On 05/08/2018 10:33 PM, Kevin Darbyshire-Bryant wrote:
>
>
>> On 8 May 2018, at 21:11, Rosen Penev <ros...@gmail.com> wrote:
>>
>>>
>>> So out of curiosity I built this for my Archer C7 v2 ar71xx device. I also
>>> modified the code to not give up on ‘OK’, so it always iterated 10 times.
>>> I then ran this repeatedly using ‘watch’. Observations:
>>>
>>> 1) Failure only occurred on 1st check, it never appeared/re-appeared on
>>> subsequent passes.
>>> 2) Failure offsets are always at 32 byte intervals. That corresponds
>>> nicely with cache-line size.
>> Yeah the L1
>>>
>>> Grabbing at straws to some extent.
>
> So I modified the user space code a little more, namely moving the usleep to
> before the check (obviously still after the mmap) - I am yet to see an error.
>
> printf("mmap addr: %p\n", addr);
> data = addr;
>
> for (i = 0; i < 10; i++) {
> usleep(500000);
> check_data(data, page_size);
> }
>
> So that smells more of a race condition between the writer filling with 0xFF
> and the reader catching up.
The open() syscall does the memset(0xff) and blocks. This ensures the memory is
initialized before the open() returns. I don't think there is a race.
>
> Again, assume that I am an idiot and am missing something fundamental.
>
>
>
--
Regards
Daniel Danzberger
embeDD GmbH, Alter Postplatz 2, CH-6370 Stans
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