Improve upon the elegant approach of using signals to cause task/pthreads
suspension and scheduler execution by using directed signals.
This fixes:
- Deadlocks in non-FreeRTOS pthreads
- Multiple FreeRTOS tasks(pthreads) incorrectly running at the same time
By directing the signals using pthread_kill() the signal handler in the presently running
FreeRTOS task/pthread will be called, ensuring that the scheduler runs both in the context
of a FreeRTOS task/pthread and from the presently executing FreeRTOS task/pthread.
Details
==============
The POSIX port uses signals to preempt FreeRTOS tasks (implemented as pthreads), a very neat and elegant
approach to forcing tasks/pthreads to suspend and run the scheduler.
Signal handlers are process global.
Posix timers generate signals when the timer expires, and the signal is sent to the currently
running pthread.
In systems where there are pthreads that are NOT a result of creating FreeRTOS tasks, such as the
entry point thread that calls main(), or user created pthreads, this poses a serious issue.
While the POSIX port only allows a single FreeRTOS pthread to run at once, by causing all suspended
threads to not be scheduled due to their waiting on a pthread condition variable,
this isn't the case with non-FreeRTOS pthreads.
Thus it is possible that a non-FreeRTOS pthread is running when the timer expires and the signal
is generated. This results in the signal handler running in the non-FreeRTOS thread.
The sequence of events results in these events from signal handler context:
- vPortSystemTickHandler() being called
- The scheduler running
- Selecting another FreeRTOS task to run and switching the active task
- The newly selected task released from suspension by pthread_cond_signal()
- The presently active thread calling event_wait()
- The pthread calling pthread_cond_wait(), suspending the thread and allowing the host OS scheduler
to schedule another thread to run.
If this occurs from a non-FreeRTOS thread this results in:
- The active FreeRTOS pthread (Task A/Thread A) continuing to run (as the signal handler that calls
event_wait() ran instead in a non-FreeRTOS pthread.
- The pthread where the signal handler did run (Thread B) will call event_wait() and pthread_cond_wait(),
but on the condition variable of the previously active FreeRTOS task, oops. This causes the
non-FreeRTOS pthread to block unexpectedly relative to what the developer might have expected.
- The newly selected FreeRTOS Task (Task C/Thread C) will resume and start running.
At this point Task A/Thread A is running concurrently with Task C/Thread C. While this may not
necessarily be an issue, it does not replicate the expected behavior of a single Task running at
once.
Note that Thread B will resume if/when Task A/ThreadA is switched to. However, this could be delayed
by an arbitrary amount of time, or could never occur.
Also note that if there are multiple non-FreeRTOS pthreads that Thread D, E, F...etc could suffer the
same fate as Thread B, if the scheduler were to suspend Task C/Thread C and resume Task E/Thread E.
Implementation
==============
Timer details
-------------
A standalone pthread for the signal generation thread was chosen, rather than using
a posix timer_settime() handler function because the latter creates a temporary
pthread for each handler callback. This makes debugging much more difficult due to
gdb detecting the creation and destruction of these temporary threads.
Signal delivery
--------------
While signal handlers are per-thread, it is possible for pthreads to selectively block
signals, rather than using thread directed signals. However, the approach of blocking
signals in non-FreeRTOS pthreads adds complexity to each of these non-FreeRTOS pthreads
including ensuring that these signals are blocked at thread creation, prior to the thread
starting up. Directed signals removes the requirement for non-FreeRTOS pthreads to be aware
of and take action to protect against these signals, reducing complexity.
* Introduce portHAS_NESTED_INTERRUPTS to identify if port has nested interrupt or not.
* Update atomic.h to use portHAS_NESTED_INTERRUPTS instead of portSET_INTERRUPT_MASK_FROM_ISR definition.
---------
Co-authored-by: Gaurav-Aggarwal-AWS <33462878+aggarg@users.noreply.github.com>
Co-authored-by: ActoryOu <jay2002824@gmail.com>
Export the PRIVILEGED_FUNCTION, PRIVILEGED_DATA, and FREERTOS_SYSTEM_CALL
attributes to make it easier for end users to add their own privileged functions and
system calls.
Remove the idle_task_static_memory.c and use the new default implementations
to allows for FreeRTOS-Kernel-Static to be used with configNUMBER_OF_CORES > 1
* GCC: MSP430F449: Add missing attributes
Apparently at some point in the past, GCC (or TI's GCC) used to define
these attributes. Define them ourselves so that we can compile the demo
application.
* GCC: MSP430F449: Make interrupts return void
If a return type of a function is not specified, it defaults to int. Set
the return type of interrupts to void to avoid warnings.
* GCC: MSP430F449: Define portPOINTER_SIZE_TYPE
portPOINTER_SIZE_TYPE defaults to uint32_t if undefined. Define it to
uint16_t, which is correct for this port.
