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SubjectRe: [RFC git tree] Userspace RCU (urcu) for Linux (repost)
* 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
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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