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

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