Hello,
I've run into a CMA-related issue while testing a DMA engine driver with
dmatest on a Renesas R-Car ARM platform.
When allocating contiguous memory through CMA the kernel prints the following
messages to the kernel log.
[ 99.770000] alloc_contig_range test_pages_isolated(6b843, 6b844) failed
[ 124.220000] alloc_contig_range test_pages_isolated(6b843, 6b844) failed
[ 127.550000] alloc_contig_range test_pages_isolated(6b845, 6b846) failed
[ 132.850000] alloc_contig_range test_pages_isolated(6b845, 6b846) failed
[ 151.390000] alloc_contig_range test_pages_isolated(6b843, 6b844) failed
[ 166.490000] alloc_contig_range test_pages_isolated(6b843, 6b844) failed
[ 181.450000] alloc_contig_range test_pages_isolated(6b845, 6b846) failed
I've stripped the dmatest module down as much as possible to remove any
hardware dependencies and came up with the following implementation.
-----------------------------------------------------------------------------
/*
* CMA test module
*
* Copyright (C) 2014 Laurent Pinchart
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*/
#include <linux/delay.h>
#include <linux/dma-mapping.h>
#include <linux/freezer.h>
#include <linux/kthread.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/slab.h>
#include <linux/wait.h>
static unsigned int num_threads = 4;
module_param(num_threads, uint, S_IRUGO | S_IWUSR);
static unsigned int iterations = 100000;
module_param(iterations, uint, S_IRUGO | S_IWUSR);
struct cma_test_thread {
struct list_head node;
struct task_struct *task;
bool done;
};
static DECLARE_WAIT_QUEUE_HEAD(thread_wait);
static LIST_HEAD(threads);
static int cma_test_thread(void *data)
{
struct cma_test_thread *thread = data;
unsigned int i = 0;
set_freezable();
while (!kthread_should_stop() && i < iterations) {
dma_addr_t dma;
void *mem;
mem = dma_alloc_coherent(NULL, 32, &dma, GFP_KERNEL);
usleep_range(1000, 2000);
if (mem)
dma_free_coherent(NULL, 32, mem, dma);
else
printk(KERN_INFO "allocation error @%u\n", i);
++i;
}
thread->done = true;
wake_up(&thread_wait);
return 0;
}
static bool cma_test_threads_done(void)
{
struct cma_test_thread *thread;
list_for_each_entry(thread, &threads, node) {
if (!thread->done)
return false;
}
return true;
}
static int cma_test_init(void)
{
struct cma_test_thread *thread, *_thread;
unsigned int i;
for (i = 0; i < num_threads; ++i) {
thread = kzalloc(sizeof(*thread), GFP_KERNEL);
if (!thread) {
pr_warn("No memory for thread %u\n", i);
break;
}
thread->task = kthread_create(cma_test_thread, thread,
"cmatest-%u", i);
if (IS_ERR(thread->task)) {
pr_warn("Failed to create thread %u\n", i);
kfree(thread);
break;
}
get_task_struct(thread->task);
list_add_tail(&thread->node, &threads);
wake_up_process(thread->task);
}
wait_event(thread_wait, cma_test_threads_done());
list_for_each_entry_safe(thread, _thread, &threads, node) {
kthread_stop(thread->task);
put_task_struct(thread->task);
list_del(&thread->node);
kfree(thread);
}
return 0;
}
module_init(cma_test_init);
static void cma_test_exit(void)
{
}
module_exit(cma_test_exit);
MODULE_AUTHOR("Laurent Pinchart");
MODULE_LICENSE("GPL v2");
-----------------------------------------------------------------------------
Loading the module will start 4 threads that will allocate and free DMA
coherent memory in a tight loop and eventually produce the error. It seems
like the probability of occurrence grows with the number of threads, which
could indicate a race condition.
The tests have been run on 3.18-rc1, but previous tests on 3.16 did exhibit
the same behaviour.
I'm not that familiar with the CMA internals, help would be appreciated to
debug the problem.
--
Regards,
Laurent Pinchart
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