Re: [PATCH] call_rcu: Fix race between rcu_barrier() and call_rcu_data_free()

Mathieu Desnoyers
2020-10-26T19:58:15Z

----- On Oct 22, 2020, at 6:30 PM, paulmck paulmck@kernel.org wrote:

> 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.

I agree with this fix, will merge in liburcu master, stable-0.12, and stable-2.11.
Out of curiosity, which test is hanging ? Is it a test which is part of the liburcu
tree or some out-of-tree test ? I wonder why we did not catch it in our CI [1].

Thanks,

Mathieu

[1] https://ci.lttng.org/view/Liburcu/

>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Stephen Hemminger <stephen@networkplumber.org>
> Cc: Alan Stern <stern@rowland.harvard.edu>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: <lttng-dev@lists.lttng.org>
> Cc: <linux-kernel@vger.kernel.org>
>
> ---
>
> 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);

--
Mathieu Desnoyers
EfficiOS Inc.