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Subjectlockdep rcu-preempt and synchronize_srcu() awareness
Hi,

I just though about the following deadlock scenario involving rcu preempt and
mutexes. I see that lockdep does not warn about it, and it actually triggers a
deadlock on my box. It might be worth addressing for TREE_PREEMPT_RCU configs.

CPU A:
mutex_lock(&test_mutex);
synchronize_rcu();
mutex_unlock(&test_mutex);

CPU B:
rcu_read_lock();
mutex_lock(&test_mutex);
mutex_unlock(&test_mutex);
rcu_read_unlock();

But given that it's not legit to take a mutex from within a rcu read lock in
non-preemptible configs, I guess it's not much of a real-life problem, but I
think SRCU is also affected, because there is no lockdep annotation around
synchronize_srcu().

So I think it would be good to mark rcu_read_lock/unlock as not permitting
"might_sleep()" in non preemptable RCU configs, and having a look at lockdep
SRCU support might be worthwhile.

The following test module triggers the problem:


/* test-rcu-lockdep.c
*
* Test RCU-awareness of lockdep. Don't look at the interface, it's aweful.
* run, in parallel:
*
* cat /proc/testa
* cat /proc/testb
*/

#include <linux/module.h>
#include <linux/mutex.h>
#include <linux/proc_fs.h>
#include <linux/sched.h>
#include <linux/delay.h>

struct proc_dir_entry *pentrya = NULL;
struct proc_dir_entry *pentryb = NULL;

static DEFINE_MUTEX(test_mutex);

static int my_opena(struct inode *inode, struct file *file)
{
mutex_lock(&test_mutex);
synchronize_rcu();
mutex_unlock(&test_mutex);

return -EPERM;
}


static struct file_operations my_operationsa = {
.open = my_opena,
};

static int my_openb(struct inode *inode, struct file *file)
{
rcu_read_lock();
mutex_lock(&test_mutex);
ssleep(1);
mutex_unlock(&test_mutex);
rcu_read_unlock();


return -EPERM;
}


static struct file_operations my_operationsb = {
.open = my_openb,
};

int init_module(void)
{
pentrya = create_proc_entry("testa", 0444, NULL);
if (pentrya)
pentrya->proc_fops = &my_operationsa;

pentryb = create_proc_entry("testb", 0444, NULL);
if (pentryb)
pentryb->proc_fops = &my_operationsb;

return 0;
}

void cleanup_module(void)
{
remove_proc_entry("testa", NULL);
remove_proc_entry("testb", NULL);
}

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Mathieu Desnoyers");
MODULE_DESCRIPTION("lockdep rcu test");



Thanks,

Mathieu


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