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

[Paul E. McKenney]

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.  ;-)

							Thanx, Paul
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