From 483c853e4227f9d0b3ad03f7a4bd8038fe26da89 Mon Sep 17 00:00:00 2001 From: Mathieu Desnoyers Date: Tue, 18 Dec 2012 19:31:21 -0500 Subject: [PATCH] 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 --- urcu-call-rcu-impl.h | 45 ++++++++++++++++++++++++++++++++++++++++---- urcu-call-rcu.h | 10 ++++++---- 2 files changed, 47 insertions(+), 8 deletions(-) diff --git a/urcu-call-rcu-impl.h b/urcu-call-rcu-impl.h index e38123b..6ea7b9a 100644 --- 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 b109f4e..35b31ab 100644 --- 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 -- 2.34.1