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rcu_read_lock() scopes in dentry eviction machinery are too wide and badly structured; we end up with too many of those, quite a few essentially identical. Worse, quite a few of the function involved are not neutral wrt that, making them harder to reason about. rcu_read_lock() scope is not the only thing establishing an RCU read-side critical area - spin_lock scope does the same and they can be mixed - the sequence rcu_read_lock() ... spin_lock() ... rcu_read_unlock() ... rcu_read_lock() ... spun_unlock() ... rcu_read_unlock() is an unbroken RCU read-side critical area. Use of that observation allows to simplify things. First of all, lock_for_kill() relies upon being in an unbroken RCU read-side critical area. It's always called with ->d_lock held, and normally returns without having ever dropped that spinlock. We would not need rcu_read_lock() at all, if not for the slow path - if trylock of inode->i_lock fails, we need to drop and retake ->d_lock. Having all calls of lock_for_kill() inside an rcu_read_lock() scope takes care of that, but to show that lock_for_kill() slow path is safe, we need to demonstrate such rcu_read_lock() scope for any call chain leading to lock_for_kill(). Which is not fun, seeing that there are 10 such scopes, with 5 distinct beginnings between them. Case 1: opens in dput() proceeds through fast_dput() grabbing ->d_lock, returning false into dput() and there a call of finish_dput() which calls dentry_kill(), which calls lock_for_kill(); ends in dentry_kill(), either right after lock_for_kill() success or right after dropping ->d_lock on lock_for_kill() failure. ->d_lock is held continuously all the way into lock_for_kill(). Case 2: opens in dentry_kill(), where we proceed to the same call of dentry_kill() as in case 1. ->d_lock is held since before the beginning of the scope and all the way into lock_for_kill(). Case 3: opens in select_collect2(), proceeds through the return to d_walk() and to shrink_dcache_tree() where we grab ->d_lock and proceed to call shrink_kill(), which calls dentry_kill(), then as in the previous scopes. Case 4: opens in shrink_dentry_list(), followed by call of shrink_kill(), then same as in case 3. ->d_lock is held since before the beginning of the scope and all the way into lock_for_kill(). Case 5: opens in shrink_kill(), where it's immediately followed by call of dentry_kill(), then same as in the previous scopes. ->d_lock is held since before the beginning of the scope all the way into lock_for_kill(). Note that in cases 2, 4 and 5 the slow path of lock_for_kill() is the only part of rcu_read_lock() scope that is not covered by spinlock scopes. In case 1 we have the area in fast_dput() as well and in case 3 - the return path from select_collect2() and chunk in shrink_dcache_tree() up to grabbing ->d_lock. Seeing that the reasons we need rcu_read_lock() in these additional areas are completely unrelated to lock_for_kill() slow path, the things get much more straightforward with * explicit rcu_read_lock() scope surrounding the area in slow path of lock_for_kill() where ->d_lock is not held * shrink_dentry_list() dropping rcu_read_lock() as soon as it has grabbed ->d_lock. * dput() dropping rcu_read_lock() just before calling finish_dput(). * rcu_read_lock() calls in finish_dput(), shrink_kill() and shrink_dentry_list() are removed, along with rcu_read_unlock() calls in dentry_kill(). RCU read-side critical areas are unchanged by that, safety of lock_for_kill() slow path is trivial to verify and a bunch of rcu_read_lock() scopes either gone or become easier to describe. Update the comments on locking conventions and memory safety considerations, including the NORCU case. Incidentally, all calls of fast_dput() are immediately preceded by rcu_read_lock() and followed by rcu_read_unlock() now, which will allow to simplify those on the next step... Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> |
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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. Quick Start ----------- * Report a bug: See Documentation/admin-guide/reporting-issues.rst * Get the latest kernel: https://kernel.org * Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst * Join the community: https://lore.kernel.org/ Essential Documentation ----------------------- All users should be familiar with: * Building requirements: Documentation/process/changes.rst * Code of Conduct: Documentation/process/code-of-conduct.rst * License: See COPYING Documentation can be built with make htmldocs or viewed online at: https://www.kernel.org/doc/html/latest/ Who Are You? ============ Find your role below: * New Kernel Developer - Getting started with kernel development * Academic Researcher - Studying kernel internals and architecture * Security Expert - Hardening and vulnerability analysis * Backport/Maintenance Engineer - Maintaining stable kernels * System Administrator - Configuring and troubleshooting * Maintainer - Leading subsystems and reviewing patches * Hardware Vendor - Writing drivers for new hardware * Distribution Maintainer - Packaging kernels for distros * AI Coding Assistant - LLMs and AI-powered development tools For Specific Users ================== New Kernel Developer -------------------- Welcome! 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. Communication and Support ========================= * Mailing Lists: https://lore.kernel.org/ * IRC: #kernelnewbies on irc.oftc.net * Bugzilla: https://bugzilla.kernel.org/ * MAINTAINERS file: Lists subsystem maintainers and mailing lists * Email Clients: Documentation/process/email-clients.rst