Messages in this thread | | | Date | Sat, 7 Feb 2009 07:10:28 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [RFC git tree] Userspace RCU (urcu) for Linux (repost) |
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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...
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?).
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!!!
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.
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. ;-)
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.
Thanx, Paul
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