[PATCH 00/11] Introduce Simple atomic and non-atomic counters

Shuah Khan
2020-09-25T23:47:33Z

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

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2.25.1