From patchwork Sat Mar 21 11:25:57 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Thomas Gleixner X-Patchwork-Id: 11450949 X-Patchwork-Delegate: johannes@sipsolutions.net Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 1936F139A for ; Sat, 21 Mar 2020 11:36:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E2FE42077D for ; Sat, 21 Mar 2020 11:36:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728829AbgCULgE (ORCPT ); Sat, 21 Mar 2020 07:36:04 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:38483 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728481AbgCULe6 (ORCPT ); Sat, 21 Mar 2020 07:34:58 -0400 Received: from p5de0bf0b.dip0.t-ipconnect.de ([93.224.191.11] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1jFcOV-0001zp-3Y; Sat, 21 Mar 2020 12:34:23 +0100 Received: from nanos.tec.linutronix.de (localhost [IPv6:::1]) by nanos.tec.linutronix.de (Postfix) with ESMTP id 92A3F1039FF; Sat, 21 Mar 2020 12:34:20 +0100 (CET) Message-Id: <20200321113242.026561244@linutronix.de> User-Agent: quilt/0.65 Date: Sat, 21 Mar 2020 12:25:57 +0100 From: Thomas Gleixner To: LKML Cc: Peter Zijlstra , Ingo Molnar , Sebastian Siewior , Linus Torvalds , Joel Fernandes , Oleg Nesterov , Davidlohr Bueso , "Paul E . McKenney" , Jonathan Corbet , Randy Dunlap , Logan Gunthorpe , Bjorn Helgaas , Kurt Schwemmer , linux-pci@vger.kernel.org, Greg Kroah-Hartman , Felipe Balbi , linux-usb@vger.kernel.org, Kalle Valo , "David S. Miller" , linux-wireless@vger.kernel.org, netdev@vger.kernel.org, Darren Hart , Andy Shevchenko , platform-driver-x86@vger.kernel.org, Zhang Rui , "Rafael J. Wysocki" , linux-pm@vger.kernel.org, Len Brown , linux-acpi@vger.kernel.org, kbuild test robot , Nick Hu , Greentime Hu , Vincent Chen , Guo Ren , linux-csky@vger.kernel.org, Brian Cain , linux-hexagon@vger.kernel.org, Tony Luck , Fenghua Yu , linux-ia64@vger.kernel.org, Michal Simek , Michael Ellerman , Arnd Bergmann , Geoff Levand , linuxppc-dev@lists.ozlabs.org, Davidlohr Bueso Subject: [patch V3 13/20] Documentation: Add lock ordering and nesting documentation References: <20200321112544.878032781@linutronix.de> MIME-Version: 1.0 Content-transfer-encoding: 8-bit X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-wireless-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-wireless@vger.kernel.org From: Thomas Gleixner The kernel provides a variety of locking primitives. The nesting of these lock types and the implications of them on RT enabled kernels is nowhere documented. Add initial documentation. Signed-off-by: Thomas Gleixner Cc: "Paul E . McKenney" Cc: Jonathan Corbet Cc: Davidlohr Bueso Cc: Randy Dunlap --- V3: Addressed review comments from Paul, Jonathan, Davidlohr V2: Addressed review comments from Randy --- Documentation/locking/index.rst | 1 Documentation/locking/locktypes.rst | 299 ++++++++++++++++++++++++++++++++++++ 2 files changed, 300 insertions(+) create mode 100644 Documentation/locking/locktypes.rst --- a/Documentation/locking/index.rst +++ b/Documentation/locking/index.rst @@ -7,6 +7,7 @@ locking .. toctree:: :maxdepth: 1 + locktypes lockdep-design lockstat locktorture --- /dev/null +++ b/Documentation/locking/locktypes.rst @@ -0,0 +1,299 @@ +.. SPDX-License-Identifier: GPL-2.0 + +.. _kernel_hacking_locktypes: + +========================== +Lock types and their rules +========================== + +Introduction +============ + +The kernel provides a variety of locking primitives which can be divided +into two categories: + + - Sleeping locks + - Spinning locks + +This document conceptually describes these lock types and provides rules +for their nesting, including the rules for use under PREEMPT_RT. + + +Lock categories +=============== + +Sleeping locks +-------------- + +Sleeping locks can only be acquired in preemptible task context. + +Although implementations allow try_lock() from other contexts, it is +necessary to carefully evaluate the safety of unlock() as well as of +try_lock(). Furthermore, it is also necessary to evaluate the debugging +versions of these primitives. In short, don't acquire sleeping locks from +other contexts unless there is no other option. + +Sleeping lock types: + + - mutex + - rt_mutex + - semaphore + - rw_semaphore + - ww_mutex + - percpu_rw_semaphore + +On PREEMPT_RT kernels, these lock types are converted to sleeping locks: + + - spinlock_t + - rwlock_t + +Spinning locks +-------------- + + - raw_spinlock_t + - bit spinlocks + +On non-PREEMPT_RT kernels, these lock types are also spinning locks: + + - spinlock_t + - rwlock_t + +Spinning locks implicitly disable preemption and the lock / unlock functions +can have suffixes which apply further protections: + + =================== ==================================================== + _bh() Disable / enable bottom halves (soft interrupts) + _irq() Disable / enable interrupts + _irqsave/restore() Save and disable / restore interrupt disabled state + =================== ==================================================== + + +rtmutex +======= + +RT-mutexes are mutexes with support for priority inheritance (PI). + +PI has limitations on non PREEMPT_RT enabled kernels due to preemption and +interrupt disabled sections. + +PI clearly cannot preempt preemption-disabled or interrupt-disabled +regions of code, even on PREEMPT_RT kernels. Instead, PREEMPT_RT kernels +execute most such regions of code in preemptible task context, especially +interrupt handlers and soft interrupts. This conversion allows spinlock_t +and rwlock_t to be implemented via RT-mutexes. + + +raw_spinlock_t and spinlock_t +============================= + +raw_spinlock_t +-------------- + +raw_spinlock_t is a strict spinning lock implementation regardless of the +kernel configuration including PREEMPT_RT enabled kernels. + +raw_spinlock_t is a strict spinning lock implementation in all kernels, +including PREEMPT_RT kernels. Use raw_spinlock_t only in real critical +core code, low level interrupt handling and places where disabling +preemption or interrupts is required, for example, to safely access +hardware state. raw_spinlock_t can sometimes also be used when the +critical section is tiny, thus avoiding RT-mutex overhead. + +spinlock_t +---------- + +The semantics of spinlock_t change with the state of CONFIG_PREEMPT_RT. + +On a non PREEMPT_RT enabled kernel spinlock_t is mapped to raw_spinlock_t +and has exactly the same semantics. + +spinlock_t and PREEMPT_RT +------------------------- + +On a PREEMPT_RT enabled kernel spinlock_t is mapped to a separate +implementation based on rt_mutex which changes the semantics: + + - Preemption is not disabled + + - The hard interrupt related suffixes for spin_lock / spin_unlock + operations (_irq, _irqsave / _irqrestore) do not affect the CPUs + interrupt disabled state + + - The soft interrupt related suffix (_bh()) still disables softirq + handlers. + + Non-PREEMPT_RT kernels disable preemption to get this effect. + + PREEMPT_RT kernels use a per-CPU lock for serialization which keeps + preemption disabled. The lock disables softirq handlers and also + prevents reentrancy due to task preemption. + +PREEMPT_RT kernels preserve all other spinlock_t semantics: + + - Tasks holding a spinlock_t do not migrate. Non-PREEMPT_RT kernels + avoid migration by disabling preemption. PREEMPT_RT kernels instead + disable migration, which ensures that pointers to per-CPU variables + remain valid even if the task is preempted. + + - Task state is preserved across spinlock acquisition, ensuring that the + task-state rules apply to all kernel configurations. Non-PREEMPT_RT + kernels leave task state untouched. However, PREEMPT_RT must change + task state if the task blocks during acquisition. Therefore, it saves + the current task state before blocking and the corresponding lock wakeup + restores it. + + Other types of wakeups would normally unconditionally set the task state + to RUNNING, but that does not work here because the task must remain + blocked until the lock becomes available. Therefore, when a non-lock + wakeup attempts to awaken a task blocked waiting for a spinlock, it + instead sets the saved state to RUNNING. Then, when the lock + acquisition completes, the lock wakeup sets the task state to the saved + state, in this case setting it to RUNNING. + +rwlock_t +======== + +rwlock_t is a multiple readers and single writer lock mechanism. + +Non-PREEMPT_RT kernels implement rwlock_t as a spinning lock and the +suffix rules of spinlock_t apply accordingly. The implementation is fair, +thus preventing writer starvation. + +rwlock_t and PREEMPT_RT +----------------------- + +PREEMPT_RT kernels map rwlock_t to a separate rt_mutex-based +implementation, thus changing semantics: + + - All the spinlock_t changes also apply to rwlock_t. + + - Because an rwlock_t writer cannot grant its priority to multiple + readers, a preempted low-priority reader will continue holding its lock, + thus starving even high-priority writers. In contrast, because readers + can grant their priority to a writer, a preempted low-priority writer + will have its priority boosted until it releases the lock, thus + preventing that writer from starving readers. + + +PREEMPT_RT caveats +================== + +spinlock_t and rwlock_t +----------------------- + +These changes in spinlock_t and rwlock_t semantics on PREEMPT_RT kernels +have a few implications. For example, on a non-PREEMPT_RT kernel the +following code sequence works as expected:: + + local_irq_disable(); + spin_lock(&lock); + +and is fully equivalent to:: + + spin_lock_irq(&lock); + +Same applies to rwlock_t and the _irqsave() suffix variants. + +On PREEMPT_RT kernel this code sequence breaks because RT-mutex requires a +fully preemptible context. Instead, use spin_lock_irq() or +spin_lock_irqsave() and their unlock counterparts. In cases where the +interrupt disabling and locking must remain separate, PREEMPT_RT offers a +local_lock mechanism. Acquiring the local_lock pins the task to a CPU, +allowing things like per-CPU irq-disabled locks to be acquired. However, +this approach should be used only where absolutely necessary. + + +raw_spinlock_t +-------------- + +Acquiring a raw_spinlock_t disables preemption and possibly also +interrupts, so the critical section must avoid acquiring a regular +spinlock_t or rwlock_t, for example, the critical section must avoid +allocating memory. Thus, on a non-PREEMPT_RT kernel the following code +works perfectly:: + + raw_spin_lock(&lock); + p = kmalloc(sizeof(*p), GFP_ATOMIC); + +But this code fails on PREEMPT_RT kernels because the memory allocator is +fully preemptible and therefore cannot be invoked from truly atomic +contexts. However, it is perfectly fine to invoke the memory allocator +while holding normal non-raw spinlocks because they do not disable +preemption on PREEMPT_RT kernels:: + + spin_lock(&lock); + p = kmalloc(sizeof(*p), GFP_ATOMIC); + + +bit spinlocks +------------- + +Bit spinlocks are problematic for PREEMPT_RT as they cannot be easily +substituted by an RT-mutex based implementation for obvious reasons. + +The semantics of bit spinlocks are preserved on PREEMPT_RT kernels and the +caveats vs. raw_spinlock_t apply. + +Some bit spinlocks are substituted by regular spinlock_t for PREEMPT_RT but +this requires conditional (#ifdef'ed) code changes at the usage site while +the spinlock_t substitution is simply done by the compiler and the +conditionals are restricted to header files and core implementation of the +locking primitives and the usage sites do not require any changes. + + +Lock type nesting rules +======================= + +The most basic rules are: + + - Lock types of the same lock category (sleeping, spinning) can nest + arbitrarily as long as they respect the general lock ordering rules to + prevent deadlocks. + + - Sleeping lock types cannot nest inside spinning lock types. + + - Spinning lock types can nest inside sleeping lock types. + +These rules apply in general independent of CONFIG_PREEMPT_RT. + +As PREEMPT_RT changes the lock category of spinlock_t and rwlock_t from +spinning to sleeping this has obviously restrictions how they can nest with +raw_spinlock_t. + +This results in the following nest ordering: + + 1) Sleeping locks + 2) spinlock_t and rwlock_t + 3) raw_spinlock_t and bit spinlocks + +Lockdep is aware of these constraints to ensure that they are respected. + + +Owner semantics +=============== + +Most lock types in the Linux kernel have strict owner semantics, i.e. the +context (task) which acquires a lock has to release it. + +There are two exceptions: + + - semaphores + - rwsems + +semaphores have no owner semantics for historical reason, and as such +trylock and release operations can be called from any context. They are +often used for both serialization and waiting purposes. That's generally +discouraged and should be replaced by separate serialization and wait +mechanisms, such as mutexes and completions. + +rwsems have grown interfaces which allow non owner release for special +purposes. This usage is problematic on PREEMPT_RT because PREEMPT_RT +substitutes all locking primitives except semaphores with RT-mutex based +implementations to provide priority inheritance for all lock types except +the truly spinning ones. Priority inheritance on ownerless locks is +obviously impossible. + +For now the rwsem non-owner release excludes code which utilizes it from +being used on PREEMPT_RT enabled kernels. In same cases this can be +mitigated by disabling portions of the code, in other cases the complete +functionality has to be disabled until a workable solution has been found.