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

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

On Sat, Feb 07, 2009 at 06:38:27PM -0500, Mathieu Desnoyers wrote:
> * 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)

A patch below to allow nested rcu_read_lock() while keeping to the Linux
kernel API, just FYI.  One can argue that the overhead of accessing the
extra per-thread variables is offset by the fact that there no longer
needs to be a return value from rcu_read_lock() nor an argument to
rcu_read_unlock(), but hard to say.

> > 	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.

Patch in earlier email.  ;-)

> > 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 ?

There is no array, just a global counter that is incremented by a modest
power of two for each grace period.  Then the outermost rcu_read_lock()
records the one greater than current value of the global counter in its
per-thread variable.

Now, rcu_read_lock() can tell that it is outermost by examining the
low-order bits of its per-thread variable -- if these bits are zero,
then this is the outermost rcu_read_lock().  So if rcu_read_lock() sees
that it is nested, it simply increments its per-thread counter.

Then rcu_read_unlock() simply decrements its per-thread variable.

If the counter is only 32 bits, it is subject to overflow.  In that case,
it is necessary to check for the counter having been incremented a huge
number of times between the time the outermost rcu_read_lock() fetched
the counter value and the time that it stored into its per-thread
variable.

An admittedly crude implementation of this approach may be found in
CodeSamples/defer/rcu_nest.[hc] in:

	git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git

Of course, if the counter is 64 bits, overflow can safely be ignored.
If you have a grace period every microsecond and allow RCU read-side
critical sections to be nested 255 deep, it would take more than 2,000
years to overflow.  ;-)

> 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.

But would the debugger's code injection take more than a minute without
doing a context switch?  Ah -- you are thinking of a probe that spins
for several minutes.  Yes, this would be strange, but not impossible.

OK, so for this usage, solution 1 it is!

> > 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.

No argument here!!!  ;-)  See the earlier patch for one way to fix.

The following patch makes rcu_read_lock() back into a void function
while still permitting nesting, for whatever it is worth.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---

 test_urcu.c |    6 +++---
 urcu.c      |    2 ++
 urcu.h      |   40 ++++++++++++++++++++++++----------------
 3 files changed, 29 insertions(+), 19 deletions(-)

diff --git a/test_urcu.c b/test_urcu.c
index db0b68c..16b212b 100644
--- a/test_urcu.c
+++ b/test_urcu.c
@@ -33,7 +33,7 @@ static struct test_array *test_rcu_pointer;
 
 void *thr_reader(void *arg)
 {
-	int qparity, i, j;
+	int i, j;
 	struct test_array *local_ptr;
 
 	printf("thread %s, thread id : %lu, pid %lu\n",
@@ -44,14 +44,14 @@ void *thr_reader(void *arg)
 
 	for (i = 0; i < 100000; i++) {
 		for (j = 0; j < 100000000; j++) {
-			qparity = rcu_read_lock();
+			rcu_read_lock();
 			local_ptr = rcu_dereference(test_rcu_pointer);
 			if (local_ptr) {
 				assert(local_ptr->a == 8);
 				assert(local_ptr->b == 12);
 				assert(local_ptr->c[55] == 2);
 			}
-			rcu_read_unlock(qparity);
+			rcu_read_unlock();
 		}
 	}
 
diff --git a/urcu.c b/urcu.c
index 1a276ce..95eea4e 100644
--- a/urcu.c
+++ b/urcu.c
@@ -23,6 +23,8 @@ pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
 int urcu_qparity;
 
 int __thread urcu_active_readers[2];
+int __thread urcu_reader_nesting;
+int __thread urcu_reader_parity;
 
 /* Thread IDs of registered readers */
 #define INIT_NUM_THREADS 4
diff --git a/urcu.h b/urcu.h
index 9431da5..6d28ea2 100644
--- a/urcu.h
+++ b/urcu.h
@@ -70,6 +70,8 @@ static inline void atomic_inc(int *v)
 extern int urcu_qparity;
 
 extern int __thread urcu_active_readers[2];
+extern int __thread urcu_reader_nesting;
+extern int __thread urcu_reader_parity;
 
 static inline int get_urcu_qparity(void)
 {
@@ -79,26 +81,32 @@ static inline int get_urcu_qparity(void)
 /*
  * returns urcu_parity.
  */
-static inline int rcu_read_lock(void)
+static inline void rcu_read_lock(void)
 {
-	int urcu_parity = get_urcu_qparity();
-	urcu_active_readers[urcu_parity]++;
-	/*
-	 * Increment active readers count before accessing the pointer.
-	 * See force_mb_all_threads().
-	 */
-	barrier();
-	return urcu_parity;
+	int urcu_parity;
+
+	if (urcu_reader_nesting++ == 0) {
+		urcu_parity = get_urcu_qparity();
+		urcu_active_readers[urcu_parity]++;
+		urcu_reader_parity = urcu_parity;
+		/*
+		 * Increment active readers count before accessing the pointer.
+		 * See force_mb_all_threads().
+		 */
+		barrier();
+	}
 }
 
-static inline void rcu_read_unlock(int urcu_parity)
+static inline void rcu_read_unlock(void)
 {
-	barrier();
-	/*
-	 * Finish using rcu before decrementing the pointer.
-	 * See force_mb_all_threads().
-	 */
-	urcu_active_readers[urcu_parity]--;
+	if (--urcu_reader_nesting == 0) {
+		barrier();
+		/*
+		 * Finish using rcu before decrementing the pointer.
+		 * See force_mb_all_threads().
+		 */
+		urcu_active_readers[urcu_reader_parity]--;
+	}
 }
 
 extern void rcu_write_lock(void);