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

Shuah Khan skhan at linuxfoundation.org
Wed Sep 23 01:43:29 UTC 2020

This patch series is a result of discussion at the refcount_t BOF
the Linux Plumbers Conference. In this discussion, we identifed
a need for looking closely and investigating atomic_t usages in
the kernel when it is used strictly as a counter wothout 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

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.

Please review and let me know if non-stat conversions e.g: probe_count,
deferred_trigger_count make sense.

Shuah Khan (11):
  counters: Introduce counter and counter_atomic counters
  selftests:lib:test_counters: add new test for counters
  drivers/base: convert deferred_trigger_count and probe_count to
  drivers/base/devcoredump: convert devcd_count to counter_atomic
  drivers/acpi: convert seqno counter_atomic
  drivers/acpi/apei: convert seqno counter_atomic
  drivers/android/binder: convert stats, transaction_log to
  drivers/base/test/test_async_driver_probe: convert to use
  drivers/char/ipmi: convert stats to use counter_atomic
  drivers/misc/vmw_vmci: convert num guest devices counter to
  drivers/edac: convert pci counters to counter_atomic

 Documentation/core-api/counters.rst          | 158 +++++++++
 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                     | 343 +++++++++++++++++++
 lib/Kconfig                                  |  10 +
 lib/Makefile                                 |   1 +
 lib/test_counters.c                          | 283 +++++++++++++++
 tools/testing/selftests/lib/Makefile         |   1 +
 tools/testing/selftests/lib/config           |   1 +
 tools/testing/selftests/lib/test_counters.sh |   5 +
 21 files changed, 897 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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