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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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