From 51b98c3cb988eca7538238e22268bdcf316d4541 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 22 Oct 2020 15:30:21 -0700 Subject: [PATCH] call_rcu: Fix race between rcu_barrier() and call_rcu_data_free() The current code can lose RCU callbacks at shutdown time, which can result in hangs. This lossage can happen as follows: o A thread invokes call_rcu_data_free(), which executes up through the wake_call_rcu_thread(). At this point, the call_rcu_data structure has been drained of callbacks, but is still on the call_rcu_data_list. Note that this thread does not hold the call_rcu_mutex. o Another thread invokes rcu_barrier(), which traverses the call_rcu_data_list under the protection of call_rcu_mutex, a list which still includes the above newly drained structure. This thread therefore adds a callback to the newly drained call_rcu_data structure. It then releases call_rcu_mutex and enters a mystifying loop that does futex stuff. o The first thread finishes executing call_rcu_data_free(), which acquires call_rcu_mutex just long enough to remove the newly drained call_rcu_data structure from call_rcu_data_list. Which causes one of the rcu_barrier() invocation's callbacks to be leaked. o The second thread's rcu_barrier() invocation never returns resulting in a hang. This commit therefore changes call_rcu_data_free() to acquire call_rcu_mutex before checking the call_rcu_data structure for callbacks. In the case where there are no callbacks, call_rcu_mutex is held across both the check and the removal from call_rcu_data_list, thus preventing rcu_barrier() from adding a callback in the meantime. In the case where there are callbacks, call_rcu_mutex must be momentarily dropped across the call to get_default_call_rcu_data(), which can itself acquire call_rcu_mutex. This momentary drop is not a problem because any callbacks that rcu_barrier() might queue during that period of time will be moved to the default call_rcu_data structure, and the lock will be held across the full time including moving those callbacks and removing the call_rcu_data structure that was passed into call_rcu_data_free() from call_rcu_data_list. With this fix, a several-hundred-CPU test successfully completes more than 5,000 executions. Without this fix, it fails within a few tens of executions. Although the failures happen more quickly on larger systems, in theory this could happen on a single-CPU system, courtesy of preemption. Signed-off-by: Paul E. McKenney Cc: Mathieu Desnoyers Cc: Stephen Hemminger Cc: Alan Stern Cc: Lai Jiangshan Cc: Cc: Signed-off-by: Mathieu Desnoyers --- src/urcu-call-rcu-impl.h | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/src/urcu-call-rcu-impl.h b/src/urcu-call-rcu-impl.h index b6ec6ba..18fd65a 100644 --- a/src/urcu-call-rcu-impl.h +++ b/src/urcu-call-rcu-impl.h @@ -772,9 +772,13 @@ void call_rcu_data_free(struct call_rcu_data *crdp) while ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOPPED) == 0) (void) poll(NULL, 0, 1); } + call_rcu_lock(&call_rcu_mutex); if (!cds_wfcq_empty(&crdp->cbs_head, &crdp->cbs_tail)) { - /* Create default call rcu data if need be */ + call_rcu_unlock(&call_rcu_mutex); + /* Create default call rcu data if need be. */ + /* CBs queued here will be handed to the default list. */ (void) get_default_call_rcu_data(); + call_rcu_lock(&call_rcu_mutex); __cds_wfcq_splice_blocking(&default_call_rcu_data->cbs_head, &default_call_rcu_data->cbs_tail, &crdp->cbs_head, &crdp->cbs_tail); @@ -783,7 +787,6 @@ void call_rcu_data_free(struct call_rcu_data *crdp) wake_call_rcu_thread(default_call_rcu_data); } - call_rcu_lock(&call_rcu_mutex); cds_list_del(&crdp->list); call_rcu_unlock(&call_rcu_mutex); -- 2.34.1