops.
In addition this patch series converts a few drivers to use the new api. The following criteria is used for select variables for conversion:
- Variable doesn't guard object lifetimes, manage state changes e.g: device usage counts, device open counts, and pm states.
- Variable is used for stats and counters.
- The conversion doesn't change the overflow behavior.
Changes since RFC: -- Thanks for reviews and reviewed-by, and Acked-by tags. Updated the patches with the tags. -- Addressed Kees's comments:
- Non-atomic counters renamed to counter_simple32 and counter_simple64 to clearly indicate size.
- Added warning for counter_simple* usage and it should be used only when there is no need for atomicity.
- Renamed counter_atomic to counter_atomic32 to clearly indicate size.
- Renamed counter_atomic_long to counter_atomic64 and it now uses atomic64_t ops and indicates size.
- Test updated for the API renames.
- Added helper functions for test results printing
- Verified that the test module compiles in kunit env. and test module can be loaded to run the test.
- Updated Documentation to reflect the intent to make the API restricted so it can never be used to guard object lifetimes and state management. I left _return ops for now, inc_return is necessary for now as per the discussion we had on this topic.
-- Updated driver patches with API name changes. -- We discussed if binder counters can be non-atomic. For now I left them the same as the RFC patch - using counter_atomic32 -- Unrelated to this patch series: The patch series review uncovered improvements could be made to test_async_driver_probe and vmw_vmci/vmci_guest. I will track these for fixing later.
Shuah Khan (11): counters: Introduce counter_simple* and counter_atomic* counters selftests:lib:test_counters: add new test for counters drivers/base: convert deferred_trigger_count and probe_count to counter_atomic32 drivers/base/devcoredump: convert devcd_count to counter_atomic32 drivers/acpi: convert seqno counter_atomic32 drivers/acpi/apei: convert seqno counter_atomic32 drivers/android/binder: convert stats, transaction_log to counter_atomic32 drivers/base/test/test_async_driver_probe: convert to use counter_atomic32 drivers/char/ipmi: convert stats to use counter_atomic32 drivers/misc/vmw_vmci: convert num guest devices counter to counter_atomic32 drivers/edac: convert pci counters to counter_atomic32
Documentation/core-api/counters.rst | 174 +++++++++ MAINTAINERS | 8 + drivers/acpi/acpi_extlog.c | 5 +- drivers/acpi/apei/ghes.c | 5 +- drivers/android/binder.c | 41 +-- drivers/android/binder_internal.h | 3 +- drivers/base/dd.c | 19 +- drivers/base/devcoredump.c | 5 +- drivers/base/test/test_async_driver_probe.c | 23 +- drivers/char/ipmi/ipmi_msghandler.c | 9 +- drivers/char/ipmi/ipmi_si_intf.c | 9 +- drivers/edac/edac_pci.h | 5 +- drivers/edac/edac_pci_sysfs.c | 28 +- drivers/misc/vmw_vmci/vmci_guest.c | 9 +- include/linux/counters.h | 350 +++++++++++++++++++ lib/Kconfig | 10 + lib/Makefile | 1 + lib/test_counters.c | 276 +++++++++++++++ tools/testing/selftests/lib/Makefile | 1 + tools/testing/selftests/lib/config | 1 + tools/testing/selftests/lib/test_counters.sh | 5 + 21 files changed, 913 insertions(+), 74 deletions(-) create mode 100644 Documentation/core-api/counters.rst create mode 100644 include/linux/counters.h create mode 100644 lib/test_counters.c create mode 100755 tools/testing/selftests/lib/test_counters.sh
-- 2.25.1
On Sun, Sep 27, 2020 at 07:35:26PM -0400, Joel Fernandes wrote:
On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
This patch series is a result of discussion at the refcount_t BOF the Linux Plumbers Conference. In this discussion, we identified a need for looking closely and investigating atomic_t usages in the kernel when it is used strictly as a counter without it controlling object lifetimes and state changes.
There are a number of atomic_t usages in the kernel where atomic_t api is used strictly for counting and not for managing object lifetime. In some cases, atomic_t might not even be needed. The purpose of these counters is twofold: 1. clearly differentiate atomic_t counters from atomic_t usages that guard object lifetimes, hence prone to overflow and underflow errors. It allows tools that scan for underflow and overflow on atomic_t usages to detect overflow and underflows to scan just the cases that are prone to errors. 2. provides non-atomic counters for cases where atomic isn't necessary.
Nice series :)
It appears there is no user of counter_simple in this series other than the selftest. Would you be planning to add any conversions in the series itself, for illustration of use? Sorry if I missed a usage.
Also how do we guard against atomicity of counter_simple RMW operations? Is the implication that it should be guarded using other synchronization to prevent lost-update problem?
Some more comments:
- atomic RMW operations that have a return value are fully ordered. Would you be adding support to counter_simple for such ordering as well, for consistency?
No -- there is no atomicity guarantee for counter_simple. I would prefer counter_simple not exist at all, specifically for this reason.
- I felt counter_atomic and counter_atomic64 would be nice equivalents to the atomic and atomic64 naming currently used (i.e. dropping the '32'). However that is just my opinion and I am ok with either naming.
I had asked that they be size-named to avoid any confusion (i.e. we're making a new API).
On Mon, Sep 28, 2020 at 01:34:31PM -0700, Kees Cook wrote:
On Sun, Sep 27, 2020 at 07:35:26PM -0400, Joel Fernandes wrote:
On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
This patch series is a result of discussion at the refcount_t BOF the Linux Plumbers Conference. In this discussion, we identified a need for looking closely and investigating atomic_t usages in the kernel when it is used strictly as a counter without it controlling object lifetimes and state changes.
There are a number of atomic_t usages in the kernel where atomic_t api is used strictly for counting and not for managing object lifetime. In some cases, atomic_t might not even be needed. The purpose of these counters is twofold: 1. clearly differentiate atomic_t counters from atomic_t usages that guard object lifetimes, hence prone to overflow and underflow errors. It allows tools that scan for underflow and overflow on atomic_t usages to detect overflow and underflows to scan just the cases that are prone to errors. 2. provides non-atomic counters for cases where atomic isn't necessary.
Nice series :)
It appears there is no user of counter_simple in this series other than the selftest. Would you be planning to add any conversions in the series itself, for illustration of use? Sorry if I missed a usage.
Also how do we guard against atomicity of counter_simple RMW operations? Is the implication that it should be guarded using other synchronization to prevent lost-update problem?
Some more comments:
- atomic RMW operations that have a return value are fully ordered. Would you be adding support to counter_simple for such ordering as well, for consistency?
No -- there is no atomicity guarantee for counter_simple. I would prefer counter_simple not exist at all, specifically for this reason.
Yeah I am ok with it not existing, especially also as there are no examples of its conversion/usage in the series.
- I felt counter_atomic and counter_atomic64 would be nice equivalents to the atomic and atomic64 naming currently used (i.e. dropping the '32'). However that is just my opinion and I am ok with either naming.
I had asked that they be size-named to avoid any confusion (i.e. we're making a new API).
Works for me.
Cheers,
- Joel
On 9/28/20 3:17 PM, Joel Fernandes wrote:
On Mon, Sep 28, 2020 at 01:34:31PM -0700, Kees Cook wrote:
On Sun, Sep 27, 2020 at 07:35:26PM -0400, Joel Fernandes wrote:
On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
This patch series is a result of discussion at the refcount_t BOF the Linux Plumbers Conference. In this discussion, we identified a need for looking closely and investigating atomic_t usages in the kernel when it is used strictly as a counter without it controlling object lifetimes and state changes.
There are a number of atomic_t usages in the kernel where atomic_t api is used strictly for counting and not for managing object lifetime. In some cases, atomic_t might not even be needed. The purpose of these counters is twofold: 1. clearly differentiate atomic_t counters from atomic_t usages that guard object lifetimes, hence prone to overflow and underflow errors. It allows tools that scan for underflow and overflow on atomic_t usages to detect overflow and underflows to scan just the cases that are prone to errors. 2. provides non-atomic counters for cases where atomic isn't necessary.
Nice series :)
Thanks.
It appears there is no user of counter_simple in this series other than the selftest. Would you be planning to add any conversions in the series itself, for illustration of use? Sorry if I missed a usage.
Also how do we guard against atomicity of counter_simple RMW operations? Is the implication that it should be guarded using other synchronization to prevent lost-update problem?
Some more comments:
- atomic RMW operations that have a return value are fully ordered. Would you be adding support to counter_simple for such ordering as well, for consistency?
No -- there is no atomicity guarantee for counter_simple. I would prefer counter_simple not exist at all, specifically for this reason.
Yeah I am ok with it not existing, especially also as there are no examples of its conversion/usage in the series.
No. counter_simple is just for counting when there is no need for atomicity with the premise that there might be some use-cases. You are right that this patch series doesn't use these. My hunch is though that atomic_t is overused and it isn't needed in all cases.
I will do some research to look for any places that can use counter_simple before I spin v2. If I don't find any, I can drop them.
thanks, -- Shuah
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Joel Fernandes -
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Shuah Khan