[NSWI004] dprintk makes my code stable

[Petr Tuma]

Hi,

check the C modifier "volatile" and think about the possible difference in optimizations the compiler can do when you (1) have a loop that calls a complex function in a different compilation unit vs (2) have a loop that is quite simple and the compiler can see all variable manipulation.

https://barrgroup.com/embedded-systems/how-to/c-volatile-keyword

Petr


On 24/10/2020 07:46, Jura Pelc wrote:
> So have here awkward behaving of code.
> 
> 
> I have a block of code where originally was `dprintk` for debugging and everything was fine.
> But when I tried to remove it (for the sanity of debug output) kernel crashed. (Both with `.configure.py --debug`)
> I can't see why this is happing and most importantly WHY IS THIS HAPPENING?
> 
> both `print` and `dprintk` should not apply any change.
> 
> 
> This is a copy of the issue: https://gitlab.mff.cuni.cz/teaching/nswi004/2020/team-please_dont/-/issues/15 <https://gitlab.mff.cuni.cz/teaching/nswi004/2020/team-please_dont/-/issues/15>
> related to branch https://gitlab.mff.cuni.cz/teaching/nswi004/2020/team-please_dont/-/tree/bugfix/K-15 <https://gitlab.mff.cuni.cz/teaching/nswi004/2020/team-please_dont/-/tree/bugfix/K-15>
> 
> file: `heap.c/search_heap_size`
> When running with `dprintk`, code will run smoothly and the result is expected.
> ```c
> *((int*)scanner) = 0xC0FFEE;
> dprintk("Scanning: 0x%x (%x)\n", scanner, *((int*)scanner));
> if(*((int*)scanner) != 0xC0FFEE)
> ```
> 
> ``` shell
> [DEBUG src/mm/heap.c:24 search_heap_size()] Scanning: 0x800fb130 (c0ffee)
> [DEBUG src/mm/heap.c:24 search_heap_size()] Scanning: 0x800fc130 (c0ffee)
> [DEBUG src/mm/heap.c:24 search_heap_size()] Scanning: 0x800fd130 (c0ffee)
> [DEBUG src/mm/heap.c:24 search_heap_size()] Scanning: 0x800fe130 (c0ffee)
> [DEBUG src/mm/heap.c:24 search_heap_size()] Scanning: 0x800ff130 (c0ffee)
> [DEBUG src/mm/heap.c:24 search_heap_size()] Scanning: 0x80100130 (c0ffee)
> [DEBUG src/mm/heap.c:27 search_heap_size()] NonMatch: 0x80100130 (ffffffff)
> [DEBUG src/mm/heap.c:149 heap_init()] heap_begin: 0x80001130
> [DEBUG src/mm/heap.c:150 heap_init()] heap_end:   0x800ff130
> [DEBUG src/mm/heap.c:151 heap_init()] heap size: 1016 bytes
> Hello world!
> <msim> Alert: XHLT: Machine halt
> 
> Cycles: 229951
> ```
> 
> 
> 
> When running **Without** `dprintk`, code will fail during the process.
> ```c
> *((int*)scanner) = 0xC0FFEE;
> //dprintk("Scanning: 0x%x (%x)\n", scanner, *((int*)scanner));
> if(*((int*)scanner) != 0xC0FFEE)
> ```
> 
> ``` shell
> <msim> Alert: XRD: Register dump
> processor 0
>     0                0   at                0   v0 ffffffffc0000f20   v1           c0ffee   a0                0
>    a1                0   a2                0   a3                0   t0                0   t1                0
>    t2                0   t3                0   t4                0   t5                0   t6                0
>    t7                0   s0                0   s1                0   s2                0   s3                0
>    s4                0   s5                0   s6                0   s7                0   t8                0
>    t9                0   k0             ff01   k1                0   gp ffffffff80000000   sp ffffffff800003e8
>    fp                0   ra ffffffff80000440   pc ffffffff80000180   lo                0   hi                0
> <msim> Alert: XINT: Interactive mode
> [msim]
> ```
> 
> 
> -- 
> 
> S pozdravem Jiří Pelc
> Matematicko-fyzikální fakulta
> Univerzita Karlova
> 
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