On Wed, May 13, 2026 at 9:31 AM Albert Esteve <[email protected]> wrote: > > Some unit tests intentionally trigger warning backtraces by passing bad > parameters to kernel API functions. Such unit tests typically check the > return value from such calls, not the existence of the warning backtrace. > > Such intentionally generated warning backtraces are neither desirable > nor useful for a number of reasons: > - They can result in overlooked real problems. > - A warning that suddenly starts to show up in unit tests needs to be > investigated and has to be marked to be ignored, for example by > adjusting filter scripts. Such filters are ad hoc because there is > no real standard format for warnings. On top of that, such filter > scripts would require constant maintenance. > > One option to address the problem would be to add messages such as > "expected warning backtraces start/end here" to the kernel log. > However, that would again require filter scripts, might result in > missing real problematic warning backtraces triggered while the test > is running, and the irrelevant backtrace(s) would still clog the > kernel log. > > Solve the problem by providing a means to suppress warning backtraces > originating from the current kthread while executing test code. > Since each KUnit test runs in its own kthread, this effectively scopes > suppression to the test that enabled it, without requiring any > architecture-specific code. > > Overview: > Patch#1 Introduces the suppression infrastructure integrated into > KUnit's hook mechanism. > Patch#2 Adds selftests to validate the functionality. > Patch#3 Demonstrates real-world usage in the DRM subsystem. > Patch#4 Documents the new API and usage guidelines. > > Design Notes: > Suppression is integrated into the existing KUnit hooks infrastructure, > reusing the kunit_running static branch for zero overhead > when no tests are running. The implementation lives entirely in the > kunit module; only a static-inline wrapper and a function pointer > slot are added to built-in code. > > Suppression is checked at three points in the warning path: > - In `warn_slowpath_fmt()` (kernel/panic.c), for architectures without > __WARN_FLAGS. The check runs before any output, fully suppressing > both message and backtrace. > - In `__warn_printk()` (kernel/panic.c), for architectures that define > __WARN_FLAGS but not their own __WARN_printf (arm64, loongarch, > parisc, powerpc, riscv, sh). The check suppresses the warning message > text that is printed before the trap enters __report_bug(). > - In `__report_bug()` (lib/bug.c), for architectures that define > __WARN_FLAGS. The check runs before `__warn()` is called, suppressing > the backtrace and stack dump. > > To avoid double-counting on architectures where both `__warn_printk()` > and `__report_bug()` run for the same warning, the hook takes a bool > parameter: true to increment the suppression counter, false to suppress > without counting. > > The suppression state is dynamically allocated via kunit_kzalloc() and > tied to the KUnit test lifecycle via `kunit_add_action()`, ensuring > automatic cleanup at test exit. Writer-side access to the global > suppression list is serialized with a spinlock; readers use RCU. > > Two API forms are provided: > - kunit_warning_suppress(test) { ... }: scoped blocks with automatic > cleanup. The suppression handle is not accessible outside the block, > so warning counts (if needed) must be checked inside. Multiple > suppression blocks are allowed. > - kunit_start/end_suppress_warning(test): direct functions that return > an explicit handle. Use when the handle needs to be retained, or passed > across helpers. Multiple suppression blocks are allowed. > > This series is based on the RFC patch and subsequent discussion at > https://patchwork.kernel.org/project/linux-kselftest/patch/[email protected]/ > and offers a more comprehensive solution of the problem discussed there. > > Changes since RFC: > - Introduced CONFIG_KUNIT_SUPPRESS_BACKTRACE > - Minor cleanups and bug fixes > - Added support for all affected architectures > - Added support for counting suppressed warnings > - Added unit tests using those counters > - Added patch to suppress warning backtraces in dev_addr_lists tests > > Changes since v1: > - Rebased to v6.9-rc1 > - Added Tested-by:, Acked-by:, and Reviewed-by: tags > [I retained those tags since there have been no functional changes] > - Introduced KUNIT_SUPPRESS_BACKTRACE configuration option, enabled by > default. > > Changes since v2: > - Rebased to v6.9-rc2 > - Added comments to drm warning suppression explaining why it is needed. > - Added patch to move conditional code in arch/sh/include/asm/bug.h > to avoid kerneldoc warning > - Added architecture maintainers to Cc: for architecture specific patches > - No functional changes > > Changes since v3: > - Rebased to v6.14-rc6 > - Dropped net: "kunit: Suppress lock warning noise at end of dev_addr_lists > tests" > since 3db3b62955cd6d73afde05a17d7e8e106695c3b9 > - Added __kunit_ and KUNIT_ prefixes. > - Tested on interessed architectures. > > Changes since v4: > - Rebased to v6.15-rc7 > - Dropped all code in __report_bug() > - Moved all checks in WARN*() macros. > - Dropped all architecture specific code. > - Made __kunit_is_suppressed_warning nice to noinstr functions. > > Changes since v5: > - Rebased to v7.0-rc3 > - Added RCU protection for the suppressed warnings list. > - Added static key and branching optimization. > - Removed custom `strcmp` implementation and reworked > __kunit_is_suppressed_warning() entrypoint function. > > Changes since v6: > - Moved suppression checks from WARN*() macros to warn_slowpath_fmt() > and __report_bug(). > - Replaced stack-allocated suppression struct with kunit_kzalloc() heap > allocation tied to the KUnit test lifecycle. > - Changed suppression strategy from function-name matching to task-scoped: > all warnings on the current task are suppressed between START and END, > rather than only warnings originating from a specific named function. > - Simplified macro API: removed KUNIT_DECLARE_SUPPRESSED_WARNING(), > the START macro now takes (test) and handles allocation internally. > - Removed static key and branching optiomization, as by the time it > was executed, callers are already in warn slowpaths. > - Link to v6: > https://lore.kernel.org/r/[email protected] > > Changes since v7: > - Integrated suppression into existing KUnit hooks infrastructure > - Removed CONFIG_KUNIT_SUPPRESS_BACKTRACE > - Added suppression check in __warn_printk() > - Added spinlock for writer-side RCU protection > - Replaced explicit rcu_read_lock/unlock with guard(rcu)() > - Added scoped API (kunit_warning_suppress) using __cleanup attribute > - Updated DRM patch to use scoped API > - Expanded self-tests: incremental counting, cross-kthread isolation > - Rewrote documentation covering all three API forms with examples > - Link to v7: > https://lore.kernel.org/r/[email protected] > > Changes since v8: > - Rebased to v7.1-rc2 > - Remove KUNIT_START/END_SUPPRESSED_WARNING() macros > - Add KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT checks to drm tests > - Link to v8: > https://lore.kernel.org/r/[email protected] > > Changes since v9: > - Fix silent false-pass when kunit_start_suppress_warning() returns NULL > - Fix RCU lockdep splat for kunit_is_suppressed_warning() calls > - Move disable_trace_on_warning() in __report_bug() > - Make suppress counter atomic > - Mark helper warn functions in selftest as noinline > - Add kunit_skip() for CONFIG_BUG=n in selftests > - Fix potentially uninitialized data.was_active in kthread seltest > - Add kthread_stop() in kthread selftest early exit > - Initialize scaling_factor to INT_MIN in DRM scaling tests > - Add include for bool in test-bug.h to fix CONFIG_KUNIT=n case
These were mostly sashiko + kernel test robot comments. The current version is available at https://sashiko.dev/#/patchset/20260513-kunit_add_support-v10-0-e379d206c8cd%40redhat.com, which again has a few comments that I'd consider potential real issues. I think `synchronize_rcu()` can be removed in patch 1, to avoid one of the mentioned issues. The struct kunit_suppressed_warning task would be safer if we increased its refcount. I'll take a reference on task_struct via get_task_struct() when starting suppression and release it on cleanup. We can address the kthread use-after-free problem with the warn suppression selftest by actively looping while waiting for kthread_stop(). Finally, I am thinking of adding something like this to the drm test (similar to what we do with the suppression selftest): ``` if (!IS_ENABLED(CONFIG_BUG)) kunit_skip(test, "requires CONFIG_BUG"); ``` While sashiko is right about this, it is a bit annoying having to add this to tests that want to use the suppression API, especially since we are setting the drm test as a "real-life example". But I'd say that it is better to warn that the configuration is required rather than fail an assertion without knowing the reason. Overall, these comments were slightly nitpicky already. So I hope I can clear them all with a new version. Sorry for the added noise, but it's better to fix these before merging! > - Link to v9: > https://lore.kernel.org/r/[email protected] > > -- > 2.34.1 > > --- > To: Brendan Higgins <[email protected]> > To: David Gow <[email protected]> > To: Rae Moar <[email protected]> > To: Andrew Morton <[email protected]> > To: Paul Walmsley <[email protected]> > To: Palmer Dabbelt <[email protected]> > To: Albert Ou <[email protected]> > To: Alexandre Ghiti <[email protected]> > To: Maarten Lankhorst <[email protected]> > To: Maxime Ripard <[email protected]> > To: Thomas Zimmermann <[email protected]> > To: David Airlie <[email protected]> > To: Simona Vetter <[email protected]> > To: Jonathan Corbet <[email protected]> > To: Shuah Khan <[email protected]> > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > > --- > Alessandro Carminati (1): > bug/kunit: Core support for suppressing warning backtraces > > Guenter Roeck (3): > kunit: Add backtrace suppression self-tests > drm: Suppress intentional warning backtraces in scaling unit tests > kunit: Add documentation for warning backtrace suppression API > > Documentation/dev-tools/kunit/usage.rst | 46 +++++++- > drivers/gpu/drm/tests/drm_rect_test.c | 32 +++++- > include/kunit/test-bug.h | 26 +++++ > include/kunit/test.h | 98 ++++++++++++++++ > kernel/panic.c | 11 ++ > lib/bug.c | 14 ++- > lib/kunit/Makefile | 4 +- > lib/kunit/backtrace-suppression-test.c | 196 > ++++++++++++++++++++++++++++++++ > lib/kunit/bug.c | 119 +++++++++++++++++++ > lib/kunit/hooks-impl.h | 2 + > 10 files changed, 538 insertions(+), 10 deletions(-) > --- > base-commit: 74fe02ce122a6103f207d29fafc8b3a53de6abaf > change-id: 20260312-kunit_add_support-2f35806b19dd > > Best regards, > -- > Albert Esteve <[email protected]> >
