Re: [PATCH] rcu: Improve detection of illegal synchronize_rcu() call from RCU read side

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Jan 05, 2012 at 02:45:20AM +0100, Frederic Weisbecker wrote:
> On Wed, Jan 04, 2012 at 01:30:35PM -0800, Paul E. McKenney wrote:
> > On Wed, Jan 04, 2012 at 08:03:39PM +0100, Frederic Weisbecker wrote:
> > > Actually for the case of RCU, the wait_for_completion() called by synchronize_rcu()
> > > has a might_sleep() call that triggers a warning in this case.
> > > 
> > > But in the case of SMP with 1 online CPU, the rcu_blocking_is_gp()
> > > checks returns right away on rcutree. So probably we need this?
> > 
> > I modified this to push the might_sleep() down into the
> > rcu_blocking_is_gp() function, queued the result, and retained your
> > Signed-off-by.  (Please let me know if there is any problem with this.)
> > 
> > This does work for TREE_PREEMPT_RCU and for synchronize_rcu_bh() in
> > TREE_RCU, but not for synchronize_sched() in TREE_RCU.  This is because
> > rcu_read_lock() and rcu_read_unlock() are no-ops in the TREE_RCU case.
> 
> Not sure about that. This calls preempt_disable() which, in any case with
> CONFIG_DEBUG_ATOMIC_SLEEP, handles the preempt count. And that even if
> !CONFIG_PREEMPT.

Ah, of course!  I keep forgetting that CONFIG_DEBUG_ATOMIC_SLEEP selects
CONFIG_PREEMPT_COUNT.

> > So I queued up a separate patch using rcu_lockdep_assert() to check for
> > illegal RCU grace period within the same-type RCU read-side critical
> > section, including for SRCU.  This is also a partial solution, as it
> > does not handle things like this:
> > 
> > 	void foo(void)
> > 	{
> > 		mutex_lock(&my_mutex);
> > 		. . .
> > 		synchronize_srcu(&my_srcu);
> > 		. . .
> > 		mutex_unlock(&my_mutex);
> > 	}
> > 
> > 	void bar(void)
> > 	{
> > 		int idx;
> > 
> > 		idx = rcu_read_lock(&m_srcu);
> > 		. . .
> > 		mutex_lock(&my_mutex);
> > 		. . .
> > 		mutex_unlock(&my_mutex);
> > 		. . .
> > 		srcu_read_unlock(&m_srcu, idx);
> > 	}
> > 
> > This can be extended into a chain of locks and a chain of SRCU instances.
> > For an example of the latter, consider an SRCU-A read-side critical
> > section containing an SRCU-B grace period, an SRCU-B read-side critical
> > section containing an SRCU-C grace period, and so on, with the SRCU-Z
> > read-side critical section containing an RCU-A grace period.
> 
> Heh! Indeed...
> 
> > But it
> > is OK to hold a mutex across one SRCU read-side critical section while
> > acquiring that same mutex within another same-flavor SRCU read-side
> > critical section.  So the analogy with reader-writer locking only goes
> > so far.
> > 
> > At the moment, a full solution seems to require some surgery on lockdep
> > itself, but perhaps there is a better way.
> 
> Ok.
> 
> > 
> > > rcutiny seems to be fine with the cond_resched() call, but srcu needs
> > > a special treatment.
> > 
> > For the moment, I just applied rcu_lockdep_assert() everywhere -- zero
> > cost on non-lockdep kernels, and fully handles all of the RCU simple
> > self-deadlock cases.
> 
> So, for RCU I'm not sure this is useful given the might_sleep() things.
> But for srcu it is.

One nice thing about the lockdep approach is that it tracks where the
conflicting RCU read-side critical section started.  But I am planning
for these to be 3.4 material, so we do have some time to refine them.

							Thanx, Paul