Re: lockdep rcu-preempt and synchronize_srcu() awareness

[Paul E. McKenney]

On Mon, Feb 08, 2010 at 04:57:48PM -0500, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) 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.

Looks good to me!

							Thanx, Paul

> 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
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