Re: [RFC PATCH 5/5 single-thread-version] implement per-domain single-thread state machine call_srcu()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Mar 10, 2012 at 11:16:48AM +0800, Lai Jiangshan wrote:
> On Fri, Mar 9, 2012 at 4:35 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Wed, Mar 07, 2012 at 11:54:02AM +0800, Lai Jiangshan wrote:
> >> This patch is on the top of the 4 previous patches(1/6, 2/6, 3/6, 4/6).
> >>
> >> o     state machine is light way and single-threaded, it is preemptible when checking.
> >>
> >> o     state machine is a work_struct. So, there is no thread occupied
> >>       by SRCU when the srcu is not actived(no callback). And it does
> >>       not sleep(avoid to occupy a thread when sleep).
> >>
> >> o     state machine is the only thread can flip/check/write(*) the srcu_struct,
> >>       so we don't need any mutex.
> >>       (write(*): except ->per_cpu_ref, ->running, ->batch_queue)
> >>
> >> o     synchronize_srcu() is always call call_srcu().
> >>       synchronize_srcu_expedited() is also.
> >>       It is OK for mb()-based srcu are extremely fast.
> >>
> >> o     In current kernel, we can expect that there are only 1 callback per gp.
> >>       so callback is probably called in the same CPU when it is queued.
> >>
> >> The trip of a callback:
> >>       1) ->batch_queue when call_srcu()
> >>
> >>       2) ->batch_check0 when try to do check_zero
> >>
> >>       3) ->batch_check1 after finish its first check_zero and the flip
> >>
> >>       4) ->batch_done after finish its second check_zero
> >>
> >> The current requirement of the callbacks:
> >>       The callback will be called inside process context.
> >>       The callback should be fast without any sleeping path.
> >>
> >> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> >> ---
> >>  include/linux/rcupdate.h |    2 +-
> >>  include/linux/srcu.h     |   28 +++++-
> >>  kernel/rcupdate.c        |   24 ++++-
> >>  kernel/rcutorture.c      |   44 ++++++++-
> >>  kernel/srcu.c            |  238 ++++++++++++++++++++++++++++++++-------------
> >>  5 files changed, 259 insertions(+), 77 deletions(-)
> >> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> >> index 9372174..d98eab2 100644
> >> --- a/include/linux/rcupdate.h
> >> +++ b/include/linux/rcupdate.h
> >> @@ -222,7 +222,7 @@ extern void rcu_irq_exit(void);
> >>   * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
> >>   */
> >>
> >> -typedef void call_rcu_func_t(struct rcu_head *head,
> >> +typedef void (*call_rcu_func_t)(struct rcu_head *head,
> >
> > I don't see what this applies against.  The old patch 5/6 created
> > a "(*call_rcu_func_t)(struct rcu_head *head," and I don't see what
> > created the "call_rcu_func_t(struct rcu_head *head,".
> 
> typedef void call_rcu_func_t(...) declares a function type, not a
> function pointer
> type. I use a line of code as following:
> 
> call_rcu_func_t crf = func;
> 
> if call_rcu_func_t is a function type, the above code can't be complied,
> I need to covert it to function pointer type.

Got it, thank you!

> >>                            void (*func)(struct rcu_head *head));
> >>  void wait_rcu_gp(call_rcu_func_t crf);
> >>
> >> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> >> index df8f5f7..56cb774 100644
> >> --- a/include/linux/srcu.h
> >> +++ b/include/linux/srcu.h
> >> @@ -29,6 +29,7 @@
> >>
> >>  #include <linux/mutex.h>
> >>  #include <linux/rcupdate.h>
> >> +#include <linux/workqueue.h>
> >>
> >>  struct srcu_struct_array {
> >>       unsigned long c[2];
> >> @@ -39,10 +40,23 @@ struct srcu_struct_array {
> >>  #define SRCU_REF_MASK                (ULONG_MAX >> SRCU_USAGE_BITS)
> >>  #define SRCU_USAGE_COUNT     (SRCU_REF_MASK + 1)
> >>
> >> +struct rcu_batch {
> >> +     struct rcu_head *head, **tail;
> >> +};
> >> +
> >>  struct srcu_struct {
> >>       unsigned completed;
> >>       struct srcu_struct_array __percpu *per_cpu_ref;
> >> -     struct mutex mutex;
> >> +     spinlock_t queue_lock; /* protect ->batch_queue, ->running */
> >> +     bool running;
> >> +     /* callbacks just queued */
> >> +     struct rcu_batch batch_queue;
> >> +     /* callbacks try to do the first check_zero */
> >> +     struct rcu_batch batch_check0;
> >> +     /* callbacks done with the first check_zero and the flip */
> >> +     struct rcu_batch batch_check1;
> >> +     struct rcu_batch batch_done;
> >> +     struct delayed_work work;
> >
> > Why not use your multiple-tail-pointer trick here?  (The one that is
> > used in treercu.)
> 
> 1) Make the code of the advance of batches simpler.
> 2) batch_queue is protected by lock, so it will be hard to use
> multiple-tail-pointer trick.
> 3) rcu_batch API do add a little more runtime overhead, but this
> overhead is just
> several cpu-instructions, I think it is OK. It is good tradeoff when
> compare to the readability.

OK, let's see how it goes.

> I think we can also use rcu_batch for rcutree/rcutiny.

Hmmm...  Readability and speed both improved when moving from something
resembing rcu_batch to the current multi-tailed lists.  ;-)

> >>       unsigned long snap[NR_CPUS];
> >>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> >>       struct lockdep_map dep_map;
> >> @@ -67,12 +81,24 @@ int init_srcu_struct(struct srcu_struct *sp);
> >>
> >>  #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> >>
> >> +/* draft
> >> + * queue callbacks which will be invoked after grace period.
> >> + * The callback will be called inside process context.
> >> + * The callback should be fast without any sleeping path.
> >> + */
> >> +void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
> >> +             void (*func)(struct rcu_head *head));
> >> +
> >> +typedef void (*call_srcu_func_t)(struct srcu_struct *sp, struct rcu_head *head,
> >> +             void (*func)(struct rcu_head *head));
> >> +void __wait_srcu_gp(struct srcu_struct *sp, call_srcu_func_t crf);
> >>  void cleanup_srcu_struct(struct srcu_struct *sp);
> >>  int __srcu_read_lock(struct srcu_struct *sp) __acquires(sp);
> >>  void __srcu_read_unlock(struct srcu_struct *sp, int idx) __releases(sp);
> >>  void synchronize_srcu(struct srcu_struct *sp);
> >>  void synchronize_srcu_expedited(struct srcu_struct *sp);
> >>  long srcu_batches_completed(struct srcu_struct *sp);
> >> +void srcu_barrier(struct srcu_struct *sp);
> >>
> >>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> >>
> >> diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
> >> index a86f174..f9b551f 100644
> >> --- a/kernel/rcupdate.c
> >> +++ b/kernel/rcupdate.c
> >> @@ -45,6 +45,7 @@
> >>  #include <linux/mutex.h>
> >>  #include <linux/export.h>
> >>  #include <linux/hardirq.h>
> >> +#include <linux/srcu.h>
> >>
> >>  #define CREATE_TRACE_POINTS
> >>  #include <trace/events/rcu.h>
> >> @@ -123,20 +124,39 @@ static void wakeme_after_rcu(struct rcu_head  *head)
> >>       complete(&rcu->completion);
> >>  }
> >>
> >> -void wait_rcu_gp(call_rcu_func_t crf)
> >> +static void __wait_rcu_gp(void *domain, void *func)
> >>  {
> >>       struct rcu_synchronize rcu;
> >>
> >>       init_rcu_head_on_stack(&rcu.head);
> >>       init_completion(&rcu.completion);
> >> +
> >>       /* Will wake me after RCU finished. */
> >> -     crf(&rcu.head, wakeme_after_rcu);
> >> +     if (!domain) {
> >> +             call_rcu_func_t crf = func;
> >> +             crf(&rcu.head, wakeme_after_rcu);
> >> +     } else {
> >> +             call_srcu_func_t crf = func;
> >> +             crf(domain, &rcu.head, wakeme_after_rcu);
> >> +     }
> >> +
> >>       /* Wait for it. */
> >>       wait_for_completion(&rcu.completion);
> >>       destroy_rcu_head_on_stack(&rcu.head);
> >>  }
> >
> > Mightn't it be simpler and faster to just have a separate wait_srcu_gp()
> > that doesn't share code with wait_rcu_gp()?  I am all for sharing code,
> > but this might be hrting more than helping.
> >
> >> +
> >> +void wait_rcu_gp(call_rcu_func_t crf)
> >> +{
> >> +     __wait_rcu_gp(NULL, crf);
> >> +}
> >>  EXPORT_SYMBOL_GPL(wait_rcu_gp);
> >>
> >> +/* srcu.c internel */
> >> +void __wait_srcu_gp(struct srcu_struct *sp, call_srcu_func_t crf)
> >> +{
> >> +     __wait_rcu_gp(sp, crf);
> >> +}
> >> +
> >>  #ifdef CONFIG_PROVE_RCU
> >>  /*
> >>   * wrapper function to avoid #include problems.
> >> diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
> >> index 54e5724..40d24d0 100644
> >
> > OK, so your original patch #6 is folded into this?  I don't have a strong
> > view either way, just need to know.
> >
> >> --- a/kernel/rcutorture.c
> >> +++ b/kernel/rcutorture.c
> >> @@ -623,6 +623,11 @@ static int srcu_torture_completed(void)
> >>       return srcu_batches_completed(&srcu_ctl);
> >>  }
> >>
> >> +static void srcu_torture_deferred_free(struct rcu_torture *rp)
> >> +{
> >> +     call_srcu(&srcu_ctl, &rp->rtort_rcu, rcu_torture_cb);
> >> +}
> >> +
> >>  static void srcu_torture_synchronize(void)
> >>  {
> >>       synchronize_srcu(&srcu_ctl);
> >> @@ -652,7 +657,7 @@ static struct rcu_torture_ops srcu_ops = {
> >>       .read_delay     = srcu_read_delay,
> >>       .readunlock     = srcu_torture_read_unlock,
> >>       .completed      = srcu_torture_completed,
> >> -     .deferred_free  = rcu_sync_torture_deferred_free,
> >> +     .deferred_free  = srcu_torture_deferred_free,
> >>       .sync           = srcu_torture_synchronize,
> >>       .call           = NULL,
> >>       .cb_barrier     = NULL,
> >> @@ -660,6 +665,21 @@ static struct rcu_torture_ops srcu_ops = {
> >>       .name           = "srcu"
> >>  };
> >>
> >> +static struct rcu_torture_ops srcu_sync_ops = {
> >> +     .init           = srcu_torture_init,
> >> +     .cleanup        = srcu_torture_cleanup,
> >> +     .readlock       = srcu_torture_read_lock,
> >> +     .read_delay     = srcu_read_delay,
> >> +     .readunlock     = srcu_torture_read_unlock,
> >> +     .completed      = srcu_torture_completed,
> >> +     .deferred_free  = rcu_sync_torture_deferred_free,
> >> +     .sync           = srcu_torture_synchronize,
> >> +     .call           = NULL,
> >> +     .cb_barrier     = NULL,
> >> +     .stats          = srcu_torture_stats,
> >> +     .name           = "srcu_sync"
> >> +};
> >> +
> >>  static int srcu_torture_read_lock_raw(void) __acquires(&srcu_ctl)
> >>  {
> >>       return srcu_read_lock_raw(&srcu_ctl);
> >> @@ -677,7 +697,7 @@ static struct rcu_torture_ops srcu_raw_ops = {
> >>       .read_delay     = srcu_read_delay,
> >>       .readunlock     = srcu_torture_read_unlock_raw,
> >>       .completed      = srcu_torture_completed,
> >> -     .deferred_free  = rcu_sync_torture_deferred_free,
> >> +     .deferred_free  = srcu_torture_deferred_free,
> >>       .sync           = srcu_torture_synchronize,
> >>       .call           = NULL,
> >>       .cb_barrier     = NULL,
> >> @@ -685,6 +705,21 @@ static struct rcu_torture_ops srcu_raw_ops = {
> >>       .name           = "srcu_raw"
> >>  };
> >>
> >> +static struct rcu_torture_ops srcu_raw_sync_ops = {
> >> +     .init           = srcu_torture_init,
> >> +     .cleanup        = srcu_torture_cleanup,
> >> +     .readlock       = srcu_torture_read_lock_raw,
> >> +     .read_delay     = srcu_read_delay,
> >> +     .readunlock     = srcu_torture_read_unlock_raw,
> >> +     .completed      = srcu_torture_completed,
> >> +     .deferred_free  = rcu_sync_torture_deferred_free,
> >> +     .sync           = srcu_torture_synchronize,
> >> +     .call           = NULL,
> >> +     .cb_barrier     = NULL,
> >> +     .stats          = srcu_torture_stats,
> >> +     .name           = "srcu_raw_sync"
> >> +};
> >> +
> >>  static void srcu_torture_synchronize_expedited(void)
> >>  {
> >>       synchronize_srcu_expedited(&srcu_ctl);
> >> @@ -1673,7 +1708,7 @@ static int rcu_torture_barrier_init(void)
> >>       for (i = 0; i < n_barrier_cbs; i++) {
> >>               init_waitqueue_head(&barrier_cbs_wq[i]);
> >>               barrier_cbs_tasks[i] = kthread_run(rcu_torture_barrier_cbs,
> >> -                                                (void *)i,
> >> +                                                (void *)(long)i,
> >>                                                  "rcu_torture_barrier_cbs");
> >>               if (IS_ERR(barrier_cbs_tasks[i])) {
> >>                       ret = PTR_ERR(barrier_cbs_tasks[i]);
> >> @@ -1857,7 +1892,8 @@ rcu_torture_init(void)
> >>       static struct rcu_torture_ops *torture_ops[] =
> >>               { &rcu_ops, &rcu_sync_ops, &rcu_expedited_ops,
> >>                 &rcu_bh_ops, &rcu_bh_sync_ops, &rcu_bh_expedited_ops,
> >> -               &srcu_ops, &srcu_raw_ops, &srcu_expedited_ops,
> >> +               &srcu_ops, &srcu_sync_ops, &srcu_raw_ops,
> >> +               &srcu_raw_sync_ops, &srcu_expedited_ops,
> >>                 &sched_ops, &sched_sync_ops, &sched_expedited_ops, };
> >>
> >>       mutex_lock(&fullstop_mutex);
> >> diff --git a/kernel/srcu.c b/kernel/srcu.c
> >> index d101ed5..532f890 100644
> >> --- a/kernel/srcu.c
> >> +++ b/kernel/srcu.c
> >> @@ -34,10 +34,60 @@
> >>  #include <linux/delay.h>
> >>  #include <linux/srcu.h>
> >>
> >> +static inline void rcu_batch_init(struct rcu_batch *b)
> >> +{
> >> +     b->head = NULL;
> >> +     b->tail = &b->head;
> >> +}
> >> +
> >> +static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head)
> >> +{
> >> +     *b->tail = head;
> >> +     b->tail = &head->next;
> >> +}
> >> +
> >> +static inline bool rcu_batch_empty(struct rcu_batch *b)
> >> +{
> >> +     return b->tail == &b->head;
> >> +}
> >> +
> >> +static inline struct rcu_head *rcu_batch_dequeue(struct rcu_batch *b)
> >> +{
> >> +     struct rcu_head *head;
> >> +
> >> +     if (rcu_batch_empty(b))
> >> +             return NULL;
> >> +
> >> +     head = b->head;
> >> +     b->head = head->next;
> >> +     if (b->tail == &head->next)
> >> +             rcu_batch_init(b);
> >> +
> >> +     return head;
> >> +}
> >> +
> >> +static inline void rcu_batch_move(struct rcu_batch *to, struct rcu_batch *from)
> >> +{
> >> +     if (!rcu_batch_empty(from)) {
> >> +             *to->tail = from->head;
> >> +             to->tail = from->tail;
> >> +             rcu_batch_init(from);
> >> +     }
> >> +}
> >
> > And perhaps this is why you don't want the multi-tailed queue?
> >
> >> +
> >> +/* single-thread state-machine */
> >> +static void process_srcu(struct work_struct *work);
> >> +
> >>  static int init_srcu_struct_fields(struct srcu_struct *sp)
> >>  {
> >>       sp->completed = 0;
> >> -     mutex_init(&sp->mutex);
> >> +     spin_lock_init(&sp->queue_lock);
> >> +     sp->running = false;
> >> +     rcu_batch_init(&sp->batch_queue);
> >> +     rcu_batch_init(&sp->batch_check0);
> >> +     rcu_batch_init(&sp->batch_check1);
> >> +     rcu_batch_init(&sp->batch_done);
> >> +     INIT_DELAYED_WORK(&sp->work, process_srcu);
> >>       sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
> >>       return sp->per_cpu_ref ? 0 : -ENOMEM;
> >>  }
> >> @@ -254,11 +304,9 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
> >>   * we repeatedly block for 1-millisecond time periods.  This approach
> >>   * has done well in testing, so there is no need for a config parameter.
> >>   */
> >> -#define SYNCHRONIZE_SRCU_READER_DELAY        5
> >> -#define SYNCHRONIZE_SRCU_TRYCOUNT    2
> >> -#define SYNCHRONIZE_SRCU_EXP_TRYCOUNT        12
> >> +#define SRCU_RETRY_CHECK_DELAY       5
> >>
> >> -static void wait_idx(struct srcu_struct *sp, int idx, int trycount)
> >> +static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
> >>  {
> >>       /*
> >>        * If a reader fetches the index before the ->completed increment,
> >> @@ -271,19 +319,12 @@ static void wait_idx(struct srcu_struct *sp, int idx, int trycount)
> >>        */
> >>       smp_mb(); /* D */
> >>
> >> -     /*
> >> -      * SRCU read-side critical sections are normally short, so wait
> >> -      * a small amount of time before possibly blocking.
> >> -      */
> >> -     if (!srcu_readers_active_idx_check(sp, idx)) {
> >> -             udelay(SYNCHRONIZE_SRCU_READER_DELAY);
> >> -             while (!srcu_readers_active_idx_check(sp, idx)) {
> >> -                     if (trycount > 0) {
> >> -                             trycount--;
> >> -                             udelay(SYNCHRONIZE_SRCU_READER_DELAY);
> >> -                     } else
> >> -                             schedule_timeout_interruptible(1);
> >> -             }
> >> +     for (;;) {
> >> +             if (srcu_readers_active_idx_check(sp, idx))
> >> +                     break;
> >> +             if (--trycount <= 0)
> >> +                     return false;
> >> +             udelay(SRCU_RETRY_CHECK_DELAY);
> >>       }
> >>
> >>       /*
> >> @@ -297,6 +338,8 @@ static void wait_idx(struct srcu_struct *sp, int idx, int trycount)
> >>        * the next flipping.
> >>        */
> >>       smp_mb(); /* E */
> >> +
> >> +     return true;
> >>  }
> >>
> >>  /*
> >> @@ -308,10 +351,27 @@ static void srcu_flip(struct srcu_struct *sp)
> >>       ACCESS_ONCE(sp->completed)++;
> >>  }
> >>
> >> +void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
> >> +             void (*func)(struct rcu_head *head))
> >> +{
> >> +     unsigned long flags;
> >> +
> >> +     head->next = NULL;
> >> +     head->func = func;
> >> +     spin_lock_irqsave(&sp->queue_lock, flags);
> >> +     rcu_batch_queue(&sp->batch_queue, head);
> >> +     if (!sp->running) {
> >> +             sp->running = true;
> >> +             queue_delayed_work(system_nrt_wq, &sp->work, 0);
> >> +     }
> >> +     spin_unlock_irqrestore(&sp->queue_lock, flags);
> >> +}
> >> +EXPORT_SYMBOL_GPL(call_srcu);
> >> +
> >>  /*
> >>   * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
> >>   */
> >> -static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
> >> +static void __synchronize_srcu(struct srcu_struct *sp)
> >>  {
> >>       rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
> >>                          !lock_is_held(&rcu_bh_lock_map) &&
> >> @@ -319,54 +379,7 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
> >>                          !lock_is_held(&rcu_sched_lock_map),
> >>                          "Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section");
> >>
> >> -     mutex_lock(&sp->mutex);
> >> -
> >> -     /*
> >> -      * Suppose that during the previous grace period, a reader
> >> -      * picked up the old value of the index, but did not increment
> >> -      * its counter until after the previous instance of
> >> -      * __synchronize_srcu() did the counter summation and recheck.
> >> -      * That previous grace period was OK because the reader did
> >> -      * not start until after the grace period started, so the grace
> >> -      * period was not obligated to wait for that reader.
> >> -      *
> >> -      * However, the current SRCU grace period does have to wait for
> >> -      * that reader.  This is handled by invoking wait_idx() on the
> >> -      * non-active set of counters (hence sp->completed - 1).  Once
> >> -      * wait_idx() returns, we know that all readers that picked up
> >> -      * the old value of ->completed and that already incremented their
> >> -      * counter will have completed.
> >> -      *
> >> -      * But what about readers that picked up the old value of
> >> -      * ->completed, but -still- have not managed to increment their
> >> -      * counter?  We do not need to wait for those readers, because
> >> -      * they will have started their SRCU read-side critical section
> >> -      * after the current grace period starts.
> >> -      *
> >> -      * Because it is unlikely that readers will be preempted between
> >> -      * fetching ->completed and incrementing their counter, wait_idx()
> >> -      * will normally not need to wait.
> >> -      */
> >> -     wait_idx(sp, (sp->completed - 1) & 0x1, trycount);
> >> -
> >> -     /*
> >> -      * Now that wait_idx() has waited for the really old readers,
> >> -      *
> >> -      * Flip the readers' index by incrementing ->completed, then wait
> >> -      * until there are no more readers using the counters referenced by
> >> -      * the old index value.  (Recall that the index is the bottom bit
> >> -      * of ->completed.)
> >> -      *
> >> -      * Of course, it is possible that a reader might be delayed for the
> >> -      * full duration of flip_idx_and_wait() between fetching the
> >> -      * index and incrementing its counter.  This possibility is handled
> >> -      * by the next __synchronize_srcu() invoking wait_idx() for such
> >> -      * readers before starting a new grace period.
> >> -      */
> >> -     srcu_flip(sp);
> >> -     wait_idx(sp, (sp->completed - 1) & 0x1, trycount);
> >> -
> >> -     mutex_unlock(&sp->mutex);
> >> +     __wait_srcu_gp(sp, call_srcu);
> >>  }
> >>
> >>  /**
> >> @@ -385,7 +398,7 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
> >>   */
> >>  void synchronize_srcu(struct srcu_struct *sp)
> >>  {
> >> -     __synchronize_srcu(sp, SYNCHRONIZE_SRCU_TRYCOUNT);
> >> +     __synchronize_srcu(sp);
> >>  }
> >>  EXPORT_SYMBOL_GPL(synchronize_srcu);
> >>
> >> @@ -406,10 +419,16 @@ EXPORT_SYMBOL_GPL(synchronize_srcu);
> >>   */
> >>  void synchronize_srcu_expedited(struct srcu_struct *sp)
> >>  {
> >> -     __synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT);
> >> +     __synchronize_srcu(sp);
> >>  }
> >
> > OK, I'll bite...  Why aren't synchronize_srcu_expedited() and
> > synchronize_srcu() different?
> 
> In mb()-based srcu, synchronize_srcu() is very fast,
> synchronize_srcu_expedited() makes less sense than before.

I am worried about expedited callbacks getting backed up behind
non-expedited callbacks (especially given Peter's point about per-VMA
SRCU callbacks) and behind other workqueue uses.

> But when wait_srcu_gp() is move back here, I will use
> a bigger "trycount" for synchronize_srcu_expedited().
> 
> And any problem for srcu_advance_batches()?

I prefer the use of "return" that you and Peter discussed later.

What sort of testing are you doing?

							Thanx, Paul

> Thanks.
> Lai
> 
> >
> >                                                        Thanx, Paul
> >
> >>  EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
> >>
> >> +void srcu_barrier(struct srcu_struct *sp)
> >> +{
> >> +     __synchronize_srcu(sp);
> >> +}
> >> +EXPORT_SYMBOL_GPL(srcu_barrier);
> >> +
> >>  /**
> >>   * srcu_batches_completed - return batches completed.
> >>   * @sp: srcu_struct on which to report batch completion.
> >> @@ -423,3 +442,84 @@ long srcu_batches_completed(struct srcu_struct *sp)
> >>       return sp->completed;
> >>  }
> >>  EXPORT_SYMBOL_GPL(srcu_batches_completed);
> >> +
> >> +#define SRCU_CALLBACK_BATCH  10
> >> +#define SRCU_INTERVAL                1
> >> +
> >> +static void srcu_collect_new(struct srcu_struct *sp)
> >> +{
> >> +     if (!rcu_batch_empty(&sp->batch_queue)) {
> >> +             spin_lock_irq(&sp->queue_lock);
> >> +             rcu_batch_move(&sp->batch_check0, &sp->batch_queue);
> >> +             spin_unlock_irq(&sp->queue_lock);
> >> +     }
> >> +}
> >> +
> >> +static void srcu_advance_batches(struct srcu_struct *sp)
> >> +{
> >> +     int idx = 1 - (sp->completed & 0x1UL);
> >> +
> >> +     /*
> >> +      * SRCU read-side critical sections are normally short, so check
> >> +      * twice after a flip.
> >> +      */
> >> +     if (!rcu_batch_empty(&sp->batch_check1) ||
> >> +         !rcu_batch_empty(&sp->batch_check0)) {
> >> +             if (try_check_zero(sp, idx, 1)) {
> >> +                     rcu_batch_move(&sp->batch_done, &sp->batch_check1);
> >> +                     rcu_batch_move(&sp->batch_check1, &sp->batch_check0);
> >> +                     if (!rcu_batch_empty(&sp->batch_check1)) {
> >> +                             srcu_flip(sp);
> >> +                             if (try_check_zero(sp, 1 - idx, 2)) {
> >> +                                     rcu_batch_move(&sp->batch_done,
> >> +                                             &sp->batch_check1);
> >> +                             }
> >> +                     }
> >> +             }
> >> +     }
> >> +}
> >> +
> >> +static void srcu_invoke_callbacks(struct srcu_struct *sp)
> >> +{
> >> +     int i;
> >> +     struct rcu_head *head;
> >> +
> >> +     for (i = 0; i < SRCU_CALLBACK_BATCH; i++) {
> >> +             head = rcu_batch_dequeue(&sp->batch_done);
> >> +             if (!head)
> >> +                     break;
> >> +             head->func(head);
> >> +     }
> >> +}
> >> +
> >> +static void srcu_reschedule(struct srcu_struct *sp)
> >> +{
> >> +     bool running = true;
> >> +
> >> +     if (rcu_batch_empty(&sp->batch_done) &&
> >> +         rcu_batch_empty(&sp->batch_check1) &&
> >> +         rcu_batch_empty(&sp->batch_check0) &&
> >> +         rcu_batch_empty(&sp->batch_queue)) {
> >> +             spin_lock_irq(&sp->queue_lock);
> >> +             if (rcu_batch_empty(&sp->batch_queue)) {
> >> +                     sp->running = false;
> >> +                     running = false;
> >> +             }
> >> +             spin_unlock_irq(&sp->queue_lock);
> >> +     }
> >> +
> >> +     if (running)
> >> +             queue_delayed_work(system_nrt_wq, &sp->work, SRCU_INTERVAL);
> >> +}
> >> +
> >> +static void process_srcu(struct work_struct *work)
> >> +{
> >> +     struct srcu_struct *sp;
> >> +
> >> +     sp = container_of(work, struct srcu_struct, work.work);
> >> +
> >> +     srcu_collect_new(sp);
> >> +     srcu_advance_batches(sp);
> >> +     srcu_invoke_callbacks(sp);
> >> +     srcu_reschedule(sp);
> >> +}
> >>
> >>
> >>
> >
> > --
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