Fix: deadlock when thread join is issued in read-side C.S.

[userspace-rcu.git] / urcu-bp.c

diff --git a/urcu-bp.c b/urcu-bp.c
index 280dd0dac4c5e00b9775e73f7b86f3a5507e5ca5..3e5be5fb59e1baf0ed7a7a7b7c26f61603654a98 100644 (file)
--- a/urcu-bp.c
+++ b/urcu-bp.c
@@ -99,7 +99,21 @@ void __attribute__((constructor)) rcu_bp_init(void);
 static
 void __attribute__((destructor)) _rcu_bp_exit(void);
 
+/*
+ * rcu_gp_lock ensures mutual exclusion between threads calling
+ * synchronize_rcu().
+ */
 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
+/*
+ * rcu_registry_lock ensures mutual exclusion between threads
+ * registering and unregistering themselves to/from the registry, and
+ * with threads reading that registry from synchronize_rcu(). However,
+ * this lock is not held all the way through the completion of awaiting
+ * for the grace period. It is sporadically released between iterations
+ * on the registry.
+ * rcu_registry_lock may nest inside rcu_gp_lock.
+ */
+static pthread_mutex_t rcu_registry_lock = PTHREAD_MUTEX_INITIALIZER;
 
 static pthread_mutex_t init_lock = PTHREAD_MUTEX_INITIALIZER;
 static int initialized;
@@ -171,6 +185,10 @@ static void mutex_unlock(pthread_mutex_t *mutex)
                urcu_die(ret);
 }
 
+/*
+ * Always called with rcu_registry lock held. Releases this lock between
+ * iterations and grabs it again. Holds the lock when it returns.
+ */
 void update_counter_and_wait(void)
 {
        CDS_LIST_HEAD(qsreaders);
@@ -209,10 +227,14 @@ void update_counter_and_wait(void)
                if (cds_list_empty(&registry)) {
                        break;
                } else {
+                       /* Temporarily unlock the registry lock. */
+                       mutex_unlock(&rcu_registry_lock);
                        if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS)
                                (void) poll(NULL, 0, RCU_SLEEP_DELAY_MS);
                        else
                                caa_cpu_relax();
+                       /* Re-lock the registry lock before the next loop. */
+                       mutex_lock(&rcu_registry_lock);
                }
        }
        /* put back the reader list in the registry */
@@ -230,6 +252,7 @@ void synchronize_rcu(void)
        assert(!ret);
 
        mutex_lock(&rcu_gp_lock);
+       mutex_lock(&rcu_registry_lock);
 
        if (cds_list_empty(&registry))
                goto out;
@@ -241,6 +264,8 @@ void synchronize_rcu(void)
 
        /*
         * Wait for previous parity to be empty of readers.
+        * update_counter_and_wait() can release and grab again
+        * rcu_registry_lock interally.
         */
        update_counter_and_wait();      /* 0 -> 1, wait readers in parity 0 */
 
@@ -253,6 +278,8 @@ void synchronize_rcu(void)
 
        /*
         * Wait for previous parity to be empty of readers.
+        * update_counter_and_wait() can release and grab again
+        * rcu_registry_lock interally.
         */
        update_counter_and_wait();      /* 1 -> 0, wait readers in parity 1 */
 
@@ -262,6 +289,7 @@ void synchronize_rcu(void)
         */
        cmm_smp_mb();
 out:
+       mutex_unlock(&rcu_registry_lock);
        mutex_unlock(&rcu_gp_lock);
        ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
        assert(!ret);
@@ -465,9 +493,9 @@ void rcu_bp_register(void)
         */
        rcu_bp_init();
 
-       mutex_lock(&rcu_gp_lock);
+       mutex_lock(&rcu_registry_lock);
        add_thread();
-       mutex_unlock(&rcu_gp_lock);
+       mutex_unlock(&rcu_registry_lock);
 end:
        ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
        if (ret)
@@ -488,9 +516,9 @@ void rcu_bp_unregister(struct rcu_reader *rcu_reader_reg)
        if (ret)
                abort();
 
-       mutex_lock(&rcu_gp_lock);
+       mutex_lock(&rcu_registry_lock);
        remove_thread(rcu_reader_reg);
-       mutex_unlock(&rcu_gp_lock);
+       mutex_unlock(&rcu_registry_lock);
        ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
        if (ret)
                abort();
@@ -553,9 +581,10 @@ void rcu_bp_exit(void)
 }
 
 /*
- * Holding the rcu_gp_lock across fork will make sure we fork() don't race with
- * a concurrent thread executing with this same lock held. This ensures that the
- * registry is in a coherent state in the child.
+ * Holding the rcu_gp_lock and rcu_registry_lock across fork will make
+ * sure we fork() don't race with a concurrent thread executing with
+ * any of those locks held. This ensures that the registry and data
+ * protected by rcu_gp_lock are in a coherent state in the child.
  */
 void rcu_bp_before_fork(void)
 {
@@ -567,6 +596,7 @@ void rcu_bp_before_fork(void)
        ret = pthread_sigmask(SIG_BLOCK, &newmask, &oldmask);
        assert(!ret);
        mutex_lock(&rcu_gp_lock);
+       mutex_lock(&rcu_registry_lock);
        saved_fork_signal_mask = oldmask;
 }
 
@@ -576,6 +606,7 @@ void rcu_bp_after_fork_parent(void)
        int ret;
 
        oldmask = saved_fork_signal_mask;
+       mutex_unlock(&rcu_registry_lock);
        mutex_unlock(&rcu_gp_lock);
        ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
        assert(!ret);
@@ -583,7 +614,7 @@ void rcu_bp_after_fork_parent(void)
 
 /*
  * Prune all entries from registry except our own thread. Fits the Linux
- * fork behavior. Called with rcu_gp_lock held.
+ * fork behavior. Called with rcu_gp_lock and rcu_registry_lock held.
  */
 static
 void urcu_bp_prune_registry(void)
@@ -611,6 +642,7 @@ void rcu_bp_after_fork_child(void)
 
        urcu_bp_prune_registry();
        oldmask = saved_fork_signal_mask;
+       mutex_unlock(&rcu_registry_lock);
        mutex_unlock(&rcu_gp_lock);
        ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
        assert(!ret);