Fix: move wait loop increment before first conditional block

The fix "Fix: high cpu usage in synchronize_rcu with long RCU read-side
C.S." has an imperfection in urcu.c and urcu-qsbr.c: when incrementing
the wait loop counter for the last time, the first conditional branch is
not taken, but the following conditionals are, and they assume the first
conditional has been taken.

Within urcu.c (urcu-mb, urcu-membarrier and urcu-signal), and
urcu-qsbr.c, this will simply skip the first wait_gp() call, without any
noticeable ill side-effect.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index cd3beff7337da0225a3c55f36b19e9499ec052e9..71e7a39e6f73795026e054bf8f0620772ec9a70f 100644 (file)
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -130,6 +130,8 @@ static void wait_for_readers(struct cds_list_head *input_readers,
         * current rcu_gp.ctr value.
         */
        for (;;) {
+               if (wait_loops < RCU_QS_ACTIVE_ATTEMPTS)
+                       wait_loops++;
                if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
                        uatomic_set(&rcu_gp.futex, -1);
                        /*
@@ -142,8 +144,6 @@ static void wait_for_readers(struct cds_list_head *input_readers,
                        }
                        /* Write futex before read reader_gp */
                        cmm_smp_mb();
-               } else {
-                       wait_loops++;
                }
                cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
                        switch (rcu_reader_state(&index->ctr)) {

diff --git a/urcu.c b/urcu.c
index d6dec1afa288a38c3faf523947ae4f9f7b9150ef..ae3490f7cb2cdc1f7cdb1d26f77c0ee22620e9eb 100644 (file)
--- a/urcu.c
+++ b/urcu.c
@@ -242,12 +242,12 @@ static void wait_for_readers(struct cds_list_head *input_readers,
         * rcu_gp.ctr value.
         */
        for (;;) {
+               if (wait_loops < RCU_QS_ACTIVE_ATTEMPTS)
+                       wait_loops++;
                if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
                        uatomic_dec(&rcu_gp.futex);
                        /* Write futex before read reader_gp */
                        smp_mb_master(RCU_MB_GROUP);
-               } else {
-                       wait_loops++;
                }
 
                cds_list_for_each_entry_safe(index, tmp, input_readers, node) {