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SubjectRe: lockdep rcu-preempt and synchronize_srcu() awareness
On Mon, Feb 08, 2010 at 02:18:58PM -0500, Mathieu Desnoyers wrote:
> 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().

Indeed, doing this with SRCU would result in deadlock, and it is quite
legal to acquire mutexes from within SRCU read-side critical sections.
And similar deadlocks can be constructed using pthread_mutex_lock() and
user-space RCU implementations.

The basic rule is "don't wait for a grace period to complete while in
the corresponding flavor of RCU read-side critical section". Your point,
that it is possible to wait indirectly, is well taken.

> 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.

Given the in-progress lockdep enhancements to RCU, the information is at
least present. I can easily check for the direct case, but must defer
to Peter Z on the indirect case.

Thanx, Paul

> 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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