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

The transitive dependency between:

RCU read-side C.S. -> synchronize_rcu -> rcu_gp_lock -> rcu_register_thread

and the dependency:

pthread_join -> awaiting for thread completion

Can block a thread on join, and thus have the side-effect of deadlocking
a thread doing a pthread_join while within a RCU read-side critical
section. This join would be awaiting for completion of register_thread or
rcu_unregister_thread, which may never complete because the rcu_gp_lock
is held by synchronize_rcu executed from another thread.

One solution to fix this is to add a new lock, rcu_registry_lock. This
lock now protects the thread registry. It is released between iterations
on the registry by synchronize_rcu, thus allowing thread
registration/unregistration to complete even though synchronize_rcu is
awaiting for RCU read-side critical sections to complete.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Eugene Ivanov <Eugene.Ivanov@orc-group.com>
CC: Lai Jiangshan <laijs@cn.fujitsu.com>
CC: Stephen Hemminger <stephen@networkplumber.org>

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);

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index a2cabb446afda2db312b6d42a00b8ea5f02a4769..1a94efa61a47fc48e12f9fc671b7c147d29382b4 100644 (file)
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -51,7 +51,21 @@
 
 void __attribute__((destructor)) rcu_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;
 
 int32_t gp_futex;
 
@@ -116,6 +130,10 @@ 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.
+ */
 static void update_counter_and_wait(void)
 {
        CDS_LIST_HEAD(qsreaders);
@@ -178,6 +196,8 @@ static 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 {
@@ -187,6 +207,8 @@ static void update_counter_and_wait(void)
                                cmm_smp_mb();
 #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
                        }
+                       /* Re-lock the registry lock before the next loop. */
+                       mutex_lock(&rcu_registry_lock);
                }
        }
        /* put back the reader list in the registry */
@@ -219,12 +241,15 @@ void synchronize_rcu(void)
                cmm_smp_mb();
 
        mutex_lock(&rcu_gp_lock);
+       mutex_lock(&rcu_registry_lock);
 
        if (cds_list_empty(&registry))
                goto out;
 
        /*
         * 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 */
 
@@ -247,9 +272,12 @@ 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 */
 out:
+       mutex_unlock(&rcu_registry_lock);
        mutex_unlock(&rcu_gp_lock);
 
        /*
@@ -279,10 +307,16 @@ void synchronize_rcu(void)
                cmm_smp_mb();
 
        mutex_lock(&rcu_gp_lock);
+       mutex_lock(&rcu_registry_lock);
        if (cds_list_empty(&registry))
                goto out;
+       /*
+        * update_counter_and_wait() can release and grab again
+        * rcu_registry_lock interally.
+        */
        update_counter_and_wait();
 out:
+       mutex_unlock(&rcu_registry_lock);
        mutex_unlock(&rcu_gp_lock);
 
        if (was_online)
@@ -326,9 +360,9 @@ void rcu_register_thread(void)
        URCU_TLS(rcu_reader).tid = pthread_self();
        assert(URCU_TLS(rcu_reader).ctr == 0);
 
-       mutex_lock(&rcu_gp_lock);
+       mutex_lock(&rcu_registry_lock);
        cds_list_add(&URCU_TLS(rcu_reader).node, &registry);
-       mutex_unlock(&rcu_gp_lock);
+       mutex_unlock(&rcu_registry_lock);
        _rcu_thread_online();
 }
 
@@ -339,9 +373,9 @@ void rcu_unregister_thread(void)
         * with a waiting writer.
         */
        _rcu_thread_offline();
-       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);
 }
 
 void rcu_exit(void)

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)
 {