X-Git-Url: https://git.liburcu.org/?a=blobdiff_plain;f=urcu.c;h=e6ff0f349e9e4fef17d6d366b5e7197acb841b84;hb=6362f68f024fd85fefe342b1f82d8787146c1ebb;hp=c42184656a3a0381b518aa15372b84a365400dea;hpb=1de4df4b770e5e90440008becc5e14a15c75c6e0;p=urcu.git

diff --git a/urcu.c b/urcu.c
index c421846..e6ff0f3 100644
--- a/urcu.c
+++ b/urcu.c
@@ -215,33 +215,17 @@ static void wait_gp(void)
 		      NULL, NULL, 0);
 }
 
-static void update_counter_and_wait(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;
 
-	/* Switch parity: 0 -> 1, 1 -> 0 */
-	CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE);
-
-	/*
-	 * Must commit rcu_gp_ctr update to memory before waiting for quiescent
-	 * state. Failure to do so could result in the writer waiting forever
-	 * while new readers are always accessing data (no progress). Enforce
-	 * compiler-order of store to rcu_gp_ctr before load rcu_reader ctr.
-	 */
-	cmm_barrier();
-
 	/*
-	 *
-	 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
-	 * model easier to understand. It does not have a big performance impact
-	 * anyway, given this is the write-side.
-	 */
-	cmm_smp_mb();
-
-	/*
-	 * Wait for each thread URCU_TLS(rcu_reader).ctr count to become 0.
+	 * Wait for each thread URCU_TLS(rcu_reader).ctr to either
+	 * indicate quiescence (not nested), or observe the current
+	 * rcu_gp_ctr value.
 	 */
 	for (;;) {
 		wait_loops++;
@@ -251,13 +235,31 @@ static void update_counter_and_wait(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);
@@ -276,7 +278,7 @@ static void update_counter_and_wait(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);
@@ -298,12 +300,13 @@ static void update_counter_and_wait(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))
@@ -316,12 +319,12 @@ void synchronize_rcu(void)
 	smp_mb_master(RCU_MB_GROUP);
 
 	/*
-	 * Wait for previous parity to be empty of readers.
+	 * Wait for readers to observe original parity or be quiescent.
 	 */
-	update_counter_and_wait();	/* 0 -> 1, wait readers in parity 0 */
+	wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
 
 	/*
-	 * Must finish waiting for quiescent state for parity 0 before
+	 * Must finish waiting for quiescent state for original parity before
 	 * committing next rcu_gp_ctr update to memory. Failure to do so could
 	 * result in the writer waiting forever while new readers are always
 	 * accessing data (no progress).  Enforce compiler-order of load
@@ -336,10 +339,34 @@ void synchronize_rcu(void)
 	 */
 	cmm_smp_mb();
 
+	/* Switch parity: 0 -> 1, 1 -> 0 */
+	CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE);
+
 	/*
-	 * Wait for previous parity to be empty of readers.
+	 * Must commit rcu_gp_ctr update to memory before waiting for quiescent
+	 * state. Failure to do so could result in the writer waiting forever
+	 * while new readers are always accessing data (no progress). Enforce
+	 * compiler-order of store to rcu_gp_ctr before load rcu_reader ctr.
 	 */
-	update_counter_and_wait();	/* 1 -> 0, wait readers in parity 1 */
+	cmm_barrier();
+
+	/*
+	 *
+	 * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
+	 * model easier to understand. It does not have a big performance impact
+	 * anyway, given this is the write-side.
+	 */
+	cmm_smp_mb();
+
+	/*
+	 * Wait for readers to observe new parity or be quiescent.
+	 */
+	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