535 lines
17 KiB
ReStructuredText
535 lines
17 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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.. _kernel_hacking_locktypes:
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==========================
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Lock types and their rules
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==========================
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Introduction
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============
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The kernel provides a variety of locking primitives which can be divided
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into three categories:
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- Sleeping locks
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- CPU local locks
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- Spinning locks
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This document conceptually describes these lock types and provides rules
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for their nesting, including the rules for use under PREEMPT_RT.
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Lock categories
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===============
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Sleeping locks
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--------------
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Sleeping locks can only be acquired in preemptible task context.
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Although implementations allow try_lock() from other contexts, it is
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necessary to carefully evaluate the safety of unlock() as well as of
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try_lock(). Furthermore, it is also necessary to evaluate the debugging
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versions of these primitives. In short, don't acquire sleeping locks from
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other contexts unless there is no other option.
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Sleeping lock types:
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- mutex
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- rt_mutex
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- semaphore
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- rw_semaphore
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- ww_mutex
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- percpu_rw_semaphore
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On PREEMPT_RT kernels, these lock types are converted to sleeping locks:
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- local_lock
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- spinlock_t
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- rwlock_t
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CPU local locks
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---------------
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- local_lock
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On non-PREEMPT_RT kernels, local_lock functions are wrappers around
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preemption and interrupt disabling primitives. Contrary to other locking
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mechanisms, disabling preemption or interrupts are pure CPU local
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concurrency control mechanisms and not suited for inter-CPU concurrency
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control.
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Spinning locks
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--------------
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- raw_spinlock_t
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- bit spinlocks
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On non-PREEMPT_RT kernels, these lock types are also spinning locks:
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- spinlock_t
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- rwlock_t
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Spinning locks implicitly disable preemption and the lock / unlock functions
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can have suffixes which apply further protections:
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=================== ====================================================
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_bh() Disable / enable bottom halves (soft interrupts)
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_irq() Disable / enable interrupts
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_irqsave/restore() Save and disable / restore interrupt disabled state
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=================== ====================================================
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Owner semantics
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===============
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The aforementioned lock types except semaphores have strict owner
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semantics:
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The context (task) that acquired the lock must release it.
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rw_semaphores have a special interface which allows non-owner release for
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readers.
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rtmutex
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=======
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RT-mutexes are mutexes with support for priority inheritance (PI).
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PI has limitations on non-PREEMPT_RT kernels due to preemption and
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interrupt disabled sections.
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PI clearly cannot preempt preemption-disabled or interrupt-disabled
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regions of code, even on PREEMPT_RT kernels. Instead, PREEMPT_RT kernels
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execute most such regions of code in preemptible task context, especially
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interrupt handlers and soft interrupts. This conversion allows spinlock_t
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and rwlock_t to be implemented via RT-mutexes.
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semaphore
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=========
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semaphore is a counting semaphore implementation.
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Semaphores are often used for both serialization and waiting, but new use
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cases should instead use separate serialization and wait mechanisms, such
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as mutexes and completions.
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semaphores and PREEMPT_RT
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----------------------------
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PREEMPT_RT does not change the semaphore implementation because counting
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semaphores have no concept of owners, thus preventing PREEMPT_RT from
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providing priority inheritance for semaphores. After all, an unknown
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owner cannot be boosted. As a consequence, blocking on semaphores can
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result in priority inversion.
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rw_semaphore
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============
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rw_semaphore is a multiple readers and single writer lock mechanism.
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On non-PREEMPT_RT kernels the implementation is fair, thus preventing
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writer starvation.
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rw_semaphore complies by default with the strict owner semantics, but there
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exist special-purpose interfaces that allow non-owner release for readers.
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These interfaces work independent of the kernel configuration.
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rw_semaphore and PREEMPT_RT
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---------------------------
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PREEMPT_RT kernels map rw_semaphore to a separate rt_mutex-based
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implementation, thus changing the fairness:
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Because an rw_semaphore writer cannot grant its priority to multiple
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readers, a preempted low-priority reader will continue holding its lock,
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thus starving even high-priority writers. In contrast, because readers
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can grant their priority to a writer, a preempted low-priority writer will
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have its priority boosted until it releases the lock, thus preventing that
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writer from starving readers.
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local_lock
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==========
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local_lock provides a named scope to critical sections which are protected
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by disabling preemption or interrupts.
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On non-PREEMPT_RT kernels local_lock operations map to the preemption and
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interrupt disabling and enabling primitives:
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=============================== ======================
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local_lock(&llock) preempt_disable()
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local_unlock(&llock) preempt_enable()
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local_lock_irq(&llock) local_irq_disable()
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local_unlock_irq(&llock) local_irq_enable()
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local_lock_irqsave(&llock) local_irq_save()
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local_unlock_irqrestore(&llock) local_irq_restore()
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=============================== ======================
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The named scope of local_lock has two advantages over the regular
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primitives:
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- The lock name allows static analysis and is also a clear documentation
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of the protection scope while the regular primitives are scopeless and
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opaque.
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- If lockdep is enabled the local_lock gains a lockmap which allows to
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validate the correctness of the protection. This can detect cases where
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e.g. a function using preempt_disable() as protection mechanism is
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invoked from interrupt or soft-interrupt context. Aside of that
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lockdep_assert_held(&llock) works as with any other locking primitive.
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local_lock and PREEMPT_RT
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-------------------------
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PREEMPT_RT kernels map local_lock to a per-CPU spinlock_t, thus changing
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semantics:
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- All spinlock_t changes also apply to local_lock.
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local_lock usage
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----------------
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local_lock should be used in situations where disabling preemption or
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interrupts is the appropriate form of concurrency control to protect
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per-CPU data structures on a non PREEMPT_RT kernel.
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local_lock is not suitable to protect against preemption or interrupts on a
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PREEMPT_RT kernel due to the PREEMPT_RT specific spinlock_t semantics.
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raw_spinlock_t and spinlock_t
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=============================
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raw_spinlock_t
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--------------
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raw_spinlock_t is a strict spinning lock implementation in all kernels,
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including PREEMPT_RT kernels. Use raw_spinlock_t only in real critical
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core code, low-level interrupt handling and places where disabling
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preemption or interrupts is required, for example, to safely access
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hardware state. raw_spinlock_t can sometimes also be used when the
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critical section is tiny, thus avoiding RT-mutex overhead.
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spinlock_t
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----------
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The semantics of spinlock_t change with the state of PREEMPT_RT.
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On a non-PREEMPT_RT kernel spinlock_t is mapped to raw_spinlock_t and has
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exactly the same semantics.
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spinlock_t and PREEMPT_RT
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-------------------------
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On a PREEMPT_RT kernel spinlock_t is mapped to a separate implementation
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based on rt_mutex which changes the semantics:
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- Preemption is not disabled.
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- The hard interrupt related suffixes for spin_lock / spin_unlock
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operations (_irq, _irqsave / _irqrestore) do not affect the CPU's
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interrupt disabled state.
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- The soft interrupt related suffix (_bh()) still disables softirq
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handlers.
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Non-PREEMPT_RT kernels disable preemption to get this effect.
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PREEMPT_RT kernels use a per-CPU lock for serialization which keeps
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preemption enabled. The lock disables softirq handlers and also
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prevents reentrancy due to task preemption.
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PREEMPT_RT kernels preserve all other spinlock_t semantics:
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- Tasks holding a spinlock_t do not migrate. Non-PREEMPT_RT kernels
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avoid migration by disabling preemption. PREEMPT_RT kernels instead
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disable migration, which ensures that pointers to per-CPU variables
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remain valid even if the task is preempted.
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- Task state is preserved across spinlock acquisition, ensuring that the
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task-state rules apply to all kernel configurations. Non-PREEMPT_RT
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kernels leave task state untouched. However, PREEMPT_RT must change
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task state if the task blocks during acquisition. Therefore, it saves
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the current task state before blocking and the corresponding lock wakeup
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restores it, as shown below::
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task->state = TASK_INTERRUPTIBLE
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lock()
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block()
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task->saved_state = task->state
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task->state = TASK_UNINTERRUPTIBLE
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schedule()
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lock wakeup
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task->state = task->saved_state
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Other types of wakeups would normally unconditionally set the task state
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to RUNNING, but that does not work here because the task must remain
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blocked until the lock becomes available. Therefore, when a non-lock
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wakeup attempts to awaken a task blocked waiting for a spinlock, it
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instead sets the saved state to RUNNING. Then, when the lock
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acquisition completes, the lock wakeup sets the task state to the saved
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state, in this case setting it to RUNNING::
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task->state = TASK_INTERRUPTIBLE
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lock()
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block()
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task->saved_state = task->state
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task->state = TASK_UNINTERRUPTIBLE
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schedule()
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non lock wakeup
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task->saved_state = TASK_RUNNING
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lock wakeup
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task->state = task->saved_state
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This ensures that the real wakeup cannot be lost.
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rwlock_t
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========
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rwlock_t is a multiple readers and single writer lock mechanism.
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Non-PREEMPT_RT kernels implement rwlock_t as a spinning lock and the
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suffix rules of spinlock_t apply accordingly. The implementation is fair,
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thus preventing writer starvation.
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rwlock_t and PREEMPT_RT
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-----------------------
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PREEMPT_RT kernels map rwlock_t to a separate rt_mutex-based
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implementation, thus changing semantics:
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- All the spinlock_t changes also apply to rwlock_t.
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- Because an rwlock_t writer cannot grant its priority to multiple
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readers, a preempted low-priority reader will continue holding its lock,
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thus starving even high-priority writers. In contrast, because readers
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can grant their priority to a writer, a preempted low-priority writer
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will have its priority boosted until it releases the lock, thus
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preventing that writer from starving readers.
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PREEMPT_RT caveats
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==================
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local_lock on RT
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----------------
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The mapping of local_lock to spinlock_t on PREEMPT_RT kernels has a few
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implications. For example, on a non-PREEMPT_RT kernel the following code
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sequence works as expected::
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local_lock_irq(&local_lock);
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raw_spin_lock(&lock);
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and is fully equivalent to::
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raw_spin_lock_irq(&lock);
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On a PREEMPT_RT kernel this code sequence breaks because local_lock_irq()
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is mapped to a per-CPU spinlock_t which neither disables interrupts nor
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preemption. The following code sequence works perfectly correct on both
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PREEMPT_RT and non-PREEMPT_RT kernels::
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local_lock_irq(&local_lock);
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spin_lock(&lock);
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Another caveat with local locks is that each local_lock has a specific
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protection scope. So the following substitution is wrong::
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func1()
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{
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local_irq_save(flags); -> local_lock_irqsave(&local_lock_1, flags);
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func3();
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local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock_1, flags);
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}
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func2()
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{
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local_irq_save(flags); -> local_lock_irqsave(&local_lock_2, flags);
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func3();
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local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock_2, flags);
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}
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func3()
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{
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lockdep_assert_irqs_disabled();
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access_protected_data();
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}
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On a non-PREEMPT_RT kernel this works correctly, but on a PREEMPT_RT kernel
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local_lock_1 and local_lock_2 are distinct and cannot serialize the callers
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of func3(). Also the lockdep assert will trigger on a PREEMPT_RT kernel
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because local_lock_irqsave() does not disable interrupts due to the
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PREEMPT_RT-specific semantics of spinlock_t. The correct substitution is::
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func1()
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{
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local_irq_save(flags); -> local_lock_irqsave(&local_lock, flags);
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func3();
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local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock, flags);
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}
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func2()
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{
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local_irq_save(flags); -> local_lock_irqsave(&local_lock, flags);
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func3();
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local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock, flags);
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}
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func3()
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{
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lockdep_assert_held(&local_lock);
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access_protected_data();
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}
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spinlock_t and rwlock_t
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-----------------------
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The changes in spinlock_t and rwlock_t semantics on PREEMPT_RT kernels
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have a few implications. For example, on a non-PREEMPT_RT kernel the
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following code sequence works as expected::
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local_irq_disable();
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spin_lock(&lock);
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and is fully equivalent to::
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spin_lock_irq(&lock);
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Same applies to rwlock_t and the _irqsave() suffix variants.
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On PREEMPT_RT kernel this code sequence breaks because RT-mutex requires a
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fully preemptible context. Instead, use spin_lock_irq() or
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spin_lock_irqsave() and their unlock counterparts. In cases where the
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interrupt disabling and locking must remain separate, PREEMPT_RT offers a
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local_lock mechanism. Acquiring the local_lock pins the task to a CPU,
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allowing things like per-CPU interrupt disabled locks to be acquired.
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However, this approach should be used only where absolutely necessary.
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A typical scenario is protection of per-CPU variables in thread context::
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struct foo *p = get_cpu_ptr(&var1);
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spin_lock(&p->lock);
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p->count += this_cpu_read(var2);
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This is correct code on a non-PREEMPT_RT kernel, but on a PREEMPT_RT kernel
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this breaks. The PREEMPT_RT-specific change of spinlock_t semantics does
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not allow to acquire p->lock because get_cpu_ptr() implicitly disables
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preemption. The following substitution works on both kernels::
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struct foo *p;
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migrate_disable();
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p = this_cpu_ptr(&var1);
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spin_lock(&p->lock);
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p->count += this_cpu_read(var2);
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migrate_disable() ensures that the task is pinned on the current CPU which
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in turn guarantees that the per-CPU access to var1 and var2 are staying on
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the same CPU while the task remains preemptible.
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The migrate_disable() substitution is not valid for the following
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scenario::
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func()
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{
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struct foo *p;
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migrate_disable();
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p = this_cpu_ptr(&var1);
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p->val = func2();
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This breaks because migrate_disable() does not protect against reentrancy from
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a preempting task. A correct substitution for this case is::
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func()
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{
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struct foo *p;
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local_lock(&foo_lock);
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p = this_cpu_ptr(&var1);
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p->val = func2();
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On a non-PREEMPT_RT kernel this protects against reentrancy by disabling
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preemption. On a PREEMPT_RT kernel this is achieved by acquiring the
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underlying per-CPU spinlock.
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raw_spinlock_t on RT
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--------------------
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Acquiring a raw_spinlock_t disables preemption and possibly also
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interrupts, so the critical section must avoid acquiring a regular
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spinlock_t or rwlock_t, for example, the critical section must avoid
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allocating memory. Thus, on a non-PREEMPT_RT kernel the following code
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works perfectly::
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raw_spin_lock(&lock);
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p = kmalloc(sizeof(*p), GFP_ATOMIC);
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But this code fails on PREEMPT_RT kernels because the memory allocator is
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fully preemptible and therefore cannot be invoked from truly atomic
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contexts. However, it is perfectly fine to invoke the memory allocator
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while holding normal non-raw spinlocks because they do not disable
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preemption on PREEMPT_RT kernels::
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spin_lock(&lock);
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p = kmalloc(sizeof(*p), GFP_ATOMIC);
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bit spinlocks
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-------------
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PREEMPT_RT cannot substitute bit spinlocks because a single bit is too
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small to accommodate an RT-mutex. Therefore, the semantics of bit
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spinlocks are preserved on PREEMPT_RT kernels, so that the raw_spinlock_t
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caveats also apply to bit spinlocks.
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Some bit spinlocks are replaced with regular spinlock_t for PREEMPT_RT
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using conditional (#ifdef'ed) code changes at the usage site. In contrast,
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usage-site changes are not needed for the spinlock_t substitution.
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Instead, conditionals in header files and the core locking implemementation
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enable the compiler to do the substitution transparently.
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Lock type nesting rules
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=======================
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The most basic rules are:
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- Lock types of the same lock category (sleeping, CPU local, spinning)
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can nest arbitrarily as long as they respect the general lock ordering
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rules to prevent deadlocks.
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- Sleeping lock types cannot nest inside CPU local and spinning lock types.
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- CPU local and spinning lock types can nest inside sleeping lock types.
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- Spinning lock types can nest inside all lock types
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These constraints apply both in PREEMPT_RT and otherwise.
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The fact that PREEMPT_RT changes the lock category of spinlock_t and
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rwlock_t from spinning to sleeping and substitutes local_lock with a
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per-CPU spinlock_t means that they cannot be acquired while holding a raw
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spinlock. This results in the following nesting ordering:
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1) Sleeping locks
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2) spinlock_t, rwlock_t, local_lock
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3) raw_spinlock_t and bit spinlocks
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Lockdep will complain if these constraints are violated, both in
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PREEMPT_RT and otherwise.
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