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

[userspace-rcu.git] / urcu.c

diff --git a/urcu.c b/urcu.c
index 8420ee494cef759bc2b87287168bacddc525315c..5cb470bcaea9e61b312a8e2d839c14ab88eacc80 100644 (file)
--- a/urcu.c
+++ b/urcu.c
@@ -81,7 +81,21 @@ void __attribute__((constructor)) rcu_init(void);
 void __attribute__((destructor)) rcu_exit(void);
 #endif
 
+/*
+ * 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;
 
 int32_t gp_futex;
 
@@ -92,7 +106,6 @@ int32_t gp_futex;
  * Written to only by writer with mutex taken. Read by both writer and readers.
  */
 unsigned long rcu_gp_ctr = RCU_GP_COUNT;
-
 /*
  * Written to only by each individual reader. Read by both the reader and the
  * writers.
@@ -215,6 +228,10 @@ static void wait_gp(void)
                      NULL, NULL, 0);
 }
 
+/*
+ * 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);
@@ -269,10 +286,14 @@ void update_counter_and_wait(void)
                        }
                        break;
                } else {
+                       /* Temporarily unlock the registry lock. */
+                       mutex_unlock(&rcu_registry_lock);
                        if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS)
                                wait_gp();
                        else
                                caa_cpu_relax();
+                       /* Re-lock the registry lock before the next loop. */
+                       mutex_lock(&rcu_registry_lock);
                }
 #else /* #ifndef HAS_INCOHERENT_CACHES */
                /*
@@ -292,12 +313,16 @@ void update_counter_and_wait(void)
                                smp_mb_master(RCU_MB_GROUP);
                                wait_gp_loops = 0;
                        }
+                       /* Temporarily unlock the registry lock. */
+                       mutex_unlock(&rcu_registry_lock);
                        if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
                                wait_gp();
                                wait_gp_loops++;
                        } else {
                                caa_cpu_relax();
                        }
+                       /* Re-lock the registry lock before the next loop. */
+                       mutex_lock(&rcu_registry_lock);
                }
 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
        }
@@ -308,18 +333,23 @@ void update_counter_and_wait(void)
 void synchronize_rcu(void)
 {
        mutex_lock(&rcu_gp_lock);
+       mutex_lock(&rcu_registry_lock);
 
        if (cds_list_empty(&registry))
                goto out;
 
-       /* All threads should read qparity before accessing data structure
-        * where new ptr points to. Must be done within rcu_gp_lock because it
-        * iterates on reader threads.*/
+       /*
+        * All threads should read qparity before accessing data structure
+        * where new ptr points to. Must be done within rcu_registry_lock
+        * because it iterates on reader threads.
+        */
        /* Write new ptr before changing the qparity */
        smp_mb_master(RCU_MB_GROUP);
 
        /*
         * 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 */
 
@@ -341,14 +371,19 @@ 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 */
 
-       /* Finish waiting for reader threads before letting the old ptr being
-        * freed. Must be done within rcu_gp_lock because it iterates on reader
-        * threads. */
+       /*
+        * Finish waiting for reader threads before letting the old ptr
+        * being freed. Must be done within rcu_registry_lock because it
+        * iterates on reader threads.
+        */
        smp_mb_master(RCU_MB_GROUP);
 out:
+       mutex_unlock(&rcu_registry_lock);
        mutex_unlock(&rcu_gp_lock);
 }
 
@@ -372,17 +407,17 @@ void rcu_register_thread(void)
        assert(URCU_TLS(rcu_reader).need_mb == 0);
        assert(!(URCU_TLS(rcu_reader).ctr & RCU_GP_CTR_NEST_MASK));
 
-       mutex_lock(&rcu_gp_lock);
+       mutex_lock(&rcu_registry_lock);
        rcu_init();     /* In case gcc does not support constructor attribute */
        cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
-       mutex_unlock(&rcu_gp_lock);
+       mutex_unlock(&rcu_registry_lock);
 }
 
 void rcu_unregister_thread(void)
 {
-       mutex_lock(&rcu_gp_lock);
+       mutex_lock(&rcu_registry_lock);
        cds_list_del(&URCU_TLS(rcu_reader).node);
-       mutex_unlock(&rcu_gp_lock);
+       mutex_unlock(&rcu_registry_lock);
 }
 
 #ifdef RCU_MEMBARRIER
@@ -413,9 +448,9 @@ static void sigrcu_handler(int signo, siginfo_t *siginfo, void *context)
  * rcu_init constructor. Called when the library is linked, but also when
  * reader threads are calling rcu_register_thread().
  * Should only be called by a single thread at a given time. This is ensured by
- * holing the rcu_gp_lock from rcu_register_thread() or by running at library
- * load time, which should not be executed by multiple threads nor concurrently
- * with rcu_register_thread() anyway.
+ * holing the rcu_registry_lock from rcu_register_thread() or by running
+ * at library load time, which should not be executed by multiple
+ * threads nor concurrently with rcu_register_thread() anyway.
  */
 void rcu_init(void)
 {