mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-23 09:19:51 +00:00
![Bjorn Helgaas](/assets/img/avatar_default.png)
Fix typos in Documentation. Signed-off-by: Bjorn Helgaas <bhelgaas@google.com> Link: https://lore.kernel.org/r/20230814212822.193684-4-helgaas@kernel.org Signed-off-by: Jonathan Corbet <corbet@lwn.net>
534 lines
17 KiB
ReStructuredText
534 lines
17 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
.. _kernel_hacking_locktypes:
|
|
|
|
==========================
|
|
Lock types and their rules
|
|
==========================
|
|
|
|
Introduction
|
|
============
|
|
|
|
The kernel provides a variety of locking primitives which can be divided
|
|
into three categories:
|
|
|
|
- Sleeping locks
|
|
- CPU local locks
|
|
- Spinning 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:
|
|
|
|
- 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
|
|
=================== ====================================================
|
|
|
|
|
|
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:
|
|
|
|
=============================== ======================
|
|
local_lock(&llock) preempt_disable()
|
|
local_unlock(&llock) preempt_enable()
|
|
local_lock_irq(&llock) local_irq_disable()
|
|
local_unlock_irq(&llock) local_irq_enable()
|
|
local_lock_irqsave(&llock) local_irq_save()
|
|
local_unlock_irqrestore(&llock) local_irq_restore()
|
|
=============================== ======================
|
|
|
|
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 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
|
|
========
|
|
|
|
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::
|
|
|
|
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(sizeof(*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(sizeof(*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.
|
|
|
|
|
|
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:
|
|
|
|
1) Sleeping locks
|
|
2) spinlock_t, rwlock_t, local_lock
|
|
3) raw_spinlock_t and bit spinlocks
|
|
|
|
Lockdep will complain if these constraints are violated, both in
|
|
PREEMPT_RT and otherwise.
|