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:

• local_lock

• spinlock_t

• rwlock_t

CPU local locks¶

• local_lock

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¶

• 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

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 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 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.

CPU local scope and bottom-half¶

Per-CPU variables that are accessed only in softirq context should not rely on the assumption that this context is implicitly protected due to being non-preemptible. In a PREEMPT_RT kernel, softirq context is preemptible, and synchronizing every bottom-half-disabled section via implicit context results in an implicit per-CPU “big kernel lock.”

A local_lock_t together with local_lock_nested_bh() and local_unlock_nested_bh() for locking operations help to identify the locking scope.

When lockdep is enabled, these functions verify that data structure access occurs within softirq context. Unlike local_lock(), local_unlock_nested_bh() does not disable preemption and does not add overhead when used without lockdep.

On a PREEMPT_RT kernel, local_lock_t behaves as a real lock and local_unlock_nested_bh() serializes access to the data structure, which allows removal of serialization via local_bh_disable().

raw_spinlock_t¶

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 enabled. 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:

  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 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:

  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 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.

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:

local_lock_irq(&local_lock);
raw_spin_lock(&lock);

and is fully equivalent to:

raw_spin_lock_irq(&lock);

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:

local_lock_irq(&local_lock);
spin_lock(&lock);

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

func1()
{
  local_irq_save(flags);    -> local_lock_irqsave(&local_lock_1, flags);
  func3();
  local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock_1, flags);
}

func2()
{
  local_irq_save(flags);    -> local_lock_irqsave(&local_lock_2, flags);
  func3();
  local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock_2, flags);
}

func3()
{
  lockdep_assert_irqs_disabled();
  access_protected_data();
}

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:

func1()
{
  local_irq_save(flags);    -> local_lock_irqsave(&local_lock, flags);
  func3();
  local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock, flags);
}

func2()
{
  local_irq_save(flags);    -> local_lock_irqsave(&local_lock, flags);
  func3();
  local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock, flags);
}

func3()
{
  lockdep_assert_held(&local_lock);
  access_protected_data();
}

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:

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 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:

struct foo *p = get_cpu_ptr(&var1);

spin_lock(&p->lock);
p->count += this_cpu_read(var2);

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:

struct foo *p;

migrate_disable();
p = this_cpu_ptr(&var1);
spin_lock(&p->lock);
p->count += this_cpu_read(var2);

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 while the task remains preemptible.

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

func()
{
  struct foo *p;

  migrate_disable();
  p = this_cpu_ptr(&var1);
  p->val = func2();

This breaks because migrate_disable() does not protect against reentrancy from a preempting task. A correct substitution for this case is:

func()
{
  struct foo *p;

  local_lock(&foo_lock);
  p = this_cpu_ptr(&var1);
  p->val = func2();

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:

raw_spin_lock(&lock);
p = kmalloc_obj(*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_obj(*p, GFP_ATOMIC);

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 implementation enable the compiler to do the substitution transparently.