From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753155AbZBHAoZ (ORCPT ); Sat, 7 Feb 2009 19:44:25 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751500AbZBHAoR (ORCPT ); Sat, 7 Feb 2009 19:44:17 -0500 Received: from e3.ny.us.ibm.com ([32.97.182.143]:32967 "EHLO e3.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750892AbZBHAoQ (ORCPT ); Sat, 7 Feb 2009 19:44:16 -0500 Date: Sat, 7 Feb 2009 16:44:16 -0800 From: "Paul E. McKenney" To: Mathieu Desnoyers Cc: ltt-dev@lists.casi.polymtl.ca, linux-kernel@vger.kernel.org, Robert Wisniewski Subject: Re: [RFC git tree] Userspace RCU (urcu) for Linux (repost) Message-ID: <20090208004416.GH7120@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090206030543.GB8560@Krystal> <20090206045841.GA12995@Krystal> <20090206130640.GB10918@linux.vnet.ibm.com> <20090206163432.GF10918@linux.vnet.ibm.com> <20090207151028.GA11150@linux.vnet.ibm.com> <20090207233827.GA3557@Krystal> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090207233827.GA3557@Krystal> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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);