wfcqueue: document first/next return values

[urcu.git] / urcu.c

diff --git a/urcu.c b/urcu.c
index 2d5c510aa6744418c5557d642b451e0afae415cd..15def09adb7e7feeeaba49f2d1d2a1c33a90bf59 100644 (file)
--- a/urcu.c
+++ b/urcu.c
@@ -43,6 +43,7 @@
 #include "urcu/tls-compat.h"
 
 #include "urcu-die.h"
+#include "urcu-wait.h"
 
 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
 #undef _LGPL_SOURCE
@@ -106,6 +107,12 @@ DEFINE_URCU_TLS(unsigned int, rcu_rand_yield);
 
 static CDS_LIST_HEAD(registry);
 
+/*
+ * Queue keeping threads awaiting to wait for a grace period. Contains
+ * struct gp_waiters_thread objects.
+ */
+static DEFINE_URCU_WAIT_QUEUE(gp_waiters);
+
 static void mutex_lock(pthread_mutex_t *mutex)
 {
        int ret;
@@ -215,9 +222,10 @@ 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 +242,31 @@ 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 +285,7 @@ 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);
@@ -281,14 +307,40 @@ 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);
+       DEFINE_URCU_WAIT_NODE(wait, URCU_WAIT_WAITING);
+       struct urcu_waiters waiters;
+
+       /*
+        * Add ourself to gp_waiters queue of threads awaiting to wait
+        * for a grace period. Proceed to perform the grace period only
+        * if we are the first thread added into the queue.
+        * The implicit memory barrier before urcu_wait_add()
+        * orders prior memory accesses of threads put into the wait
+        * queue before their insertion into the wait queue.
+        */
+       if (urcu_wait_add(&gp_waiters, &wait) != 0) {
+               /* Not first in queue: will be awakened by another thread. */
+               urcu_adaptative_busy_wait(&wait);
+               /* Order following memory accesses after grace period. */
+               cmm_smp_mb();
+               return;
+       }
+       /* We won't need to wake ourself up */
+       urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
+
        mutex_lock(&rcu_gp_lock);
 
+       /*
+        * Move all waiters into our local queue.
+        */
+       urcu_move_waiters(&waiters, &gp_waiters);
+
        if (cds_list_empty(&registry))
                goto out;
 
@@ -301,7 +353,7 @@ 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 +393,12 @@ 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
@@ -349,6 +406,13 @@ void synchronize_rcu(void)
        smp_mb_master(RCU_MB_GROUP);
 out:
        mutex_unlock(&rcu_gp_lock);
+
+       /*
+        * Wakeup waiters only after we have completed the grace period
+        * and have ensured the memory barriers at the end of the grace
+        * period have been issued.
+        */
+       urcu_wake_all_waiters(&waiters);
 }
 
 /*