Re: linux rt priority thread corrupt global variable?

[Jörn Engel]

On Thu, 8 May 2003 02:03:35 -0700, Ming Lei wrote:
> 
> Platform:
> Intel Pentium II; RedHat 7.2 with kernel version 2.4.7-10, libc 2.2.4-13 and
> gcc 2.96.

You should either upgrade to 2.4.20 or similar or post the question to
RedHat for their kernels. If the problem can be reproduced with
2.4.20, come back here. :)

> Problem description:
> 
> a program has 3 threads of priority 12, 10, 6 respectively, and the main
> process at priority 0. All the threads except main process is created with
> pthread_create, and defined SCHED_FIFO as real time scheduler policy.
> 
> There is a global variable I define with 'int cpl'. All the threads and main
> process may alter cpl at any time. cpl may have one of these values {0,
> 0xf000006e, 0xf0000068, 0xe0000000, 0xe0000060}. cpl is protected by mutex
> for any access.
> 
> <Problem=> at some point of execution which cpl should be a value say
> e0000060, but the actual value retained at cpl is another say e0000000; that
> is, the value is changed without the program actually done anything on it.
> The retained value I observed is kind of historic value(one of these value
> in the above set), not the arbituary value. The problem had occured just
> after context switch, also occured during a thread execution.
> 
> <Confirm> I used Intel debug register to track any writing to the cpl memory
> address globally, which is the way GDB use for x86 hardware watchpoint
> implementation. I could see all the writing from my program to change cpl,
> but failed to see the source from which the problem occured. So I dont know
> what cause the problem.
> 
> Can anyone listening give me a direction or hint on this annoying situation?

Sounds a bit like a caching problem. Old value in cache, new value
written to memory, chache line dirty => flushed, old value written to
memory again. But it could also be something else.

Jörn

-- 
Simplicity is prerequisite for reliability.
-- Edsger W. Dijkstra