From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EC9A2C54FD1 for ; Tue, 24 Mar 2020 23:14:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BE45D205ED for ; Tue, 24 Mar 2020 23:14:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727325AbgCXXO1 (ORCPT ); Tue, 24 Mar 2020 19:14:27 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:46547 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726204AbgCXXO1 (ORCPT ); Tue, 24 Mar 2020 19:14:27 -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 1jGsjn-0007lV-Qr; Wed, 25 Mar 2020 00:13:36 +0100 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id DC4A4100C51; Wed, 25 Mar 2020 00:13:34 +0100 (CET) From: Thomas Gleixner To: paulmck@kernel.org Cc: LKML , Peter Zijlstra , Ingo Molnar , Sebastian Siewior , Linus Torvalds , Joel Fernandes , Oleg Nesterov , Davidlohr Bueso , 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: Re: [patch V3 13/20] Documentation: Add lock ordering and nesting documentation In-Reply-To: <20200323025501.GE3199@paulmck-ThinkPad-P72> Date: Wed, 25 Mar 2020 00:13:34 +0100 Message-ID: <87r1xhz6qp.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain 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-usb-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-usb@vger.kernel.org Paul, "Paul E. McKenney" writes: > On Sat, Mar 21, 2020 at 12:25:57PM +0100, Thomas Gleixner wrote: > In the normal case where the task sleeps through the entire lock > acquisition, the sequence of events is as follows: > > state = UNINTERRUPTIBLE > lock() > block() > real_state = state > state = SLEEPONLOCK > > lock wakeup > state = real_state == UNINTERRUPTIBLE > > This sequence of events can occur when the task acquires spinlocks > on its way to sleeping, for example, in a call to wait_event(). > > The non-lock wakeup can occur when a wakeup races with this wait_event(), > which can result in the following sequence of events: > > state = UNINTERRUPTIBLE > lock() > block() > real_state = state > state = SLEEPONLOCK > > non lock wakeup > real_state = RUNNING > > lock wakeup > state = real_state == RUNNING > > Without this real_state subterfuge, the wakeup might be lost. I added this with a few modifications which reflect the actual implementation. Conceptually the same. > rwsems have grown special-purpose interfaces that allow non-owner release. > This non-owner release prevents PREEMPT_RT from substituting RT-mutex > implementations, for example, by defeating priority inheritance. > After all, if the lock has no owner, whose priority should be boosted? > As a result, PREEMPT_RT does not currently support rwsem, which in turn > means that code using it must therefore be disabled until a workable > solution presents itself. > > [ Note: Not as confident as I would like to be in the above. ] I'm not confident either especially not after looking at the actual code. In fact I feel really stupid because the rw_semaphore reader non-owner restriction on RT simply does not exist anymore and my history biased memory tricked me. The first rw_semaphore implementation of RT was simple and restricted the reader side to a single reader to support PI on both the reader and the writer side. That obviosuly did not scale well and made mmap_sem heavy use cases pretty unhappy. The short interlude with multi-reader boosting turned out to be a failed experiment - Steven might still disagree though :) At some point we gave up and I myself (sic!) reimplemented the RT variant of rw_semaphore with a reader biased mechanism. The reader never holds the underlying rt_mutex accross the read side critical section. It merily increments the reader count and drops it on release. The only time a reader takes the rt_mutex is when it blocks on a writer. Writers hold the rt_mutex across the write side critical section to allow incoming readers to boost them. Once the writer releases the rw_semaphore it unlocks the rt_mutex which is then handed off to the readers. They increment the reader count and then drop the rt_mutex before continuing in the read side critical section. So while I changed the implementation it did obviously not occur to me that this also lifted the non-owner release restriction. Nobody else noticed either. So we kept dragging this along in both memory and implementation. Both will be fixed now :) The owner semantics of down/up_read() are only enforced by lockdep. That applies to both RT and !RT. The up/down_read_non_owner() variants are just there to tell lockdep about it. So, I picked up your other suggestions with slight modifications and adjusted the owner, semaphore and rw_semaphore docs accordingly. Please have a close look at the patch below (applies on tip core/locking). Thanks, tglx, who is searching a brown paperbag 8<---------- Documentation/locking/locktypes.rst | 148 +++++++++++++++++++++++------------- 1 file changed, 98 insertions(+), 50 deletions(-) --- a/Documentation/locking/locktypes.rst +++ b/Documentation/locking/locktypes.rst @@ -67,6 +67,17 @@ Spinning locks implicitly disable preemp _irqsave/restore() Save and disable / restore interrupt disabled state =================== ==================================================== +Owner semantics +=============== + +The aforementioned lock types except semaphores have strict owner +semantics: + + The context (task) that acquired the lock must release it. + +rw_semaphores have a special interface which allows non-owner release for +readers. + rtmutex ======= @@ -83,6 +94,51 @@ interrupt handlers and soft interrupts. and rwlock_t to be implemented via RT-mutexes. +sempahore +========= + +semaphore is a counting semaphore implementation. + +Semaphores are often used for both serialization and waiting, but new use +cases should instead use separate serialization and wait mechanisms, such +as mutexes and completions. + +sempahores and PREEMPT_RT +---------------------------- + +PREEMPT_RT does not change the sempahore implementation. That's impossible +due to the counting semaphore semantics which have no concept of owners. +The lack of an owner conflicts with priority inheritance. After all an +unknown owner cannot be boosted. As a consequence blocking on semaphores +can be subject to priority inversion. + + +rw_sempahore +============ + +rw_semaphore is a multiple readers and single writer lock mechanism. + +On non-PREEMPT_RT kernels the implementation is fair, thus preventing +writer starvation. + +rw_semaphore complies by default with the strict owner semantics, but there +exist special-purpose interfaces that allow non-owner release for readers. +These work independent of the kernel configuration. + +rw_sempahore and PREEMPT_RT +--------------------------- + +PREEMPT_RT kernels map rw_sempahore to a separate rt_mutex-based +implementation, thus changing the fairness: + + Because an rw_sempaphore 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. + + raw_spinlock_t and spinlock_t ============================= @@ -140,7 +196,16 @@ On a PREEMPT_RT enabled kernel spinlock_ 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. + restores it:: + + task->state = TASK_INTERRUPTIBLE + lock() + block() + task->saved_state = task->state + task->state = TASK_UNINTERRUPTIBLE + schedule() + lock wakeup + task->state = task->saved_state Other types of wakeups would normally unconditionally set the task state to RUNNING, but that does not work here because the task must remain @@ -148,7 +213,22 @@ On a PREEMPT_RT enabled kernel spinlock_ 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. + state, in this case setting it to RUNNING:: + + task->state = TASK_INTERRUPTIBLE + lock() + block() + task->saved_state = task->state + task->state = TASK_UNINTERRUPTIBLE + schedule() + non lock wakeup + task->saved_state = TASK_RUNNING + + lock wakeup + task->state = task->saved_state + + This ensures that the real wakeup cannot be lost. + rwlock_t ======== @@ -228,17 +308,16 @@ while holding normal non-raw spinlocks b 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. +PREEMPT_RT cannot substitute bit spinlocks because a single bit is too +small to accommodate an RT-mutex. Therefore, the semantics of bit +spinlocks are preserved on PREEMPT_RT kernels, so that the raw_spinlock_t +caveats also apply to bit spinlocks. + +Some bit spinlocks are replaced with regular spinlock_t for PREEMPT_RT +using conditional (#ifdef'ed) code changes at the usage site. In contrast, +usage-site changes are not needed for the spinlock_t substitution. +Instead, conditionals in header files and the core locking implemementation +enable the compiler to do the substitution transparently. Lock type nesting rules @@ -254,46 +333,15 @@ Lock type nesting rules - Spinning lock types can nest inside sleeping lock types. -These rules apply in general independent of CONFIG_PREEMPT_RT. +These constraints apply both in CONFIG_PREEMPT_RT and otherwise. -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: +The fact that PREEMPT_RT changes the lock category of spinlock_t and +rwlock_t from spinning to sleeping means that they cannot be acquired while +holding a raw spinlock. This results in the following nesting 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. +Lockdep will complain if these constraints are violated, both in +CONFIG_PREEMPT_RT and otherwise.