[ltt-dev] [PATCH 0/3] Fixes for threads that are readers and writers, and lock-free queue example

[Paolo Bonzini]

On 03/01/2010 08:16 PM, Mathieu Desnoyers wrote:
> It is illegal to call rcu_unregister_thread from within a RCU read-side
> critical section. So I don't see the point in adding support for
> explicit thread offlining. Maybe it's a documentation issue that lead
> you to assume that unregistering a thread from a read-side C.S. is
> valid ?

It was not within, but still it deadlocked.  You can try with patch 3/3 
alone, and I debugged it to two issues.

The first one is synchronize_rcu not offlining the current thread.  Was 
the "the same thread reads and then (outside a read-side CS) writes" 
scenario ever tested outside urcu-qsbr?

The second is that synchronize_rcu will wait for the current critical 
sections to finish (using the futex), but this has two problems in turn:

- if no reader thread has a critical section anymore, no one will call 
wake_up_gp and the writer will keep waiting on the futex forever.  This 
is fixed by _rcu_thread_offline calling wake_up_gp

- if a single thread has already exited, that one thread will never 
manage to toggle the parity twice!  This is fixed by offlining the 
thread before rcu_unregister_thread.  It is still broken even after my 
patches for urcu-bp, I suspect we need to move rcu_gc_registry somewhere 
within update_counter_and_wait but I wasn't brave enough.

> By the way, it's also illegal to call synchronize_rcu() from a RCU
> read-side C.S.

I know that. ;-)  My code unlocks before calling synchronize_rcu.

> But I don't see exactly where your lock-free list needs
> to do any of these operations that would result in deadlocks ?

No, it doesn't do anything invalid. :-)

Paolo