sd_mutex implementation question

Question

I am using a mutex to protect a serial debug printout. 
 Is it safe to use sd_mutex_acquire in a blocking manner, provided it is only called from main and softdevice callbacks  (interrupt pri level 3), or is there a scenario in which this could cause a problem? 
 What happens if a softdevice callback functions needs a mutex-protected resource that main currently has locked; will the callback task be suspended and main (eventually) scheduled?

RK · Accepted Answer

The answer to the second paragraph is no. There's no operating system or preemptive scheduling here, if main (thread mode) is running, acquires the mutex, and is holding it whilst it's doing something, and anything of higher priority interrupts it, that handler mode code will run to completion before control returns to main. If the handler mode code spins waiting to get the mutex, it will spin forever because main will never get to run and release it again. 
 There's not much you can do there, you can't yield or sleep and wait for the mutex, you have to exit the interrupt routine and schedule some way to go return to it later after the code holding it has released it. That's not easy either, if you just re-raise the interrupt, then you'll just instantly be called again and the other code still won't get a chance to run 
 Spinlocks don't work on cortex processors (without RTXs) 
 Thinking about it more, if you are using a mutex for the usual case, grabbing it, doing a small amount of work, and then releasing it again; if you ever find the mutex locked when you need it, it must be currently owned by something of lower priority than you, because if it wasn't, you wouldn't be running, the other piece of code would be. 
 This makes mutexes less useful than you'd immediately think. The only real use case I can think of is a process which runs periodically and can defer work to the next 'tick'. if it doesn't get the mutex, it exits and tries again the next time. When it eventually does get it, it runs all the work it has available.

sd_mutex implementation question

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