Re: lockdep rcu-preempt and synchronize_srcu() awareness

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> 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.

Meanwhile, I'll add this to the Userspace RCU README:

Interaction with mutexes

        One must be careful to do not cause deadlocks due to interaction of
        synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu()
        is called with a mutex held, this mutex (or any mutex which has this
        mutex in its dependency chain) should not be acquired from within a RCU
        read-side critical section.

Thanks,

Mathieu

> 
> > 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