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Re: Cortex-M: share an int between two tasks

From David Brown <david.brown@hesbynett.no>
Newsgroups comp.arch.embedded
Subject Re: Cortex-M: share an int between two tasks
Date 2020-03-18 09:17 +0100
Organization A noiseless patient Spider
Message-ID <r4slfn$i3$1@dont-email.me> (permalink)
References <r4g5sc$8jr$1@dont-email.me> <r4gdi2$tn$1@dont-email.me> <r4skvb$tkq$1@dont-email.me>

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On 18/03/2020 09:09, pozz wrote:
> Il 13/03/2020 17:49, David Brown ha scritto:
>> On 13/03/2020 15:38, pozz wrote:
>>> I'm working on a Cortex-M4 MCU and using FreeRTOS.
>>>
>>> One task:
>>>
>>> uint32_t wait_period;
>>> while(1) {
>>>    // make some things
>>>    vTaskDelay(pdMS_TO_TICKS(wait_period));
>>> }
>>>
>>> The following function can be called from another task:
>>>
>>> void set_waiting_period(uint32_t new_period) {
>>>    wait_period = new_period;
>>> }
>>>
>>>
>>> In this case, is it needed to protect the access of the shared variable
>>> wait_period with a mutex? I don't think, we are dealing with an integer
>>> variable that should be accessed (reading and writing) atomically.
>>>
>>
>> You don't need to use a mutex, as you won't have race conditions when
>> accessing it (race conditions happen when at least one thread can be
>> writing, and at least one thread reading, and the accesses are not
>> atomic). 
> 
> Ok, I got the point.
> 
> 
>> However, you /do/ need to make the accesses volatile, or there
>> will be no guarantee that the variable will be read or written in the
>> spots where you have accessed it.
> 
> Let me explain just to be sure I got it.
> 
> If wait_period is not volatile, in the task endless loop, the compiler 
> could make the assumption the variable doesn't ever change, so for 
> example it could "cache" its value in a register. In this case, when 
> set_waiting_period() is called from another task, the variable in RAM is 
> changed, but the value in the register doesn't change, so the task will 
> continue using the old value.

Exactly right.

(The same can work when writing variables in a loop - the compiler knows 
that it has full control of any normal variables it is using, so there 
is no need to write out the variable to ram until later, if that would 
give more efficient code.)

> 
> I think the access could be volatile only in the loop and not in 
> set_waiting_period:
> 
> uint32_t wait_period;
> while(1) {
>      // make some things
>      volatile uint32_t *vwait_period = (volatile uint32_t *)&wait_period;
>      vTaskDelay(pdMS_TO_TICKS(*vwait_period));
>   }
> 
> void set_waiting_period(uint32_t new_period) {
>     wait_period = new_period;
> }

Bad idea - get in the habit of making such writes volatile too.  If the 
"set_waiting_period" function is compiled and linked separately, then 
the compiler will have to generate the write instruction before the 
return.  But if it has access to the definition (because it is in the 
same file as the one using it, or you have link-time optimisation), and 
you write something like:

	set_waiting_period(100);
	// more code
	set_waiting_period(1000);

then the compiler can skip the first write to "wait_period" if it knows 
"more code" does not use it.

Most problems with missing volatiles are seen from a failure to do 
volatile reads in a loop.  The second most common failure (IME) is 
assuming that volatile accesses place an order on non-volatile accesses 
(they don't).  But failure to use volatile on writes occurs too, and is 
often more subtle, with code working fine until you change something 
else that appears independent.

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Thread

Cortex-M: share an int between two tasks pozz <pozzugno@gmail.com> - 2020-03-13 15:38 +0100
  Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-13 17:49 +0100
    Re: Cortex-M: share an int between two tasks pozz <pozzugno@gmail.com> - 2020-03-18 09:09 +0100
      Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-18 09:17 +0100
      Re: Cortex-M: share an int between two tasks Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2020-03-18 17:37 +0100
        Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-18 19:23 +0100
    Re: Cortex-M: share an int between two tasks Les Cargill <lcargill99@comcast.com> - 2020-03-22 13:42 -0500
      Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-22 21:45 +0100
      Re: Cortex-M: share an int between two tasks Richard Damon <Richard@Damon-Family.org> - 2020-03-22 20:50 -0400
        Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-23 09:09 +0100
          Re: Cortex-M: share an int between two tasks Richard Damon <Richard@Damon-Family.org> - 2020-03-23 08:00 -0400
            Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-23 14:21 +0100
              Re: Cortex-M: share an int between two tasks Clifford Heath <no.spam@please.net> - 2020-03-25 09:55 +1100
                Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-30 16:07 +0200
                Re: Cortex-M: share an int between two tasks Clifford Heath <no.spam@please.net> - 2020-03-31 09:02 +1100
                Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-31 10:57 +0200
                Re: Cortex-M: share an int between two tasks Clifford Heath <no.spam@please.net> - 2020-04-01 12:05 +1100
                Re: Cortex-M: share an int between two tasks Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2020-03-30 17:49 +0300
                Re: Cortex-M: share an int between two tasks kegs@provalid.com (Kent Dickey) - 2020-05-19 00:34 -0500
                Re: Cortex-M: share an int between two tasks Clifford Heath <no.spam@please.net> - 2020-05-19 17:40 +1000
                Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-05-19 12:00 +0200
                Re: Cortex-M: share an int between two tasks Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-05-19 13:48 +0100
  Re: Cortex-M: share an int between two tasks pozz <pozzugno@gmail.com> - 2020-03-18 09:20 +0100
    Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-18 10:06 +0100
      Re: Cortex-M: share an int between two tasks pozz <pozzugno@gmail.com> - 2020-03-18 10:59 +0100
        Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-18 11:48 +0100
          Re: Cortex-M: share an int between two tasks upsidedown@downunder.com - 2020-03-18 20:37 +0200
            Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-18 21:24 +0100
              Re: Cortex-M: share an int between two tasks Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2020-03-19 14:05 +0200
                Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-19 13:16 +0100
                Re: Cortex-M: share an int between two tasks Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2020-03-19 14:27 +0200
                Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-19 14:13 +0100
            Re: Cortex-M: share an int between two tasks pozz <pozzugno@gmail.com> - 2020-03-19 08:03 +0100
              Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-19 09:10 +0100
              Re: Cortex-M: share an int between two tasks upsidedown@downunder.com - 2020-03-19 12:51 +0200
          Re: Cortex-M: share an int between two tasks pozz <pozzugno@gmail.com> - 2020-03-19 07:40 +0100
            Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-19 09:38 +0100
              Re: Cortex-M: share an int between two tasks pozz <pozzugno@gmail.com> - 2020-03-19 11:26 +0100
                Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-19 12:12 +0100
                Re: Cortex-M: share an int between two tasks pozz <pozzugno@gmail.com> - 2020-03-19 15:31 +0100
                Re: Cortex-M: share an int between two tasks pozz <pozzugno@gmail.com> - 2020-03-23 09:22 +0100
                Re: Cortex-M: share an int between two tasks Richard Damon <Richard@Damon-Family.org> - 2020-03-23 08:10 -0400
                Re: Cortex-M: share an int between two tasks pozz <pozzugno@gmail.com> - 2020-03-23 13:47 +0100
                Re: Cortex-M: share an int between two tasks David Brown <david.brown@hesbynett.no> - 2020-03-23 14:31 +0100
                Re: Cortex-M: share an int between two tasks Richard Damon <Richard@Damon-Family.org> - 2020-03-23 13:50 -0400
            Re: Cortex-M: share an int between two tasks Niklas Holsti <niklas.holsti@tidorum.invalid> - 2020-03-19 12:27 +0200
              Re: Cortex-M: share an int between two tasks pozz <pozzugno@gmail.com> - 2020-03-19 11:45 +0100
                Re: Cortex-M: share an int between two tasks upsidedown@downunder.com - 2020-03-19 13:27 +0200
    Re: Cortex-M: share an int between two tasks antispam@math.uni.wroc.pl - 2020-03-18 16:24 +0000

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