Messages in this thread | | | Date | Fri, 26 Feb 2010 01:46:23 +0100 | From | "J.A. Magallón" <> | Subject | Re: Buggy variable-length array code...or compiler? |
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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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