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SubjectRe: Buggy variable-length array code...or compiler?
On Thu, 25 Feb 2010, Steven J. Magnani 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.
>

This could probably become the first kernel user of the flexible array
library (see Documentation/flexible-arrays.txt). Dan?


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