Add missing memory barriers to ensure progress and remove an unnecessary ACCESS_ONCE

[urcu.git] / urcu.c

diff --git a/urcu.c b/urcu.c
index ac1a4d6c9314499d339fdd2a939479ec96deed5f..9542b264a6dbf53d36d2fb63e7578b4e7ff2b2f5 100644 (file)
--- a/urcu.c
+++ b/urcu.c
@@ -125,41 +125,70 @@ void wait_for_quiescent_state(void)
                while (rcu_old_gp_ongoing(index->urcu_active_readers))
                        barrier();
        }
-       /*
-        * Locally : read *index->urcu_active_readers before freeing old
-        * pointer.
-        * Remote (reader threads) : Order urcu_qparity update and other
-        * thread's quiescent state counter read.
-        */
-       force_mb_all_threads();
 }
 
-static void switch_qparity(void)
+void synchronize_rcu(void)
 {
-       /* All threads should read qparity before accessing data structure. */
-       /* Write ptr before changing the qparity */
+       /* All threads should read qparity before accessing data structure
+        * where new ptr points to. */
+       /* Write new ptr before changing the qparity */
        force_mb_all_threads();
        debug_yield_write();
-       switch_next_urcu_qparity();
+
+       internal_urcu_lock();
+       debug_yield_write();
+
+       switch_next_urcu_qparity();     /* 0 -> 1 */
        debug_yield_write();
 
        /*
-        * Wait for previous parity to be empty of readers.
+        * Must commit qparity update to memory before waiting for parity
+        * 0 quiescent state. Failure to do so could result in the writer
+        * waiting forever while new readers are always accessing data (no
+        * progress).
         */
-       wait_for_quiescent_state();
-}
+       mb();
 
-void synchronize_rcu(void)
-{
-       debug_yield_write();
-       internal_urcu_lock();
+       /*
+        * Wait for previous parity to be empty of readers.
+        */
+       wait_for_quiescent_state();     /* Wait readers in parity 0 */
        debug_yield_write();
-       switch_qparity();
+
+       /*
+        * 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).
+        */
+       mb();
+
+       switch_next_urcu_qparity();     /* 1 -> 0 */
        debug_yield_write();
-       switch_qparity();
+
+       /*
+        * Must commit qparity update to memory before waiting for parity
+        * 1 quiescent state. Failure to do so could result in the writer
+        * waiting forever while new readers are always accessing data (no
+        * progress).
+        */
+       mb();
+
+       /*
+        * Wait for previous parity to be empty of readers.
+        */
+       wait_for_quiescent_state();     /* Wait readers in parity 1 */
        debug_yield_write();
+
        internal_urcu_unlock();
        debug_yield_write();
+
+       /* All threads should finish using the data referred to by old ptr
+        * before decrementing their urcu_active_readers count */
+       /* Finish waiting for reader threads before letting the old ptr being
+        * freed. */
+       force_mb_all_threads();
+       debug_yield_write();
 }
 
 void urcu_add_reader(pthread_t id)