add a lock-free bounded spsc interthread queue

Not entirely wait-free: allows waiting on a fd, and in general fd
notifications for poll-loop integration between threads.

Change-Id: I4f17042baf76d086ce6b20eb99402dc64c22c657

include/osmocom/mgcp/mgcp_threads_queue.h

@@ -0,0 +1,52 @@
+/*
+ * (C) 2021 by sysmocom s.f.m.c. GmbH <info@sysmocom.de>
+ * All Rights Reserved
+ *
+ * Author: Eric Wild
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include <stdatomic.h>
+#include <stdbool.h>
+#include <stdlib.h>
+
+struct spsc {
+	atomic_uint readptr;
+	atomic_uint writeptr;
+
+	int efd_r, efd_w; /* eventfds used to block/notify readers/writers */
+
+	int count;
+	int size_per_buf;
+
+	void *buf; /* buffer size count*size_per_buf */
+	uintptr_t data[0]; /* count sized array of pointers to size_per_buf chunks in buf array*/
+};
+
+struct qchan {
+	struct spsc *a;
+	struct spsc *b;
+};
+
+bool spsc_push(struct spsc *q, void *elem);
+bool spsc_pop(struct spsc *q, void *elem);
+ssize_t spsc_prep_pop(struct spsc *q);
+int spsc_get_a_rdfd(struct qchan *q);
+
+struct qchan spsc_chan_init(void *talloc_ctx, unsigned int count, unsigned int size_per_buf);
+struct qchan spsc_chan_init_ex(void *talloc_ctx, unsigned int count, unsigned int size_per_buf, bool blockr_a,
+			       bool blockw_a, bool blockr_b, bool blockw_b);
+void spsc_chan_close(struct qchan *q);

src/libosmo-mgcp/mgcp_threads_queue.c

@@ -0,0 +1,207 @@
+/*
+ * (C) 2021 by sysmocom s.f.m.c. GmbH <info@sysmocom.de>
+ * All Rights Reserved
+ *
+ * Author: Eric Wild
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include <inttypes.h>
+#include <stdatomic.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/eventfd.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <talloc.h>
+
+#include <osmocom/mgcp/mgcp_threads_queue.h>
+
+/*
+classic lamport circular lockfree spsc queue:
+every "side" only writes its own ptr, but may read the other sides ptr
+
+notify reader using eventfd as soon as element is added, reader then reads until
+read fails
+-> reader pops in a loop until FALSE and might get spurious events because it
+read before it was notified, which is fine
+-> writing pushes *the same data* in a loop until TRUE, blocks
+
+shutting this down requires
+1) to stop reading and pushing
+2) ONE side to take care of the eventfds
+*/
+
+static struct spsc *spsc_init(void *talloc_ctx, unsigned int count, unsigned int size_per_buf, bool blockr, bool blockw)
+{
+	struct spsc *q = talloc_zero_size(talloc_ctx, sizeof(struct spsc) + sizeof(uintptr_t) * count);
+	atomic_init(&q->readptr, 0);
+	atomic_init(&q->writeptr, 0);
+	q->efd_r = eventfd(0, blockr ? 0 : EFD_NONBLOCK);
+	q->efd_w = eventfd(1, blockw ? 0 : EFD_NONBLOCK);
+	q->count = count;
+	q->size_per_buf = size_per_buf;
+	q->buf = talloc_zero_size(q, size_per_buf * count);
+
+	for (int i = 0; i < count; i++)
+		q->data[i] = (uintptr_t)q->buf + i * size_per_buf;
+	return q;
+}
+
+static void spsc_deinit(struct spsc *q)
+{
+	talloc_free(q->buf);
+	close(q->efd_r);
+	close(q->efd_w);
+	talloc_free(q);
+}
+
+static ssize_t spsc_check_r(struct spsc *q)
+{
+	uint64_t efdr;
+	return read(q->efd_r, &efdr, sizeof(uint64_t));
+}
+static ssize_t spsc_check_w(struct spsc *q)
+{
+	uint64_t efdr;
+	return read(q->efd_w, &efdr, sizeof(uint64_t));
+}
+static void spsc_notify_r(struct spsc *q)
+{
+	uint64_t efdu = 1;
+	write(q->efd_r, &efdu, sizeof(uint64_t));
+}
+static void spsc_notify_w(struct spsc *q)
+{
+	uint64_t efdu = 1;
+	write(q->efd_w, &efdu, sizeof(uint64_t));
+}
+
+/*! Adds element to the queue by copying the data.
+ *  \param[in] q queue.
+ *  \param[in] elem input buffer, must match the originally configured queue buffer size!.
+ *  \returns true if queue was not full and element was successfully pushed */
+bool spsc_push(struct spsc *q, void *elem)
+{
+	size_t cur_wp, cur_rp;
+	cur_wp = atomic_load_explicit(&q->writeptr, memory_order_relaxed);
+	cur_rp = atomic_load_explicit(&q->readptr, memory_order_acquire);
+	if ((cur_wp + 1) % q->count == cur_rp) {
+		spsc_check_w(q); /* blocks, ensures next (!) call succeeds */
+		return false;
+	}
+	memcpy((void *)q->data[cur_wp], elem, q->size_per_buf);
+	atomic_store_explicit(&q->writeptr, (cur_wp + 1) % q->count, memory_order_release);
+	spsc_notify_r(q); /* fine after release */
+	return true;
+}
+
+/*! Reads the read-fd of the queue, which, depending on settings passed on queue creation, blocks.
+ * This function can be used to deliberately wait for a non-empty queue on the read side.
+ *  \param[in] q queue.
+ *  \returns result of reading the fd. */
+ssize_t spsc_prep_pop(struct spsc *q)
+{
+	return spsc_check_r(q);
+}
+
+/*! Removes element from the queue by copying the data.
+ *  \param[in] q queue.
+ *  \param[in] elem output buffer, must match the originally configured queue buffer size!.
+ *  \returns true if queue was not empty and element was successfully removed */
+bool spsc_pop(struct spsc *q, void *elem)
+{
+	size_t cur_wp, cur_rp;
+	cur_wp = atomic_load_explicit(&q->writeptr, memory_order_acquire);
+	cur_rp = atomic_load_explicit(&q->readptr, memory_order_relaxed);
+
+	if (cur_wp == cur_rp) /* blocks via prep_pop */
+		return false;
+	memcpy(elem, (void *)q->data[cur_rp], q->size_per_buf);
+	atomic_store_explicit(&q->readptr, (cur_rp + 1) % q->count, memory_order_release);
+	spsc_notify_w(q);
+	return true;
+}
+
+/*! Creates a bidirectional queue channel that consists of two queues, one in each direction,
+ *  commonly referred to as a and b side.
+ *  \param[in] talloc_ctx allocation context.
+ *  \param[in] count number of buffers per queue.
+ *  \param[in] size_per_buf size of buffers per queue.
+ *  \param[in] blockr_a should reading the a-side read fd block?.
+ *  \param[in] blockw_a should reading the a-side write fd block?.
+ *  \param[in] blockr_b should reading the b-side read fd block?.
+ *  \param[in] blockw_b should reading the b-side write fd block?.
+  *  \returns queue channel */
+struct qchan spsc_chan_init_ex(void *talloc_ctx, unsigned int count, unsigned int size_per_buf, bool blockr_a,
+			       bool blockw_a, bool blockr_b, bool blockw_b)
+{
+	struct qchan q;
+	q.a = spsc_init(talloc_ctx, count, size_per_buf, blockr_a, blockw_a);
+	q.b = spsc_init(talloc_ctx, count, size_per_buf, blockr_b, blockw_b);
+	return q;
+}
+
+/*! Creates a bidirectional queue channel that consists of two queues, one in each direction,
+ *  commonly referred to as a and b side.
+ *  \param[in] talloc_ctx allocation context.
+ *  \param[in] count number of buffers per queue.
+ *  \param[in] size_per_buf size of buffers per queue.
+ *  \returns queue channel */
+struct qchan spsc_chan_init(void *talloc_ctx, unsigned int count, unsigned int size_per_buf)
+{
+	return spsc_chan_init_ex(talloc_ctx, count, size_per_buf, false, true, false, true);
+}
+
+/*! Closes a bidirectional queue channel.
+ *  \param[in] q queue */
+void spsc_chan_close(struct qchan *q)
+{
+	spsc_deinit(q->a);
+	spsc_deinit(q->b);
+	free(q);
+}
+
+/*! Gets queue channel read/write fd for a/b side according to function name.
+ *  \param[in] q queue channel.
+ *  \returns fd */
+int spsc_get_a_rdfd(struct qchan *q)
+{
+	return q->a->efd_r;
+}
+/*! Gets queue channel read/write fd for a/b side according to function name.
+ *  \param[in] q queue channel.
+ *  \returns fd */
+int spsc_get_b_rdfd(struct qchan *q)
+{
+	return q->b->efd_r;
+}
+/*! Gets queue channel read/write fd for a/b side according to function name.
+ *  \param[in] q queue channel.
+ *  \returns fd */
+int spsc_get_a_wrfd(struct qchan *q)
+{
+	return q->a->efd_w;
+}
+/*! Gets queue channel read/write fd for a/b side according to function name.
+ *  \param[in] q queue channel.
+ *  \returns fd */
+int spsc_get_b_wrfd(struct qchan *q)
+{
+	return q->b->efd_w;
+}