What is spinlock linux

Locking lessons¶

Lesson 1: Spin locks¶

The most basic primitive for locking is spinlock:

The above is always safe. It will disable interrupts _locally_, but the spinlock itself will guarantee the global lock, so it will guarantee that there is only one thread-of-control within the region(s) protected by that lock. This works well even under UP also, so the code does _not_ need to worry about UP vs SMP issues: the spinlocks work correctly under both.

NOTE! Implications of spin_locks for memory are further described in:

The above is usually pretty simple (you usually need and want only one spinlock for most things — using more than one spinlock can make things a lot more complex and even slower and is usually worth it only for sequences that you know need to be split up: avoid it at all cost if you aren’t sure).

This is really the only really hard part about spinlocks: once you start using spinlocks they tend to expand to areas you might not have noticed before, because you have to make sure the spinlocks correctly protect the shared data structures everywhere they are used. The spinlocks are most easily added to places that are completely independent of other code (for example, internal driver data structures that nobody else ever touches).

NOTE! The spin-lock is safe only when you also use the lock itself to do locking across CPU’s, which implies that EVERYTHING that touches a shared variable has to agree about the spinlock they want to use.

Lesson 2: reader-writer spinlocks.В¶

If your data accesses have a very natural pattern where you usually tend to mostly read from the shared variables, the reader-writer locks (rw_lock) versions of the spinlocks are sometimes useful. They allow multiple readers to be in the same critical region at once, but if somebody wants to change the variables it has to get an exclusive write lock.

NOTE! reader-writer locks require more atomic memory operations than simple spinlocks. Unless the reader critical section is long, you are better off just using spinlocks.

The routines look the same as above:

The above kind of lock may be useful for complex data structures like linked lists, especially searching for entries without changing the list itself. The read lock allows many concurrent readers. Anything that changes the list will have to get the write lock.

NOTE! RCU is better for list traversal, but requires careful attention to design detail (see Using RCU to Protect Read-Mostly Linked Lists ).

Also, you cannot “upgrade” a read-lock to a write-lock, so if you at _any_ time need to do any changes (even if you don’t do it every time), you have to get the write-lock at the very beginning.

NOTE! We are working hard to remove reader-writer spinlocks in most cases, so please don’t add a new one without consensus. (Instead, see RCU Concepts for complete information.)

Lesson 3: spinlocks revisited.В¶

The single spin-lock primitives above are by no means the only ones. They are the most safe ones, and the ones that work under all circumstances, but partly because they are safe they are also fairly slow. They are slower than they’d need to be, because they do have to disable interrupts (which is just a single instruction on a x86, but it’s an expensive one — and on other architectures it can be worse).

If you have a case where you have to protect a data structure across several CPU’s and you want to use spinlocks you can potentially use cheaper versions of the spinlocks. IFF you know that the spinlocks are never used in interrupt handlers, you can use the non-irq versions:

(and the equivalent read-write versions too, of course). The spinlock will guarantee the same kind of exclusive access, and it will be much faster. This is useful if you know that the data in question is only ever manipulated from a “process context”, ie no interrupts involved.

The reasons you mustn’t use these versions if you have interrupts that play with the spinlock is that you can get deadlocks:

where an interrupt tries to lock an already locked variable. This is ok if the other interrupt happens on another CPU, but it is _not_ ok if the interrupt happens on the same CPU that already holds the lock, because the lock will obviously never be released (because the interrupt is waiting for the lock, and the lock-holder is interrupted by the interrupt and will not continue until the interrupt has been processed).

(This is also the reason why the irq-versions of the spinlocks only need to disable the _local_ interrupts — it’s ok to use spinlocks in interrupts on other CPU’s, because an interrupt on another CPU doesn’t interrupt the CPU that holds the lock, so the lock-holder can continue and eventually releases the lock).

Reference information:В¶

For dynamic initialization, use spin_lock_init() or rwlock_init() as appropriate:

For static initialization, use DEFINE_SPINLOCK() / DEFINE_RWLOCK() or __SPIN_LOCK_UNLOCKED() / __RW_LOCK_UNLOCKED() as appropriate.

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Lock types and their rules¶

Introduction¶

The kernel provides a variety of locking primitives which can be divided into three categories:

CPU local 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:

On PREEMPT_RT kernels, these lock types are converted to sleeping locks:

CPU local locks¶

On non-PREEMPT_RT kernels, local_lock functions are wrappers around preemption and interrupt disabling primitives. Contrary to other locking mechanisms, disabling preemption or interrupts are pure CPU local concurrency control mechanisms and not suited for inter-CPU concurrency control.

Spinning locks¶

On non-PREEMPT_RT kernels, these lock types are also spinning locks:

Spinning locks implicitly disable preemption and the lock / unlock functions can have suffixes which apply further protections:

Disable / enable bottom halves (soft interrupts)

Disable / enable interrupts

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¶

RT-mutexes are mutexes with support for priority inheritance (PI).

PI has limitations on non-PREEMPT_RT 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.

semaphore¶

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.

semaphores and PREEMPT_RT¶

PREEMPT_RT does not change the semaphore implementation because counting semaphores have no concept of owners, thus preventing PREEMPT_RT from providing priority inheritance for semaphores. After all, an unknown owner cannot be boosted. As a consequence, blocking on semaphores can result in priority inversion.

rw_semaphore¶

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 interfaces work independent of the kernel configuration.

rw_semaphore and PREEMPT_RT¶

PREEMPT_RT kernels map rw_semaphore to a separate rt_mutex-based implementation, thus changing the fairness:

Because an rw_semaphore 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.

local_lock¶

local_lock provides a named scope to critical sections which are protected by disabling preemption or interrupts.

On non-PREEMPT_RT kernels local_lock operations map to the preemption and interrupt disabling and enabling primitives:

The named scope of local_lock has two advantages over the regular primitives:

The lock name allows static analysis and is also a clear documentation of the protection scope while the regular primitives are scopeless and opaque.

If lockdep is enabled the local_lock gains a lockmap which allows to validate the correctness of the protection. This can detect cases where e.g. a function using preempt_disable() as protection mechanism is invoked from interrupt or soft-interrupt context. Aside of that lockdep_assert_held(&llock) works as with any other locking primitive.

local_lock and PREEMPT_RT¶

PREEMPT_RT kernels map local_lock to a per-CPU spinlock_t, thus changing semantics:

All spinlock_t changes also apply to local_lock.

local_lock usage¶

local_lock should be used in situations where disabling preemption or interrupts is the appropriate form of concurrency control to protect per-CPU data structures on a non PREEMPT_RT kernel.

local_lock is not suitable to protect against preemption or interrupts on a PREEMPT_RT kernel due to the PREEMPT_RT specific spinlock_t semantics.

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 PREEMPT_RT.

On a non-PREEMPT_RT kernel spinlock_t is mapped to raw_spinlock_t and has exactly the same semantics.

spinlock_t and PREEMPT_RT¶

On a PREEMPT_RT 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 CPU’s 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, as shown below:

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:

This ensures that the real wakeup cannot be lost.

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¶

local_lock on RT¶

The mapping of local_lock to spinlock_t on PREEMPT_RT kernels has a few implications. For example, on a non-PREEMPT_RT kernel the following code sequence works as expected:

and is fully equivalent to:

On a PREEMPT_RT kernel this code sequence breaks because local_lock_irq() is mapped to a per-CPU spinlock_t which neither disables interrupts nor preemption. The following code sequence works perfectly correct on both PREEMPT_RT and non-PREEMPT_RT kernels:

Another caveat with local locks is that each local_lock has a specific protection scope. So the following substitution is wrong:

On a non-PREEMPT_RT kernel this works correctly, but on a PREEMPT_RT kernel local_lock_1 and local_lock_2 are distinct and cannot serialize the callers of func3(). Also the lockdep assert will trigger on a PREEMPT_RT kernel because local_lock_irqsave() does not disable interrupts due to the PREEMPT_RT-specific semantics of spinlock_t. The correct substitution is:

spinlock_t and rwlock_t¶

The 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:

and is fully equivalent to:

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 interrupt disabled locks to be acquired. However, this approach should be used only where absolutely necessary.

A typical scenario is protection of per-CPU variables in thread context:

This is correct code on a non-PREEMPT_RT kernel, but on a PREEMPT_RT kernel this breaks. The PREEMPT_RT-specific change of spinlock_t semantics does not allow to acquire p->lock because get_cpu_ptr() implicitly disables preemption. The following substitution works on both kernels:

On a non-PREEMPT_RT kernel migrate_disable() maps to preempt_disable() which makes the above code fully equivalent. On a PREEMPT_RT kernel migrate_disable() ensures that the task is pinned on the current CPU which in turn guarantees that the per-CPU access to var1 and var2 are staying on the same CPU.

The migrate_disable() substitution is not valid for the following scenario:

While correct on a non-PREEMPT_RT kernel, this breaks on PREEMPT_RT because here migrate_disable() does not protect against reentrancy from a preempting task. A correct substitution for this case is:

On a non-PREEMPT_RT kernel this protects against reentrancy by disabling preemption. On a PREEMPT_RT kernel this is achieved by acquiring the underlying per-CPU spinlock.

raw_spinlock_t on RT¶

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:

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:

bit spinlocks¶

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¶

The most basic rules are:

Lock types of the same lock category (sleeping, CPU local, spinning) can nest arbitrarily as long as they respect the general lock ordering rules to prevent deadlocks.

Sleeping lock types cannot nest inside CPU local and spinning lock types.

CPU local and spinning lock types can nest inside sleeping lock types.

Spinning lock types can nest inside all lock types

These constraints apply both in PREEMPT_RT and otherwise.

The fact that PREEMPT_RT changes the lock category of spinlock_t and rwlock_t from spinning to sleeping and substitutes local_lock with a per-CPU spinlock_t means that they cannot be acquired while holding a raw spinlock. This results in the following nesting ordering:

spinlock_t, rwlock_t, local_lock

raw_spinlock_t and bit spinlocks

Lockdep will complain if these constraints are violated, both in PREEMPT_RT and otherwise.

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