X-Git-Url: https://git.liburcu.org/?a=blobdiff_plain;f=urcu-qsbr.c;h=b043b8592e7f04f1606bd3b734b2130e09c260e9;hb=d40fde2c4f6170c3ace770354d99acc9e9732ec5;hp=155505abc7843385fafef29de57509ceb096519c;hpb=2dfb8b5e30bc2d268bcbc57eceb9c54d21b36d90;p=urcu.git

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index 155505a..b043b85 100644
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -113,13 +113,28 @@ static void update_counter_and_wait(void)
 	struct rcu_reader *index, *tmp;
 
 #if (BITS_PER_LONG < 64)
-	/* Switch parity: 1 -> 0, 0 -> 1 */
+	/* Switch parity: 0 -> 1, 1 -> 0 */
 	STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
 #else	/* !(BITS_PER_LONG < 64) */
 	/* Increment current G.P. */
 	STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
 #endif	/* !(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 rcu_reader ctr.
+	 */
+	barrier();
+
+	/*
+	 * Adding a 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.
+	 */
+	smp_mb();
+
 	/*
 	 * Wait for each thread rcu_reader_qs_gp count to become 0.
 	 */
@@ -197,11 +212,12 @@ void synchronize_rcu(void)
 
 	/*
 	 * Must finish waiting for quiescent state for parity 0 before
-	 * committing qparity update to memory. Failure to do so could result in
-	 * the writer waiting forever while new readers are always accessing
-	 * data (no progress).
-	 * Ensured by STORE_SHARED and LOAD_SHARED.
+	 * 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
+	 * rcu_reader ctr before store to rcu_gp_ctr.
 	 */
+	barrier();
 
 	/*
 	 * Adding a smp_mb() which is _not_ formally required, but makes the