urcu-qsbr: improve 2-phase wait scheme

In the single-bit, 2-phase grace period scheme, all we need to do is to
observe each reader going through a quiescent state while we are in the
grace period.

We therefore only need to perform one global counter update, surrounded
by 2 iterations on readers to observe change in their snapshot.

We can therefore remove the first counter update (prior to the first
iteration on readers): it was useless and was only slowing down the
grace period.

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

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index c6a1b18a10a4f84a4d4e76d7b5b6bcdef4bcce6e..eb167d128be77125ce450b1bac808870fb994975 100644 (file)
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -116,38 +116,16 @@ static void wait_gp(void)
                      NULL, NULL, 0);
 }
 
-static void update_counter_and_wait(void)
+static void wait_for_readers(void)
 {
        CDS_LIST_HEAD(qsreaders);
        int wait_loops = 0;
        struct rcu_reader *index, *tmp;
 
-#if (CAA_BITS_PER_LONG < 64)
-       /* Switch parity: 0 -> 1, 1 -> 0 */
-       CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
-#else  /* !(CAA_BITS_PER_LONG < 64) */
-       /* Increment current G.P. */
-       CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
-#endif /* !(CAA_BITS_PER_LONG < 64) */
-
-       /*
-        * 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 URCU_TLS(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 rcu_reader_qs_gp count to become 0.
+        * Wait for each thread URCU_TLS(rcu_reader).ctr to either
+        * indicate quiescence (offline), or for them to observe the
+        * current rcu_gp_ctr value.
         */
        for (;;) {
                wait_loops++;
@@ -223,17 +201,17 @@ void synchronize_rcu(void)
                goto out;
 
        /*
-        * 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();
 
        /*
-        * Must finish waiting for quiescent state for parity 0 before
-        * committing next rcu_gp_ctr update to memory. Failure to
-        * do so could result in the writer waiting forever while new
+        * 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 URCU_TLS(rcu_reader).ctr before store to
-        * rcu_gp_ctr.
+        * compiler-order of load URCU_TLS(rcu_reader).ctr before store
+        * to rcu_gp_ctr.
         */
        cmm_barrier();
 
@@ -244,10 +222,29 @@ 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);
+
        /*
-        * 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 URCU_TLS(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();
 out:
        mutex_unlock(&rcu_gp_lock);
 
@@ -280,7 +277,30 @@ void synchronize_rcu(void)
        mutex_lock(&rcu_gp_lock);
        if (cds_list_empty(&registry))
                goto out;
-       update_counter_and_wait();
+
+       /* Increment current G.P. */
+       CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
+
+       /*
+        * 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 URCU_TLS(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 readers to observe new count of be quiescent.
+        */
+       wait_for_readers();
 out:
        mutex_unlock(&rcu_gp_lock);