[lttng-dev] [RFC PATCH] Fix call_rcu fork handling
Mathieu Desnoyers
mathieu.desnoyers at efficios.com
Wed Dec 26 12:37:22 EST 2012
* Mathieu Desnoyers (mathieu.desnoyers at efficios.com) wrote:
> 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 playing 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.
I'll rework the patch so the modification to tests/test_urcu_fork.c is
moved to a separate patch, and merge the fix into the master branch,
since it solves a reproduceable problem.
Thanks,
Mathieu
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> ---
> diff --git a/tests/test_urcu_fork.c b/tests/test_urcu_fork.c
> index 07c521a..6e454b5 100644
> --- a/tests/test_urcu_fork.c
> +++ b/tests/test_urcu_fork.c
> @@ -85,6 +85,8 @@ int main(int argc, char **argv)
> pid_t pid;
> int ret;
>
> +#if 0
> + /* pthread_atfork does not work with malloc/free in callbacks */
> ret = pthread_atfork(call_rcu_before_fork,
> call_rcu_after_fork_parent,
> call_rcu_after_fork_child);
> @@ -93,6 +95,7 @@ int main(int argc, char **argv)
> perror("pthread_atfork");
> exit(EXIT_FAILURE);
> }
> +#endif
>
> test_rcu();
>
> @@ -101,10 +104,12 @@ int main(int argc, char **argv)
> fprintf(stderr, "%s parent pid: %d, before fork\n",
> argv[0], (int) getpid());
>
> + call_rcu_before_fork();
> pid = fork();
>
> if (pid == 0) {
> /* child */
> + call_rcu_after_fork_child();
> fprintf(stderr, "%s child pid: %d, after fork\n",
> argv[0], (int) getpid());
> test_rcu();
> @@ -114,6 +119,7 @@ int main(int argc, char **argv)
> int status;
>
> /* parent */
> + call_rcu_after_fork_parent();
> fprintf(stderr, "%s parent pid: %d, after fork\n",
> argv[0], (int) getpid());
> test_rcu();
> diff --git a/urcu-call-rcu-impl.h b/urcu-call-rcu-impl.h
> index 6580397..f6625ba 100644
> --- 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 1dad0e2..997bb2f 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
> --
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com
>
> _______________________________________________
> lttng-dev mailing list
> lttng-dev at lists.lttng.org
> http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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