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 6580397b1ee42b04a8e48b5c29a76220ea39b6d0..f6625ba14e2d05689443caec171605c13b120a1f 100644 (file)
--- a/urcu-call-rcu-impl.h
+++ b/urcu-call-rcu-impl.h
@@ -75,8 +75,9 @@ static CDS_LIST_HEAD(call_rcu_data_list);
 
 static DEFINE_URCU_TLS(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. */
@@ -254,6 +255,21 @@ static void *call_rcu_thread(void *arg)
                struct cds_wfcq_node *cbs, *cbs_tmp_n;
                enum cds_wfcq_ret splice_ret;
 
+               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();
+               }
+
                cds_wfcq_init(&cbs_tmp_head, &cbs_tmp_tail);
                splice_ret = __cds_wfcq_splice_blocking(&cbs_tmp_head,
                        &cbs_tmp_tail, &crdp->cbs_head, &crdp->cbs_tail);
@@ -705,12 +721,25 @@ 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);
+       }
 }
 
 /*
@@ -720,6 +749,10 @@ 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);
 }
 
@@ -752,7 +785,11 @@ void call_rcu_after_fork_child(void)
        rcu_set_pointer(&per_cpu_call_rcu_data, NULL);
        URCU_TLS(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 1dad0e2c46cc88d691827208c38c6be092f2bf1e..997bb2f19a7a0e1547f4cb009b48db8d42d4ee72 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