Re: deadlock in synchronize_srcu() in debugfs?

[Paul E. McKenney]

On Fri, Mar 24, 2017 at 07:51:47PM +0100, Johannes Berg wrote:
> 
> > Yes.  CPU2 has a pre-existing reader that CPU1's synchronize_srcu()
> > must wait for.  But CPU2's reader cannot end until CPU1 releases
> > its lock, which it cannot do until after CPU2's reader ends.  Thus,
> > as you say, deadlock.
> > 
> > The rule is that if you are within any kind of RCU read-side critical
> > section, you cannot directly or indirectly wait for a grace period
> > from that same RCU flavor.
> 
> Right. This is indirect then, in a way.

Agreed, in a way.  ;-)

> > There are some challenges, though.  This is OK:
> > 
> >	CPU1				CPU2
> >	i = srcu_read_lock(&mysrcu);	mutex_lock(&my_lock);
> >	mutex_lock(&my_lock);		i = srcu_read_lock(&mysrcu);
> >	srcu_read_unlock(&mysrcu, i);	mutex_unlock(&my_lock);
> >	mutex_unlock(&my_lock);		srcu_read_unlock(&mysrcu, i);
> > 
> > 	CPU3
> > 	synchronize_srcu(&mylock);
> > 
> > This could be a deadlock for reader-writer locking, but not for SRCU.
> 
> Hmm, yes, that's a good point. If srcu_read_lock() was read_lock, and
> synchronize_srcu() was write_lock(), then the write_lock() could stop
> CPU2's read_lock() from acquiring the lock, and thus cause a deadlock.

Yes.

> However, I'm not convinced that lockdep handles reader/writer locks
> correctly to start with, right now, since it *didn't* actually trigger
> any warnings when I annotated SRCU as a reader/writer lock.

I haven't looked into lockdep enough to know either way.

> > This is also OK:
> >	CPU1				CPU2
> >	i = srcu_read_lock(&mysrcu);	mutex_lock(&my_lock);
> >	mutex_lock(&my_lock);		synchronize_srcu(&yoursrc
> u);
> >	srcu_read_unlock(&mysrcu, i);	mutex_unlock(&my_lock);
> >	mutex_unlock(&my_lock);
> > 
> > Here CPU1's read-side critical sections are for mysrcu, which is
> > independent of CPU2's grace period for yoursrcu.
> 
> Right, but that's already covered by having separate a lockdep_map for
> each SRCU subsystem (mysrcu, yoursrcu).

I hope so, but haven't proved that this would work in all possible cases.

> > So you could flag any lockdep cycle that contained a reader and a
> > synchronous grace period for the same flavor of RCU, where for SRCU
> > the identity of the srcu_struct structure is part of the flavor.
> 
> Right. Basically, I think SRCU should be like a reader/writer lock
> (perhaps fixed to work right). The only difference seems to be the
> scenario you outlined above (first of the two)?
> 
> Actually, given the scenario above, for lockdep purposes the
> reader/writer lock is actually the same as a recursive lock, I guess?

Except that a recursive reader/writer lock can still have deadlocks
involving the outermost reader that would not be deadlocks for the
equivalent SRCU scenarios.

> You outlined a scenario in which the reader gets blocked due to a
> writer (CPU3 doing a write_lock()) so the reader can still participate
> in a deadlock cycle since it can - without any other locks being held
> by CPU3 that participate - cause a deadlock between CPU1 and CPU2 here.
> For lockdep then, even seeing the CPU1 and CPU2 scenarios should be
> sufficient to flag a deadlock (*).

Might this be one of the reasons why lockdep has problems with
reader-writer locks?

> This part then isn't true for SRCU, because there forward progress will
> still be made. So for SRCU, the "reader" side really needs to be
> connected with a "writer" side to form a deadlock cycle, unlike for a
> reader/writer lock.

Yes, for SRCU, srcu_read_lock() itself never blocks, so it never
participates directly in a deadlock cycle.  It has to be the case
that something within the SRCU read-side critical section blocks
and takes its place in the deadlock cycle.

Then again, if you didn't have something blocking within your SRCU
read-side critical section, why would you be using SRCU instead of
just plain RCU?  ;-)

> johannes
> 
> (*) technically only after checking that write_lock() is ever used, but
> ... seems reasonable enough to assume that it will be used, since why
> would anyone ever use a reader/writer lock if there are only readers?
> That's a no-op.

Makes sense to me!  The only reasons I can come up with are things like
shutting lockdep up when it wants a given lock read-held or some such.

							Thanx, Paul