Fix call_rcu fork handling

Fix call_rcu fork handling by putting all call_rcu threads in a
quiescent state before fork (paused state), and unpausing them when the
parent returns from fork.

On the child, everything will run fine as long as we don't issue fork()
from a call_rcu callback.

Side-note: pthread_atfork is not appropriate when using with multithread
and malloc/free. The glibc malloc implementation sadly expects that all
malloc/free are executed from the context of a single thread while
pthread atfork handlers are running, which leads to interesting hang in
glibc.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/urcu-call-rcu-impl.h b/urcu-call-rcu-impl.h
index e38123b3fca45ecf1e068b798a7226200abbb876..6ea7b9a05d74bdfb0c2760d426306d663cb0ade8 100644 (file)
--- a/urcu-call-rcu-impl.h
+++ b/urcu-call-rcu-impl.h
@@ -64,8 +64,9 @@ CDS_LIST_HEAD(call_rcu_data_list);
 
 static __thread struct call_rcu_data *thread_call_rcu_data;
 
-/* Guard call_rcu thread creation. */
-
+/*
+ * Guard call_rcu thread creation and atfork handlers.
+ */
 static pthread_mutex_t call_rcu_mutex = PTHREAD_MUTEX_INITIALIZER;
 
 /* If a given thread does not have its own call_rcu thread, this is default. */
@@ -239,6 +240,21 @@ static void *call_rcu_thread(void *arg)
                cmm_smp_mb();
        }
        for (;;) {
+               if (uatomic_read(&crdp->flags) & URCU_CALL_RCU_PAUSE) {
+                       /*
+                        * Pause requested. Become quiescent: remove
+                        * ourself from all global lists, and don't
+                        * process any callback. The callback lists may
+                        * still be non-empty though.
+                        */
+                       rcu_unregister_thread();
+                       cmm_smp_mb__before_uatomic_or();
+                       uatomic_or(&crdp->flags, URCU_CALL_RCU_PAUSED);
+                       while ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_PAUSE) != 0)
+                               poll(NULL, 0, 1);
+                       rcu_register_thread();
+               }
+
                if (&crdp->cbs.head != _CMM_LOAD_SHARED(crdp->cbs.tail)) {
                        while ((cbs = _CMM_LOAD_SHARED(crdp->cbs.head)) == NULL)
                                poll(NULL, 0, 1);
@@ -702,12 +718,25 @@ void free_all_cpu_call_rcu_data(void)
 
 /*
  * Acquire the call_rcu_mutex in order to ensure that the child sees
- * all of the call_rcu() data structures in a consistent state.
+ * all of the call_rcu() data structures in a consistent state. Ensure
+ * that all call_rcu threads are in a quiescent state across fork.
  * Suitable for pthread_atfork() and friends.
  */
 void call_rcu_before_fork(void)
 {
+       struct call_rcu_data *crdp;
+
        call_rcu_lock(&call_rcu_mutex);
+
+       cds_list_for_each_entry(crdp, &call_rcu_data_list, list) {
+               uatomic_or(&crdp->flags, URCU_CALL_RCU_PAUSE);
+               cmm_smp_mb__after_uatomic_or();
+               wake_call_rcu_thread(crdp);
+       }
+       cds_list_for_each_entry(crdp, &call_rcu_data_list, list) {
+               while ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_PAUSED) == 0)
+                       poll(NULL, 0, 1);
+       }
 }
 
 /*
@@ -717,6 +746,10 @@ void call_rcu_before_fork(void)
  */
 void call_rcu_after_fork_parent(void)
 {
+       struct call_rcu_data *crdp;
+
+       cds_list_for_each_entry(crdp, &call_rcu_data_list, list)
+               uatomic_and(&crdp->flags, ~URCU_CALL_RCU_PAUSE);
        call_rcu_unlock(&call_rcu_mutex);
 }
 
@@ -749,7 +782,11 @@ void call_rcu_after_fork_child(void)
        rcu_set_pointer(&per_cpu_call_rcu_data, NULL);
        thread_call_rcu_data = NULL;
 
-       /* Dispose of all of the rest of the call_rcu_data structures. */
+       /*
+        * Dispose of all of the rest of the call_rcu_data structures.
+        * Leftover call_rcu callbacks will be merged into the new
+        * default call_rcu thread queue.
+        */
        cds_list_for_each_entry_safe(crdp, next, &call_rcu_data_list, list) {
                if (crdp == default_call_rcu_data)
                        continue;

diff --git a/urcu-call-rcu.h b/urcu-call-rcu.h
index b109f4e4d16f250b1f5fdcb2e01e196f3539f238..35b31ab9c80299da23951de02d67dab6d14a7ae8 100644 (file)
--- a/urcu-call-rcu.h
+++ b/urcu-call-rcu.h
@@ -44,10 +44,12 @@ struct call_rcu_data;
 
 /* Flag values. */
 
-#define URCU_CALL_RCU_RT       0x1
-#define URCU_CALL_RCU_RUNNING  0x2
-#define URCU_CALL_RCU_STOP     0x4
-#define URCU_CALL_RCU_STOPPED  0x8
+#define URCU_CALL_RCU_RT       (1U << 0)
+#define URCU_CALL_RCU_RUNNING  (1U << 1)
+#define URCU_CALL_RCU_STOP     (1U << 2)
+#define URCU_CALL_RCU_STOPPED  (1U << 3)
+#define URCU_CALL_RCU_PAUSE    (1U << 4)
+#define URCU_CALL_RCU_PAUSED   (1U << 5)
 
 /*
  * The rcu_head data structure is placed in the structure to be freed