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SubjectRe: Buggy variable-length array code...or compiler?
On Thu, 25 Feb 2010 17:17:29 -0600, "Steven J. Magnani" <steve@digidescorp.com> wrote:

> When I run a memcpy dmatest with a Microblaze 2.6.33 noMMU kernel, the
> system crashes after about 400 iterations. After much head scratching, I
> believe I've narrowed the problem to this fragment of code in
> drivers/dma/dmatest.c:
>
> static int dmatest_func(void *data)
> {
> struct dmatest_thread *thread = data;
> ...
> unsigned int total_tests = 0;
> int src_cnt;
> int dst_cnt;
>
> ...
> if (thread->type == DMA_MEMCPY)
> src_cnt = dst_cnt = 1;
> ...
>
> while (!kthread_should_stop()
> && !(iterations && total_tests >= iterations)) {
> struct dma_device *dev = chan->device;
> struct dma_async_tx_descriptor *tx = NULL;
> dma_addr_t dma_srcs[src_cnt];
> dma_addr_t dma_dsts[dst_cnt];
>
> ...
> total_tests++;
>
> /* CODE ADDED BY ME FOR DEBUG */
> printk("dmatest: Iteration %d, dma_srcs = %p\n",
> total_tests, dma_srcs);
>
> ...
> }
>
> With this code I get output like this:
>
> dmatest: Iteration 1, dma_srcs = 2c963ee8
> dmatest: Iteration 2, dma_srcs = 2c963ed8
> dmatest: Iteration 3, dma_srcs = 2c963ec8
> dmatest: Iteration 4, dma_srcs = 2c963eb8
> ...
> dmatest: Iteration 420, dma_srcs = 2c9624b8
>
> ...and then the stack detonates and the kernel crashes with some strange
> error or other.
>
> Are there any language lawyers in the house who'd care to weigh in on
> which of these possibilities is the right one?
>
> 1. There is a coding error in dmatest
> 2. There is a bug specific to Microblaze gcc compiler(s) [mine is 4.1.2]
> 3. There is a bug generic to specific versions of gcc compilers
> 4. There is a bug generic to all gcc compilers
>
> Obviously, the options get more disturbing the higher you go. I don't
> know if VLAs are used elsewhere in the kernel; a 'smatch' search might
> be helpful.

Can you try this in userspace ? I compiled in CentOS gcc 4.1.2 (just the
same), and the addresses are always the same:

#include <stdio.h>

int main()
{
int c = 2;
while (1)
{
int a[c];
int b[c];
a[0] = b[0];
printf("%p %p\n",a,b);
}
}

Could you post the full contents of the while loop ? Perhaps there's a
buglet in other piece of code that leaves something on the stack.
Which is the size of dma_addr_t ? Does it match the difference of 16 bytes
on each iteration ? cnt's are always 1, isn't it ?
Can you switch the size to a fixed '1' to see if this hangs again ?

TIA

--
J.A. Magallon <jamagallon()ono!com> \ Software is like sex:
\ It's better when it's free


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