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cocoos.net • View topic - Safe scheduling with ISRs
Page 1 of 1

Safe scheduling with ISRs

PostPosted: Wed Jan 02, 2013 12:13 pm
by alexey

Re: Safe scheduling with ISRs

PostPosted: Wed Jan 02, 2013 12:16 pm
by alexey

Re: Safe scheduling with ISRs

PostPosted: Fri Jan 11, 2013 1:05 pm
by peter
I see what you mean, but I don't think the solution is to put a disable/enable around the event wait call. The problem is that the task code must finish its processing before next event is signalled from the ISR. Otherwise it will not reach the event wait call before next event is signalled.

You should not send an event for each received character. Let the ISR do some of processing and signal the event when a complete message is received, or N characters has been received.

Re: Safe scheduling with ISRs

PostPosted: Tue Jan 15, 2013 4:38 pm
by alexey
Assume that command timing can not be controlled. Generally commands may be streamed from host and be recieved in a software buffer on a controller. Processing task uses events or counting semaphores (seem more suitable) to synchronize with a command stream.
Processing task flow:
1a. Check whether we need to wait (there is no ready command; semaphore is zero).
1b. Put a task in a waiting state if needed
1c. Return to the scheduler.
2. Process the command (after an event has fired or semaphore signalled).

Interrupt may occur anytime. The worst scenario is:
1a (need to wait; moving to 1b)
>> ISR (command received; processing task state is ready at the moment => nothing to be done)
1b (the task is switched to a waiting state)
1c (the command is stalled)

So it is worth disabling interrupts before 1a and enabling them just before 1c. Otherwise command processing might be messed up under unfortunate circumstances.
cocoOS API event_wait() and sem_wait() can't be simply wrapped with disable/enable interrupts since we leave the task without enabling interrupts...