urcu-mb/signal/membarrier: move quiescent threads to separate list

Accelerate 2-phase grace period by not having to iterate twice on
threads not nested within a RCU read-side lock.

CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Lai Jiangshan <laijs@cn.fujitsu.com>
CC: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/urcu.c b/urcu.c
index 2d5c510aa6744418c5557d642b451e0afae415cd..e6ff0f349e9e4fef17d6d366b5e7197acb841b84 100644 (file)
--- a/urcu.c
+++ b/urcu.c
@@ -215,9 +215,10 @@ static void wait_gp(void)
                      NULL, NULL, 0);
 }
 
-static void wait_for_readers(void)
+static void wait_for_readers(struct cds_list_head *input_readers,
+                       struct cds_list_head *cur_snap_readers,
+                       struct cds_list_head *qsreaders)
 {
-       CDS_LIST_HEAD(qsreaders);
        int wait_loops = 0;
        struct rcu_reader *index, *tmp;
 
@@ -234,13 +235,31 @@ static void wait_for_readers(void)
                        smp_mb_master(RCU_MB_GROUP);
                }
 
-               cds_list_for_each_entry_safe(index, tmp, &registry, node) {
-                       if (!rcu_gp_ongoing(&index->ctr))
-                               cds_list_move(&index->node, &qsreaders);
+               cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
+                       switch (rcu_reader_state(&index->ctr)) {
+                       case RCU_READER_ACTIVE_CURRENT:
+                               if (cur_snap_readers) {
+                                       cds_list_move(&index->node,
+                                               cur_snap_readers);
+                                       break;
+                               }
+                               /* Fall-through */
+                       case RCU_READER_INACTIVE:
+                               cds_list_move(&index->node, qsreaders);
+                               break;
+                       case RCU_READER_ACTIVE_OLD:
+                               /*
+                                * Old snapshot. Leaving node in
+                                * input_readers will make us busy-loop
+                                * until the snapshot becomes current or
+                                * the reader becomes inactive.
+                                */
+                               break;
+                       }
                }
 
 #ifndef HAS_INCOHERENT_CACHES
-               if (cds_list_empty(&registry)) {
+               if (cds_list_empty(input_readers)) {
                        if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
                                /* Read reader_gp before write futex */
                                smp_mb_master(RCU_MB_GROUP);
@@ -259,7 +278,7 @@ static void wait_for_readers(void)
                 * URCU_TLS(rcu_reader).ctr update to memory if we wait
                 * for too long.
                 */
-               if (cds_list_empty(&registry)) {
+               if (cds_list_empty(input_readers)) {
                        if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
                                /* Read reader_gp before write futex */
                                smp_mb_master(RCU_MB_GROUP);
@@ -281,12 +300,13 @@ static void wait_for_readers(void)
                }
 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
        }
-       /* put back the reader list in the registry */
-       cds_list_splice(&qsreaders, &registry);
 }
 
 void synchronize_rcu(void)
 {
+       CDS_LIST_HEAD(cur_snap_readers);
+       CDS_LIST_HEAD(qsreaders);
+
        mutex_lock(&rcu_gp_lock);
 
        if (cds_list_empty(&registry))
@@ -301,7 +321,7 @@ void synchronize_rcu(void)
        /*
         * Wait for readers to observe original parity or be quiescent.
         */
-       wait_for_readers();
+       wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
 
        /*
         * Must finish waiting for quiescent state for original parity before
@@ -341,7 +361,12 @@ void synchronize_rcu(void)
        /*
         * Wait for readers to observe new parity or be quiescent.
         */
-       wait_for_readers();
+       wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
+
+       /*
+        * Put quiescent reader list back into registry.
+        */
+       cds_list_splice(&qsreaders, &registry);
 
        /* Finish waiting for reader threads before letting the old ptr being
         * freed. Must be done within rcu_gp_lock because it iterates on reader

diff --git a/urcu/static/urcu.h b/urcu/static/urcu.h
index f32f300e599d95b922cbf043cc3a839e234190d5..973826a52bfa021423bbb8cce12545357d2df48b 100644 (file)
--- a/urcu/static/urcu.h
+++ b/urcu/static/urcu.h
@@ -96,6 +96,12 @@ extern "C" {
 #define SIGRCU SIGUSR1
 #endif
 
+enum rcu_state {
+       RCU_READER_ACTIVE_CURRENT,
+       RCU_READER_ACTIVE_OLD,
+       RCU_READER_INACTIVE,
+};
+
 #ifdef DEBUG_RCU
 #define rcu_assert(args...)    assert(args)
 #else
@@ -239,7 +245,7 @@ static inline void wake_up_gp(void)
        }
 }
 
-static inline int rcu_gp_ongoing(unsigned long *ctr)
+static inline enum rcu_state rcu_reader_state(unsigned long *ctr)
 {
        unsigned long v;
 
@@ -248,8 +254,11 @@ static inline int rcu_gp_ongoing(unsigned long *ctr)
         * to insure consistency.
         */
        v = CMM_LOAD_SHARED(*ctr);
-       return (v & RCU_GP_CTR_NEST_MASK) &&
-                ((v ^ rcu_gp_ctr) & RCU_GP_CTR_PHASE);
+       if (!(v & RCU_GP_CTR_NEST_MASK))
+               return RCU_READER_INACTIVE;
+       if (!((v ^ rcu_gp_ctr) & RCU_GP_CTR_PHASE))
+               return RCU_READER_ACTIVE_CURRENT;
+       return RCU_READER_ACTIVE_OLD;
 }
 
 /*