pw_sync#
Stable
The pw_sync
module contains utilities for synchronizing between threads
and/or interrupts through signaling primitives and critical section lock
primitives.
Note
The objects in this module do not have an Init() style public API which is common in many RTOS C APIs. Instead, they rely on being able to invoke the native initialization APIs for synchronization primitives during C++ construction.
In order to support global statically constructed synchronization without constexpr constructors, the user and/or backend MUST ensure that any initialization required in your environment is done prior to the creation and/or initialization of the native synchronization primitives (e.g. kernel initialization).
Critical Section Lock Primitives#
The critical section lock primitives provided by this module comply with
BasicLockable,
Lockable, and where
relevant
TimedLockable C++
named requirements. This means that they are compatible with existing helpers in
the STL’s <mutex>
thread support library. For example std::lock_guard and std::unique_lock can be directly used.
Mutex#
The Mutex is a synchronization primitive that can be used to protect shared data from being simultaneously accessed by multiple threads. It offers exclusive, non-recursive ownership semantics where priority inheritance is used to solve the classic priority-inversion problem.
The Mutex’s API is C++11 STL std::mutex like, meaning it is a BasicLockable and Lockable.
Supported on |
Backend module |
---|---|
FreeRTOS |
|
Zephyr |
|
ThreadX |
|
embOS |
|
STL |
|
Baremetal |
Planned |
CMSIS-RTOS API v2 & RTX5 |
Planned |
C++#
Examples in C++#
#include "pw_sync/mutex.h"
pw::sync::Mutex mutex;
void ThreadSafeCriticalSection() {
mutex.lock();
NotThreadSafeCriticalSection();
mutex.unlock();
}
Alternatively you can use C++’s RAII helpers to ensure you always unlock.
#include <mutex>
#include "pw_sync/mutex.h"
pw::sync::Mutex mutex;
void ThreadSafeCriticalSection() {
std::lock_guard lock(mutex);
NotThreadSafeCriticalSection();
}
C#
The Mutex must be created in C++, however it can be passed into C using the
pw_sync_Mutex
opaque struct alias.
Safe to use in context |
Thread |
Interrupt |
NMI |
---|---|---|---|
✔ |
|||
✔ |
|||
✔ |
Example in C#
#include "pw_sync/mutex.h"
pw::sync::Mutex mutex;
extern pw_sync_Mutex mutex; // This can only be created in C++.
void ThreadSafeCriticalSection(void) {
pw_sync_Mutex_Lock(&mutex);
NotThreadSafeCriticalSection();
pw_sync_Mutex_Unlock(&mutex);
}
TimedMutex#
The TimedMutex is an extension of the Mutex which offers timeout and deadline based semantics.
The TimedMutex’s API is C++11 STL std::timed_mutex like, meaning it is a BasicLockable, Lockable, and TimedLockable.
Note that the TimedMutex is a derived Mutex class, meaning that a TimedMutex can be used by someone who needs the basic Mutex. This is in contrast to the C++ STL’s std::timed_mutex.
Supported on |
Backend module |
---|---|
FreeRTOS |
|
ThreadX |
|
embOS |
|
STL |
|
Zephyr |
Planned |
CMSIS-RTOS API v2 & RTX5 |
Planned |
C++#
Safe to use in context |
Thread |
Interrupt |
NMI |
---|---|---|---|
Constructor |
✔ |
||
Destructor |
✔ |
||
|
✔ |
||
|
✔ |
||
✔ |
|||
✔ |
|||
|
✔ |
Examples in C++#
#include "pw_chrono/system_clock.h"
#include "pw_sync/timed_mutex.h"
pw::sync::TimedMutex mutex;
bool ThreadSafeCriticalSectionWithTimeout(
const SystemClock::duration timeout) {
if (!mutex.try_lock_for(timeout)) {
return false;
}
NotThreadSafeCriticalSection();
mutex.unlock();
return true;
}
Alternatively you can use C++’s RAII helpers to ensure you always unlock.
#include <mutex>
#include "pw_chrono/system_clock.h"
#include "pw_sync/timed_mutex.h"
pw::sync::TimedMutex mutex;
bool ThreadSafeCriticalSectionWithTimeout(
const SystemClock::duration timeout) {
std::unique_lock lock(mutex, std::defer_lock);
if (!lock.try_lock_for(timeout)) {
return false;
}
NotThreadSafeCriticalSection();
return true;
}
C#
The TimedMutex must be created in C++, however it can be passed into C using the
pw_sync_TimedMutex
opaque struct alias.
Safe to use in context |
Thread |
Interrupt |
NMI |
---|---|---|---|
✔ |
|||
✔ |
|||
✔ |
|||
✔ |
|||
✔ |
Example in C#
#include "pw_chrono/system_clock.h"
#include "pw_sync/timed_mutex.h"
pw::sync::TimedMutex mutex;
extern pw_sync_TimedMutex mutex; // This can only be created in C++.
bool ThreadSafeCriticalSectionWithTimeout(
const pw_chrono_SystemClock_Duration timeout) {
if (!pw_sync_TimedMutex_TryLockFor(&mutex, timeout)) {
return false;
}
NotThreadSafeCriticalSection();
pw_sync_TimedMutex_Unlock(&mutex);
return true;
}
RecursiveMutex#
pw_sync
provides pw::sync::RecursiveMutex
, a recursive mutex
implementation. At this time, this facade can only be used internally by
Pigweed.
InterruptSpinLock#
InterruptSpinLock is a synchronization primitive that can be used to protect shared data from being simultaneously accessed by multiple threads and/or interrupts as a targeted global lock, with the exception of Non-Maskable Interrupts (NMIs). It offers exclusive, non-recursive ownership semantics where IRQs up to a backend defined level of “NMIs” will be masked to solve priority-inversion.
This InterruptSpinLock relies on built-in local interrupt masking to make it interrupt safe without requiring the caller to separately mask and unmask interrupts when using this primitive.
Unlike global interrupt locks, this also works safely and efficiently on SMP systems. On systems which are not SMP, spinning is not required but some state may still be used to detect recursion.
The InterruptSpinLock is a BasicLockable and Lockable.
Supported on |
Backend module |
---|---|
FreeRTOS |
|
ThreadX |
|
embOS |
|
STL |
|
Baremetal |
Planned, not ready for use |
Zephyr |
Planned |
CMSIS-RTOS API v2 & RTX5 |
Planned |
C++#
Safe to use in context |
Thread |
Interrupt |
NMI |
---|---|---|---|
Constructor |
✔ |
✔ |
|
Destructor |
✔ |
✔ |
|
✔ |
✔ |
||
✔ |
✔ |
||
✔ |
✔ |
Examples in C++#
#include "pw_sync/interrupt_spin_lock.h"
pw::sync::InterruptSpinLock interrupt_spin_lock;
void InterruptSafeCriticalSection() {
interrupt_spin_lock.lock();
NotThreadSafeCriticalSection();
interrupt_spin_lock.unlock();
}
Alternatively you can use C++’s RAII helpers to ensure you always unlock.
#include <mutex>
#include "pw_sync/interrupt_spin_lock.h"
pw::sync::InterruptSpinLock interrupt_spin_lock;
void InterruptSafeCriticalSection() {
std::lock_guard lock(interrupt_spin_lock);
NotThreadSafeCriticalSection();
}
C#
The InterruptSpinLock must be created in C++, however it can be passed into C
using the pw_sync_InterruptSpinLock
opaque struct alias.
Safe to use in context |
Thread |
Interrupt |
NMI |
---|---|---|---|
✔ |
✔ |
||
✔ |
✔ |
||
✔ |
✔ |
Example in C#
#include "pw_chrono/system_clock.h"
#include "pw_sync/interrupt_spin_lock.h"
pw::sync::InterruptSpinLock interrupt_spin_lock;
extern pw_sync_InterruptSpinLock interrupt_spin_lock; // This can only be created in C++.
void InterruptSafeCriticalSection(void) {
pw_sync_InterruptSpinLock_Lock(&interrupt_spin_lock);
NotThreadSafeCriticalSection();
pw_sync_InterruptSpinLock_Unlock(&interrupt_spin_lock);
}
Optional locking#
NoLock is a no-op lock that can be used to
satisfy a lock interface without providing any synchronization. This can be
useful for templated code that is lock-agnostic, but may be used in a context
that does not require any synchronization. NoLock
is a BasicLockable.
MaybeLock selects between a real lock type and
NoLock
based on a boolean template parameter. This may be helpful when
locking is conditionally enabled by a config macro.
Thread Safety Lock Annotations#
Pigweed’s critical section lock primitives support Clang’s thread safety analysis extension for C++. The analysis is completely static at compile-time. This is only supported when building with Clang. The annotations are no-ops when using different compilers.
Pigweed provides the pw_sync/lock_annotations.h
header file with macro
definitions to allow developers to document the locking policies of
multi-threaded code. The annotations can also help program analysis tools to
identify potential thread safety issues.
More information on Clang’s thread safety analysis system can be found here.
Enabling Clang’s Analysis#
In order to enable the analysis, Clang requires that the -Wthread-safety
compilation flag be used. To also enable PW_ACQUIRED_AFTER and/or
PW_ACQUIRED_BEFORE, it also requires the -Wthread-safety-beta
compilation flag. And if any STL components like std::lock_guard
are used,
the STL’s built in annotations should be manually enabled, typically by setting
the _LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS
macro.
If using GN, the pw_build:clang_thread_safety_warnings
config is provided
to do all of the above for you, when added to your clang toolchain definition’s
default configs.
Why use lock annotations?#
Lock annotations can help warn you about potential race conditions in your code when using locks: you have to remember to grab lock(s) before entering a critical section, yuou have to remember to unlock it when you leave, and you have to avoid deadlocks.
Clang’s lock annotations let you inform the compiler and anyone reading your code which variables are guarded by which locks, which locks should or cannot be held when calling which function, which order locks should be acquired in, etc.
Using Lock Annotations#
When referring to locks in the arguments of the attributes, you should
use variable names or more complex expressions (e.g. my_object->lock_
)
that evaluate to a concrete lock object whenever possible. If the lock
you want to refer to is not in scope, you may use a member pointer
(e.g. &MyClass::lock_
) to refer to a lock in some (unknown) object.
Annotating Lock Usage#
Annotating Lock Objects#
In order of lock usage annotation to work, the lock objects themselves need to be annotated as well. In case you are providing your own lock or psuedo-lock object, you can use the macros in this section to annotate it.
As an example we’ve annotated a Lock and a RAII ScopedLocker object for you, see the macro documentation after for more details:
class PW_LOCKABLE("Lock") Lock {
public:
void Lock() PW_EXCLUSIVE_LOCK_FUNCTION();
void ReaderLock() PW_SHARED_LOCK_FUNCTION();
void Unlock() PW_UNLOCK_FUNCTION();
void ReaderUnlock() PW_SHARED_TRYLOCK_FUNCTION();
bool TryLock() PW_EXCLUSIVE_TRYLOCK_FUNCTION(true);
bool ReaderTryLock() PW_SHARED_TRYLOCK_FUNCTION(true);
void AssertHeld() PW_ASSERT_EXCLUSIVE_LOCK();
void AssertReaderHeld() PW_ASSERT_SHARED_LOCK();
};
// Tag types for selecting a constructor.
struct adopt_lock_t {} inline constexpr adopt_lock = {};
struct defer_lock_t {} inline constexpr defer_lock = {};
struct shared_lock_t {} inline constexpr shared_lock = {};
class PW_SCOPED_LOCKABLE ScopedLocker {
// Acquire lock, implicitly acquire *this and associate it with lock.
ScopedLocker(Lock *lock) PW_EXCLUSIVE_LOCK_FUNCTION(lock)
: lock_(lock), locked(true) {
lock->Lock();
}
// Assume lock is held, implicitly acquire *this and associate it with lock.
ScopedLocker(Lock *lock, adopt_lock_t) PW_EXCLUSIVE_LOCKS_REQUIRED(lock)
: lock_(lock), locked(true) {}
// Acquire lock in shared mode, implicitly acquire *this and associate it
// with lock.
ScopedLocker(Lock *lock, shared_lock_t) PW_SHARED_LOCK_FUNCTION(lock)
: lock_(lock), locked(true) {
lock->ReaderLock();
}
// Assume lock is held in shared mode, implicitly acquire *this and associate
// it with lock.
ScopedLocker(Lock *lock, adopt_lock_t, shared_lock_t)
PW_SHARED_LOCKS_REQUIRED(lock) : lock_(lock), locked(true) {}
// Assume lock is not held, implicitly acquire *this and associate it with
// lock.
ScopedLocker(Lock *lock, defer_lock_t) PW_LOCKS_EXCLUDED(lock)
: lock_(lock), locked(false) {}
// Release *this and all associated locks, if they are still held.
// There is no warning if the scope was already unlocked before.
~ScopedLocker() PW_UNLOCK_FUNCTION() {
if (locked)
lock_->GenericUnlock();
}
// Acquire all associated locks exclusively.
void Lock() PW_EXCLUSIVE_LOCK_FUNCTION() {
lock_->Lock();
locked = true;
}
// Try to acquire all associated locks exclusively.
bool TryLock() PW_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
return locked = lock_->TryLock();
}
// Acquire all associated locks in shared mode.
void ReaderLock() PW_SHARED_LOCK_FUNCTION() {
lock_->ReaderLock();
locked = true;
}
// Try to acquire all associated locks in shared mode.
bool ReaderTryLock() PW_SHARED_TRYLOCK_FUNCTION(true) {
return locked = lock_->ReaderTryLock();
}
// Release all associated locks. Warn on double unlock.
void Unlock() PW_UNLOCK_FUNCTION() {
lock_->Unlock();
locked = false;
}
// Release all associated locks. Warn on double unlock.
void ReaderUnlock() PW_UNLOCK_FUNCTION() {
lock_->ReaderUnlock();
locked = false;
}
private:
Lock* lock_;
bool locked_;
};
Critical Section Lock Helpers#
Virtual Lock Interfaces#
Virtual lock interfaces can be useful when lock selection cannot be templated.
Why use virtual locks?#
Virtual locks enable depending on locks without templating implementation code on the type, while retaining flexibility with respect to the concrete lock type. Pigweed tries to avoid pushing policy on to users, and virtual locks are one way to accomplish that without templating everything.
A case when virtual locks are useful is when the concrete lock type changes at run time. For example, access to flash may be protected at run time by an internal mutex, however at crash time we may want to switch to a no-op lock. A virtual lock interface could be used here to minimize the code-size cost that would occur otherwise if the flash driver were templated.
VirtualBasicLockable#
The VirtualBasicLockable
interface meets the
BasicLockable C++
named requirement. Our critical section lock primitives offer optional virtual
versions, including:
GenericBasicLockable#
GenericBasicLockable
is a helper construct that can be used to declare
virtual versions of a critical section lock primitive that meets the
BasicLockable
C++ named requirement. For example, given a Mutex
type with lock()
and
unlock()
methods, a VirtualMutex
type that derives from
VirtualBasicLockable
can be declared as follows:
class VirtualMutex : public GenericBasicLockable<Mutex> {};
Borrowable#
Borrowable
is a helper construct that enables callers to borrow an object
which is guarded by a lock, enabling a containerized style of external locking.
Users who need access to the guarded object can ask to acquire a BorrowedPointer which permits access while the lock is held.
This class is compatible with locks which comply with BasicLockable, Lockable, and TimedLockable C++ named requirements.
By default the selected lock type is a pw::sync::VirtualBasicLockable
. If
this virtual interface is used, the templated lock parameter can be skipped.
External vs Internal locking#
Before we explain why Borrowable is useful, it’s important to understand the trade-offs when deciding on using internal and/or external locking.
Internal locking is when the lock is hidden from the caller entirely and is used internally to the API. For example:
class BankAccount {
public:
void Deposit(int amount) {
std::lock_guard lock(mutex_);
balance_ += amount;
}
void Withdraw(int amount) {
std::lock_guard lock(mutex_);
balance_ -= amount;
}
void Balance() const {
std::lock_guard lock(mutex_);
return balance_;
}
private:
int balance_ PW_GUARDED_BY(mutex_);
pw::sync::Mutex mutex_;
};
Internal locking guarantees that any concurrent calls to its public member functions don’t corrupt an instance of that class. This is typically ensured by having each member function acquire a lock on the object upon entry. This way, for any instance, there can only be one member function call active at any moment, serializing the operations.
One common issue that pops up is that member functions may have to call other member functions which also require locks. This typically results in a duplication of the public API into an internal mirror where the lock is already held. This along with having to modify every thread-safe public member function may results in an increased code size.
However, with the per-method locking approach, it is not possible to perform a multi-method thread-safe transaction. For example, what if we only wanted to withdraw money if the balance was high enough? With the current API there would be a risk that money is withdrawn after we’ve checked the balance.
This is usually why external locking is used. This is when the lock is exposed to the caller and may be used externally to the public API. External locking can take may forms which may even include mixing internal and external locking. In its most simplistic form it is an external lock used along side each instance, e.g.:
class BankAccount {
public:
void Deposit(int amount) {
balance_ += amount;
}
void Withdraw(int amount) {
balance_ -= amount;
}
void Balance() const {
return balance_;
}
private:
int balance_;
};
pw::sync::Mutex bobs_account_mutex;
BankAccount bobs_account PW_GUARDED_BY(bobs_account_mutex);
The lock is acquired before the bank account is used for a transaction. In addition, we do not have to modify every public function and its trivial to call other public member functions from a public member function. However, as you can imagine instantiating and passing around the instances and their locks can become error prone.
This is why Borrowable
exists.
Why use Borrowable?#
Borrowable offers code-size efficient way to enable external locking that is easy and safe to use. It is effectively a container which holds references to a protected instance and its lock which provides RAII-style access.
pw::sync::Mutex bobs_account_mutex;
BankAccount bobs_account PW_GUARDED_BY(bobs_account_mutex);
pw::sync::Borrowable<BankAccount, pw::sync::Mutex> bobs_acount(
bobs_account, bobs_account_mutex);
This construct is useful when sharing objects or data which are transactional in nature where making individual operations threadsafe is insufficient. See the section on internal vs external locking tradeoffs above.
It can also offer a code-size and stack-usage efficient way to separate timeout
constraints between the acquiring of the shared object and timeouts used for the
shared object’s API. For example, imagine you have an I2c bus which is used by
several threads and you’d like to specify an ACK timeout of 50ms. It’d be ideal
if the duration it takes to gain exclusive access to the I2c bus does not eat
into the ACK timeout you’d like to use for the transaction. Borrowable can help
you do exactly this if you provide access to the I2c bus through a
Borrowable
.
Note
Borrowable
has semantics similar to a pointer and should be passed by
value. Furthermore, a Borrowable<U>
can be assigned to a
Borrowable<T>
if U
is a subclass of T
.
Example in C++#
#include <chrono>
#include "pw_bytes/span.h"
#include "pw_i2c/initiator.h"
#include "pw_status/try.h"
#include "pw_status/result.h"
#include "pw_sync/borrow.h"
#include "pw_sync/mutex.h"
class ExampleI2c : public pw::i2c::Initiator;
pw::sync::VirtualMutex i2c_mutex;
ExampleI2c i2c;
pw::sync::Borrowable<ExampleI2c> borrowable_i2c(i2c, i2c_mutex);
pw::Result<ConstByteSpan> ReadI2cData(ByteSpan buffer) {
// Block indefinitely waiting to borrow the i2c bus.
pw::sync::BorrowedPointer<ExampleI2c> borrowed_i2c =
borrowable_i2c.acquire();
// Execute a sequence of transactions to get the needed data.
PW_TRY(borrowed_i2c->WriteFor(kFirstWrite, std::chrono::milliseconds(50)));
PW_TRY(borrowed_i2c->WriteReadFor(kSecondWrite, buffer,
std::chrono::milliseconds(10)));
// Borrowed i2c pointer is returned when the scope exits.
return buffer;
}
InlineBorrowable#
InlineBorrowable is a helper to
simplify the common use case where an object is wrapped in a
Borrowable for its entire lifetime. The
InlineBorrowable
owns the guarded object and the lock object.
InlineBorrowable
has a separate parameter for the concrete lock type that is
instantiated and a (possibly virtual) lock interface type that is referenced by
users of the guarded object. The default lock is VirtualMutex and the default lock interface is
VirtualBasicLockable.
An InlineBorrowable
is a Borrowable
with the same guarded object and
lock interface types, and it can be passed directly to APIs that expect a
Borrowable reference.
Why use InlineBorrowable?#
It is a safer and simpler way to guard an object for its entire lifetime. The unguarded object is never exposed and doesn’t need to be stored in a separate variable or data member. The guarded object and its lock are guaranteed to have the same lifetime, and the lock cannot be re-used for any other purpose.
Constructing objects in-place#
The guarded object and its lock are constructed in-place by the InlineBorrowable, and any constructor parameters required by the object or its lock must be passed through the InlineBorrowable constructor. There are several ways to do this:
Pass the parameters for the guarded object inline to the constructor. This is the recommended way to construct the object when the lock does not require any constructor parameters. Use the
std::in_place
marker to invoke the inline constructor.InlineBorrowable<Foo> foo(std::in_place, foo_arg1, foo_arg2); InlineBorrowable<std::array<int, 2>> foo_array(std::in_place, 1, 2);
Pass the parameters inside tuples:
InlineBorrowable<Foo> foo(std::forward_as_tuple(foo_arg1, foo_arg2)); InlineBorrowable<Foo, MyLock> foo_lock( std::forward_as_tuple(foo_arg1, foo_arg2), std::forward_as_tuple(lock_arg1, lock_arg2));
Note
This approach only supports list initialization starting with C++20.
Use callables to construct the guarded object and lock object:
InlineBorrowable<Foo> foo([&]{ return Foo{foo_arg1, foo_arg2}; }); InlineBorrowable<Foo, MyLock> foo_lock( [&]{ return Foo{foo_arg1, foo_arg2}; } [&]{ return MyLock{lock_arg1, lock_arg2}; }
Note
It is possible to construct and return objects that are not copyable or movable, thanks to mandatory copy ellision (return value optimization).
Example in C++#
#include <utility>
#include "pw_bytes/span.h"
#include "pw_i2c/initiator.h"
#include "pw_status/result.h"
#include "pw_sync/inline_borrowable.h"
struct I2cOptions;
class ExampleI2c : public pw::i2c::Initiator {
public:
ExampleI2c(int bus_id, I2cOptions options);
// ...
};
int kBusId;
I2cOptions opts;
pw::sync::InlineBorrowable<ExampleI2c> i2c(std::in_place, kBusId, opts);
pw::Result<ConstByteSpan> ReadI2cData(
pw::sync::Borrowable<pw::i2c::Initiator> initiator,
ByteSpan buffer);
pw::Result<ConstByteSpan> ReadData(ByteSpan buffer) {
return ReadI2cData(i2c, buffer);
}
Signaling Primitives#
Native signaling primitives tend to vary more compared to critial section locks across different platforms. For example, although common signaling primtives like semaphores are in most if not all RTOSes and even POSIX, it was not in the STL before C++20. Likewise many C++ developers are surprised that conditional variables tend to not be natively supported on RTOSes. Although you can usually build any signaling primitive based on other native signaling primitives, this may come with non-trivial added overhead in ROM, RAM, and execution efficiency.
For this reason, Pigweed intends to provide some simpler signaling primitives which exist to solve a narrow programming need but can be implemented as efficiently as possible for the platform that it is used on.
This simpler but highly portable class of signaling primitives is intended to ensure that a portability efficiency tradeoff does not have to be made up front. Today this is class of simpler signaling primitives is limited to the ThreadNotification and TimedThreadNotification.
ThreadNotification#
The ThreadNotification is a synchronization primitive that can be used to permit a SINGLE thread to block and consume a latching, saturating notification from multiple notifiers.
Note
Although only a single thread can block on a ThreadNotification at a time, many instances may be used by a single thread just like binary semaphores. This is in contrast to some native RTOS APIs, such as direct task notifications, which re-use the same state within a thread’s context.
Warning
This is a single consumer/waiter, multiple producer/notifier API! The acquire APIs must only be invoked by a single consuming thread. As a result, having multiple threads receiving notifications via the acquire API is unsupported.
This is effectively a subset of the BinarySemaphore API, except that only a single thread can be notified and block at a time.
The single consumer aspect of the API permits the use of a smaller and/or faster native APIs such as direct thread signaling. This should be backed by the most efficient native primitive for a target, regardless of whether that is a semaphore, event flag group, condition variable, or something else.
The ThreadNotification is initialized to being empty (latch is not set).
Generic BinarySemaphore-based Backend#
This module provides a generic backend for ThreadNotification via
pw_sync:binary_semaphore_thread_notification
which uses a
BinarySemaphore as the backing
primitive. See BinarySemaphore for
backend availability.
Optimized Backend#
Supported on |
Optimized backend module |
---|---|
FreeRTOS |
|
ThreadX |
Not possible, use |
embOS |
Not needed, use |
STL |
Not planned, use |
Baremetal |
Planned |
Zephyr |
Planned |
CMSIS-RTOS API v2 & RTX5 |
Planned |
C++#
Safe to use in context |
Thread |
Interrupt |
NMI |
---|---|---|---|
Constructor |
✔ |
||
Destructor |
✔ |
||
✔ |
|||
✔ |
|||
✔ |
✔ |
Examples in C++#
#include "pw_sync/thread_notification.h"
#include "pw_thread/thread_core.h"
class FooHandler() {
public:
// Public API invoked by other threads and/or interrupts.
void NewFooAvailable() {
new_foo_notification_.release();
}
// Thread function.
void Run() {
while (true) {
new_foo_notification_.acquire();
HandleFoo();
}
}
private:
void HandleFoo();
pw::sync::ThreadNotification new_foo_notification_;
};
TimedThreadNotification#
The TimedThreadNotification is an extension of the ThreadNotification which offers timeout and deadline based semantics.
The TimedThreadNotification is initialized to being empty (latch is not set).
Warning
This is a single consumer/waiter, multiple producer/notifier API! The acquire APIs must only be invoked by a single consuming thread. As a result, having multiple threads receiving notifications via the acquire API is unsupported.
Generic BinarySemaphore-based Backend#
This module provides a generic backend for TimedThreadNotification via
pw_sync:binary_semaphore_timed_thread_notification
which uses a
BinarySemaphore as the backing
primitive. See BinarySemaphore for
backend availability.
Optimized Backend#
Supported on |
Backend module |
---|---|
FreeRTOS |
|
ThreadX |
Not possible, use |
embOS |
Not needed, use |
STL |
Not planned, use |
Zephyr |
Planned |
CMSIS-RTOS API v2 & RTX5 |
Planned |
C++#
Safe to use in context |
Thread |
Interrupt |
NMI |
---|---|---|---|
Constructor |
✔ |
||
Destructor |
✔ |
||
|
✔ |
||
|
✔ |
||
✔ |
|||
✔ |
|||
|
✔ |
✔ |
Examples in C++#
#include "pw_sync/timed_thread_notification.h"
#include "pw_thread/thread_core.h"
class FooHandler() {
public:
// Public API invoked by other threads and/or interrupts.
void NewFooAvailable() {
new_foo_notification_.release();
}
// Thread function.
void Run() {
while (true) {
if (new_foo_notification_.try_acquire_for(kNotificationTimeout)) {
HandleFoo();
}
DoOtherStuff();
}
}
private:
void HandleFoo();
void DoOtherStuff();
pw::sync::TimedThreadNotification new_foo_notification_;
};
CountingSemaphore#
The CountingSemaphore is a synchronization primitive that can be used for counting events and/or resource management where receiver(s) can block on acquire until notifier(s) signal by invoking release.
Note that unlike Mutex, priority inheritance is not used by semaphores meaning semaphores are subject to unbounded priority inversions. Due to this, Pigweed does not recommend semaphores for mutual exclusion.
The CountingSemaphore is initialized to being empty or having no tokens.
The entire API is thread safe, but only a subset is interrupt safe.
Note
If there is only a single consuming thread, use a ThreadNotification instead which can be much more efficient on some RTOSes such as FreeRTOS.
Warning
Releasing multiple tokens is often not natively supported, meaning you may end up invoking the native kernel API many times, i.e. once per token you are releasing!
Supported on |
Backend module |
---|---|
FreeRTOS |
|
ThreadX |
|
embOS |
|
STL |
|
Zephyr |
Planned |
CMSIS-RTOS API v2 & RTX5 |
Planned |
C++#
Safe to use in context |
Thread |
Interrupt |
NMI |
---|---|---|---|
Constructor |
✔ |
||
Destructor |
✔ |
||
✔ |
|||
✔ |
✔ |
||
✔ |
|||
✔ |
|||
✔ |
✔ |
||
✔ |
✔ |
✔ |
Examples in C++#
As an example, a counting sempahore can be useful to run periodic tasks at frequencies near or higher than the system clock tick rate in a way which lets you detect whether you ever fall behind.
#include "pw_sync/counting_semaphore.h"
#include "pw_thread/thread_core.h"
class PeriodicWorker() : public pw::thread::ThreadCore {
// Public API invoked by a higher frequency timer interrupt.
void TimeToExecute() {
periodic_run_semaphore_.release();
}
private:
pw::sync::CountingSemaphore periodic_run_semaphore_;
// Thread function.
void Run() override {
while (true) {
size_t behind_by_n_cycles = 0;
periodic_run_semaphore_.acquire(); // Wait to run until it's time.
while (periodic_run_semaphore_.try_acquire()) {
++behind_by_n_cycles;
}
if (behind_by_n_cycles > 0) {
PW_LOG_WARNING("Not keeping up, behind by %d cycles",
behind_by_n_cycles);
}
DoPeriodicWork();
}
}
void DoPeriodicWork();
}
BinarySemaphore#
BinarySemaphore is a specialization of
CountingSemaphore with an
arbitrary token limit of 1. Note that that max()
is >= 1, meaning it may be
released up to max()
times but only acquired once for those N releases.
Implementations of BinarySemaphore
are typically more
efficient than the default implementation of CountingSemaphore
.
BinarySemaphore
is initialized to being empty or having no
tokens.
The entire API is thread safe, but only a subset is interrupt safe.
Note
If there is only a single consuming thread, use a ThreadNotification instead which can be much more efficient on some RTOSes such as FreeRTOS.
Supported on |
Backend module |
---|---|
FreeRTOS |
|
ThreadX |
|
embOS |
|
STL |
|
Zephyr |
Planned |
CMSIS-RTOS API v2 & RTX5 |
Planned |
C++#
Safe to use in context |
Thread |
Interrupt |
NMI |
---|---|---|---|
Constructor |
✔ |
||
Destructor |
✔ |
||
✔ |
|||
✔ |
✔ |
||
✔ |
|||
✔ |
|||
✔ |
✔ |
||
✔ |
✔ |
✔ |
Examples in C++#
#include "pw_sync/binary_semaphore.h"
#include "pw_thread/thread_core.h"
class FooHandler() {
public:
// Public API invoked by other threads and/or interrupts.
void NewFooAvailable() {
new_foo_semaphore_.release();
}
// Thread function.
void Run() {
while (true) {
if (new_foo_semaphore_.try_acquire_for(kNotificationTimeout)) {
HandleFoo();
}
DoOtherStuff();
}
}
private:
void HandleFoo();
void DoOtherStuff();
pw::sync::BinarySemaphore new_foo_semaphore_;
};
Condition Variables#
ConditionVariable provides a condition variable implementation that provides semantics and an API very similar to std::condition_variable in the C++ Standard Library.
Warning
Condition variables are not a good abstraction for embedded due to spurious
wakeups. As a result, the only pw_sync
backend provided by Pigweed that
supports condition variables is pw_sync_stl. Consider using
a ThreadNotification
instead, as these do not cause spurious wakeups and
can be used in an interrupt context.
Limitations#
As a blocking operation, condition variables should not be waited on in an interrupt context. Less intuitively, condition variables should not be notified in an interrupt context. Notifying a condition variable involves checking the corresponding condition to decide whether to resume waiting threads. This check can happen either on the signaling thread or the waiting thread:
If the signaling thread checks the condition, it needs to exclusively access the waiters and their associated conditions. Access to this list must be synchronized with calls to wait on the variable. Additional state checked by the conditions may also need to be synchronized. As a result, checking the conditions on the signaling thread may involve blocking and is not suitable for a interrupt context.
If the waiting threads check their conditions, access to the list of waiters still needs to be synchronized. Additionally, a thread may find that its condition is not satisfied, and that it needs to resume waiting. Waking threads only to resume waiting is costly in terms of both power and performance.
The second approach leads to spurious wakeups in a thread context as well. The first approach may also have spurious wakeups if the condition changes between signaling the waiter and the waiter reacquiring its lock.
API reference#
Moved: pw_sync