Dmitry Ilvokhin
a13ab9dd6e
The unconditional guard destructors check whether the lock pointer is
NULL before unlocking. This check is dead code because unconditional
guards guarantee a non-NULL lock pointer at destructor time.
DEFINE_GUARD() runs the lock operation unconditionally in the
constructor. If the pointer were NULL, the lock operation (e.g.
mutex_lock(NULL)) would crash before the constructor returns. The
destructor never runs with a NULL pointer. All DEFINE_GUARD() users
dereference the pointer in their lock. Verified by auditing every
instance found by: git grep -n -A 1 'DEFINE_GUARD('. The only exception
is xe_pm_runtime_release_only, whose constructor is a noop, but it has
no callers.
__DEFINE_UNLOCK_GUARD() has only a few usages outside of
include/linux/cleanup.h: tty_port_tty (NULL-checks in its tty_kref_put()
call), irqdesc_lock (fixed earlier) and two guards in
kernel/sched/sched.h (dereference the pointer unconditionally in their
lock constructors).
DEFINE_LOCK_GUARD_1() sets .lock from its argument and runs the lock
operation in the constructor. Same reasoning applies. All
DEFINE_LOCK_GUARD_1() users dereference the pointer in their lock. Also,
verified by auditing every match of: git grep -n 'DEFINE_LOCK_GUARD_1('.
DEFINE_LOCK_GUARD_0() hardcodes .lock = (void *)1 in the constructor,
so it is never NULL by construction.
Conditional (_try) variants: DEFINE_GUARD_COND() and
DEFINE_LOCK_GUARD_1_COND() use EXTEND_CLASS_COND(), whose wrapper
destructor returns early when the lock was not acquired, before reaching
the base destructor since commit 2deccd5c86 ("cleanup: Optimize
guards"):
if (_cond) return; class_##_name##_destructor(_T);
As compiled by GCC-11 with defconfig on top of the locking/core:
Total: Before=23889980, After=23834334, chg -0.23%
Signed-off-by: Dmitry Ilvokhin <d@ilvokhin.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/0503a089389b2270c478a873e095cf0a4ff26d24.1780064327.git.d@ilvokhin.com
Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. 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Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst AI Coding Assistant ------------------- CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and follow the AI coding assistants documentation before contributing to the Linux kernel: * Documentation/process/coding-assistants.rst This documentation contains essential requirements about licensing, attribution, and the Developer Certificate of Origin that all AI tools must comply with. 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