From: Thomas Gleixner <email@example.com> To: firstname.lastname@example.org Cc: LKML <email@example.com>, Peter Zijlstra <firstname.lastname@example.org>, Ingo Molnar <email@example.com>, Sebastian Siewior <firstname.lastname@example.org>, Linus Torvalds <email@example.com>, Joel Fernandes <firstname.lastname@example.org>, Oleg Nesterov <email@example.com>, Davidlohr Bueso <firstname.lastname@example.org>, Jonathan Corbet <email@example.com>, Randy Dunlap <firstname.lastname@example.org>, Logan Gunthorpe <email@example.com>, Bjorn Helgaas <firstname.lastname@example.org>, Kurt Schwemmer <email@example.com>, firstname.lastname@example.org, Greg Kroah-Hartman <email@example.com>, Felipe Balbi <firstname.lastname@example.org>, email@example.com, Kalle Valo <firstname.lastname@example.org>, "David S. Miller" <email@example.com>, firstname.lastname@example.org, email@example.com, Darren Hart <firstname.lastname@example.org>, Andy Shevchenko <email@example.com>, firstname.lastname@example.org, Zhang Rui <email@example.com>, "Rafael J. Wysocki" <firstname.lastname@example.org>, email@example.com, Len Brown <firstname.lastname@example.org>, email@example.com, kbuild test robot <firstname.lastname@example.org>, Nick Hu <email@example.com>, Greentime Hu <firstname.lastname@example.org>, Vincent Chen <email@example.com>, Guo Ren <firstname.lastname@example.org>, email@example.com, Brian Cain <firstname.lastname@example.org>, email@example.com, Tony Luck <firstname.lastname@example.org>, Fenghua Yu <email@example.com>, firstname.lastname@example.org, Michal Simek <email@example.com>, Michael Ellerman <firstname.lastname@example.org>, Arnd Bergmann <email@example.com>, Geoff Levand <firstname.lastname@example.org>, email@example.com, Davidlohr Bueso <firstname.lastname@example.org> Subject: Re: [patch V3 13/20] Documentation: Add lock ordering and nesting documentation Date: Wed, 25 Mar 2020 00:13:34 +0100 Message-ID: <email@example.com> (raw) In-Reply-To: <20200323025501.GE3199@paulmck-ThinkPad-P72> Paul, "Paul E. McKenney" <firstname.lastname@example.org> 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.
next prev parent reply index Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-03-21 11:25 [patch V3 00/20] Lock ordering documentation and annotation for lockdep Thomas Gleixner 2020-03-21 11:25 ` [patch V3 01/20] PCI/switchtec: Fix init_completion race condition with poll_wait() Thomas Gleixner 2020-03-21 11:25 ` [patch V3 02/20] pci/switchtec: Replace completion wait queue usage for poll Thomas Gleixner 2020-03-21 11:25 ` [patch V3 03/20] usb: gadget: Use completion interface instead of open coding it Thomas Gleixner 2020-03-25 8:37 ` Felipe Balbi 2020-03-27 12:14 ` Sebastian Siewior 2020-03-21 11:25 ` [patch V3 04/20] orinoco_usb: Use the regular completion interfaces Thomas Gleixner 2020-03-21 11:25 ` [patch V3 05/20] acpi: Remove header dependency Thomas Gleixner 2020-03-21 11:25 ` [patch V3 06/20] nds32: Remove mm.h from asm/uaccess.h Thomas Gleixner 2020-03-21 11:25 ` [patch V3 07/20] csky: " Thomas Gleixner 2020-03-21 11:25 ` [patch V3 08/20] hexagon: " Thomas Gleixner 2020-03-23 21:46 ` Brian Cain 2020-03-21 11:25 ` [patch V3 09/20] ia64: " Thomas Gleixner 2020-03-21 11:25 ` [patch V3 10/20] microblaze: " Thomas Gleixner 2020-03-21 11:25 ` [patch V3 11/20] rcuwait: Add @state argument to rcuwait_wait_event() Thomas Gleixner 2020-03-21 11:25 ` [patch V3 12/20] powerpc/ps3: Convert half completion to rcuwait Thomas Gleixner 2020-03-21 13:22 ` Thomas Gleixner 2020-03-27 19:14 ` Geoff Levand 2020-03-21 11:25 ` [patch V3 13/20] Documentation: Add lock ordering and nesting documentation Thomas Gleixner 2020-03-23 2:55 ` Paul E. McKenney 2020-03-24 23:13 ` Thomas Gleixner [this message] 2020-03-25 0:28 ` Paul E. McKenney 2020-03-25 12:27 ` Documentation/locking/locktypes: Further clarifications and wordsmithing Thomas Gleixner 2020-03-25 16:02 ` Sebastian Siewior 2020-03-25 16:39 ` Paul E. McKenney 2020-03-25 16:54 ` Sebastian Siewior 2020-03-25 16:58 ` [PATCH v2] Documentation/locking/locktypes: minor copy editor fixes Randy Dunlap 2020-03-26 2:40 ` Paul E. McKenney 2020-03-21 11:25 ` [patch V3 14/20] timekeeping: Split jiffies seqlock Thomas Gleixner 2020-03-21 11:25 ` [patch V3 15/20] sched/swait: Prepare usage in completions Thomas Gleixner 2020-03-21 11:26 ` [patch V3 16/20] completion: Use simple wait queues Thomas Gleixner 2020-03-23 15:20 ` [PATCH] completion: Use lockdep_assert_RT_in_threaded_ctx() in complete_all() Sebastian Siewior 2020-03-21 11:26 ` [patch V3 17/20] lockdep: Introduce wait-type checks Thomas Gleixner 2020-03-21 11:26 ` [patch V3 18/20] lockdep: Add hrtimer context tracing bits Thomas Gleixner 2020-03-21 11:26 ` [patch V3 19/20] lockdep: Annotate irq_work Thomas Gleixner 2020-03-21 11:26 ` [patch V3 20/20] lockdep: Add posixtimer context tracing bits Thomas Gleixner 2020-03-21 17:19 ` [patch V3 00/20] Lock ordering documentation and annotation for lockdep Davidlohr Bueso 2020-03-21 17:45 ` Thomas Gleixner
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