Re: [RFC git tree] Userspace RCU (urcu) for Linux (repost)

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Fri, Feb 06, 2009 at 08:34:32AM -0800, Paul E. McKenney wrote:
> > On Fri, Feb 06, 2009 at 05:06:40AM -0800, Paul E. McKenney wrote:
> > > On Thu, Feb 05, 2009 at 11:58:41PM -0500, Mathieu Desnoyers wrote:
> > > > (sorry for repost, I got the ltt-dev email wrong in the previous one)
> > > > 
> > > > Hi Paul,
> > > > 
> > > > I figured out I needed some userspace RCU for the userspace tracing part
> > > > of LTTng (for quick read access to the control variables) to trace
> > > > userspace pthread applications. So I've done a quick-and-dirty userspace
> > > > RCU implementation.
> > > > 
> > > > It works so far, but I have not gone through any formal verification
> > > > phase. It seems to work on paper, and the tests are also OK (so far),
> > > > but I offer no guarantee for this 300-lines-ish 1-day hack. :-) If you
> > > > want to comment on it, it would be welcome. It's a userland-only
> > > > library. It's also currently x86-only, but only a few basic definitions
> > > > must be adapted in urcu.h to port it.
> > > > 
> > > > Here is the link to my git tree :
> > > > 
> > > > git://lttng.org/userspace-rcu.git
> > > > 
> > > > http://lttng.org/cgi-bin/gitweb.cgi?p=userspace-rcu.git;a=summary
> > > 
> > > Very cool!!!  I will take a look!
> > > 
> > > I will also point you at a few that I have put together:
> > > 
> > > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git
> > > 
> > > (In the CodeSamples/defer directory.)
> > 
> > Interesting approach, using the signal to force memory-barrier execution!
> > 
> > o	One possible optimization would be to avoid sending a signal to
> > 	a blocked thread, as the context switch leading to blocking
> > 	will have implied a memory barrier -- otherwise it would not
> > 	be safe to resume the thread on some other CPU.  That said,
> > 	not sure whether checking to see whether a thread is blocked is
> > 	any faster than sending it a signal and forcing it to wake up.
> > 
> > 	Of course, this approach does require that the enclosing
> > 	application be willing to give up a signal.  I suspect that most
> > 	applications would be OK with this, though some might not.
> > 
> > 	Of course, I cannot resist pointing to an old LKML thread:
> > 
> > 		http://lkml.org/lkml/2001/10/8/189
> > 
> > 	But I think that the time is now right.  ;-)
> > 
> > o	I don't understand the purpose of rcu_write_lock() and
> > 	rcu_write_unlock().  I am concerned that it will lead people
> > 	to decide that a single global lock must protect RCU updates,
> > 	which is of course absolutely not the case.  I strongly
> > 	suggest making these internal to the urcu.c file.  Yes,
> > 	uses of urcu_publish_content() would then hit two locks (the
> > 	internal-to-urcu.c one and whatever they are using to protect
> > 	their data structure), but let's face it, if you are sending a
> > 	signal to each and every thread, the additional overhead of the
> > 	extra lock is the least of your worries.
> > 
> > 	If you really want to heavily optimize this, I would suggest
> > 	setting up a state machine that permits multiple concurrent
> > 	calls to urcu_publish_content() to share the same set of signal
> > 	invocations.  That way, if the caller has partitioned the
> > 	data structure, global locking might be avoided completely
> > 	(or at least greatly restricted in scope).
> > 
> > 	Of course, if updates are rare, the optimization would not
> > 	help, but in that case, acquiring two locks would be even less
> > 	of a problem.
> > 
> > o	Is urcu_qparity relying on initialization to zero?  Or on the
> > 	fact that, for all x, 1-x!=x mod 2^32?  Ah, given that this is
> > 	used to index urcu_active_readers[], you must be relying on
> > 	initialization to zero.
> > 
> > o	In rcu_read_lock(), why is a non-atomic increment of the
> > 	urcu_active_readers[urcu_parity] element safe?  Are you
> > 	relying on the compiler generating an x86 add-to-memory
> > 	instruction?
> > 
> > 	Ditto for rcu_read_unlock().
> > 
> > 	Ah, never mind!!!  I now see the __thread specification,
> > 	and the keeping of references to it in the reader_data list.
> > 
> > o	Combining the equivalent of rcu_assign_pointer() and
> > 	synchronize_rcu() into urcu_publish_content() is an interesting
> > 	approach.  Not yet sure whether or not it is a good idea.  I
> > 	guess trying it out on several applications would be the way
> > 	to find out.  ;-)
> > 
> > 	That said, I suspect that it would be very convenient in a
> > 	number of situations.
> > 
> > o	It would be good to avoid having to pass the return value
> > 	of rcu_read_lock() into rcu_read_unlock().  It should be
> > 	possible to avoid this via counter value tricks, though this
> > 	would add a bit more code in rcu_read_lock() on 32-bit machines.
> > 	(64-bit machines don't have to worry about counter overflow.)
> > 
> > 	See the recently updated version of CodeSamples/defer/rcu_nest.[ch]
> > 	in the aforementioned git archive for a way to do this.
> > 	(And perhaps I should apply this change to SRCU...)
> > 
> > o	Your test looks a bit strange, not sure why you test all the
> > 	different variables.  It would be nice to take a test duration
> > 	as an argument and run the test for that time.
> > 
> > 	I killed the test after better part of an hour on my laptop,
> > 	will retry on a larger machine (after noting the 18 threads
> > 	created!).  (And yes, I first tried Power, which objected
> > 	strenously to the "mfence" and "lock; incl" instructions,
> > 	so getting an x86 machine to try on.)
> > 
> > Again, looks interesting!  Looks plausible, although I have not 100%
> > convinced myself that it is perfectly bug-free.  But I do maintain
> > a healthy skepticism of purported RCU algorithms, especially ones that
> > I have written.  ;-)
> 
> OK, here is one sequence of concern...
> 

Let's see..

> o	Thread 0 starts rcu_read_lock(), picking up the current
> 	get_urcu_qparity() into the local variable urcu_parity().
> 	Assume that the value returned is zero.
> 
> o	Thread 0 is now preempted.
> 
> o	Thread 1 invokes urcu_publish_content():
> 
> 	o	It substitutes the pointer.
> 
> 	o	It forces all threads to execute a memory barrier
> 		(thread 0 runs just long enough to process its signal
> 		and then is immediately preempted again).
> 
> 	o	It switches the parity, which is now one.
> 
> 	o	It waits for all readers on parity zero, and there are
> 		none, because thread 0 has not yet registered itself.
> 
> 	o	It therefore returns the old pointer.  So far, so good.
> 
> o	Thread 0 now resumes:
> 
> 	o	It increments its urcu_active_readers[0].
> 
> 	o	It forces a compiler barrier.
> 
> 	o	It returns zero (why not store this in thread-local
> 		storage rather than returning?).
> 

To support nested rcu_read_locks. (that's the only reason)

> 	o	It enters its critical section, obtaining a reference
> 		to the new pointer that thread 1 just published.
> 
> o	Thread 1 now again invokes urcu_publish_content():
>  
> 	o	It substitutes the pointer.
> 
> 	o	It forces all threads to execute a memory barrier,
> 		including thread 0.
> 
> 	o	It switches the parity, which is now zero.
> 
> 	o	It waits for all readers on parity one, and there are
> 		none, because thread 0 has registered itself on parity
> 		zero!!!
> 
> 	o	Thread 1 therefore returns the old pointer.
> 
> 	o	Thread 1 frees the old pointer, which thread 0 is still
> 		using!!!
> 

Ah, yes, you are right.

> So, how to fix?  Here are some approaches:
> 
> o	Make urcu_publish_content() do two parity flips rather than one.
> 	I use this approach in my rcu_rcpg, rcu_rcpl, and rcu_rcpls
> 	algorithms in CodeSamples/defer.
> 

This approach seems very interesting.

> o	Use a single free-running counter, in a manner similar to rcu_nest,
> 	as suggested earlier.  This one is interesting, as I rely on a
> 	read-side memory barrier to handle the long-preemption case.
> 	However, if you believe that any thread that waits several minutes
> 	between executing adjacent instructions must have been preempted
> 	(which the memory barriers that are required to do a context
> 	switch), then a compiler barrier suffices.  ;-)

Hrm, I'm trying to figure out what kind of memory backend you need to
put your counters for each quiescent state period. Is this free-running
counter indexing a very large array ? I doubt it does. Then how does it
make sure we don't roll back to the old array entries ?

This latter solution could break jump-based probing of programs
soon-to-be available in gcc. The probes are meant to be of short
duration, but the fact is that this design lets the debugger inject code
without resorting to a breakpoint, which might therefore break your
"short time between instructions" assumption. It's very unlikely, but
possible.


> 
> Of course, the probability of seeing this failure during test is quite
> low, since it is unlikely that thread 0 would run just long enough to
> execute its signal handler.  However, it could happen.  And if you were
> to adapt this algorithm for use in a real-time application, then priority
> boosting could cause this to happen naturally.
> 

Yes. It's not because we are not able to create the faulty condition
that it will _never_ happen. It must therefore be taken care of.

Mathieu

> 							Thanx, Paul
> 

-- 
Mathieu Desnoyers
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