lib: add "seqlock" wait/broadcast primitive

Manually tested rather extensively in addition to included unit tests,
should work as intended.

NB: The OpenBSD futex() code is "future"; it's not actually in OpenBSD
(yet?) and thus untested.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

1 files changed, 106 insertions, 0 deletions

diff --git a/lib/seqlock.h b/lib/seqlock.h
new file mode 100644
index 000000000..eef05a430
--- /dev/null
+++ b/lib/seqlock.h
@@ -0,0 +1,106 @@
+/*
+ * "Sequence" lock primitive
+ *
+ * Copyright (C) 2015  David Lamparter <equinox@diac24.net>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ * Boston, MA  02110-1301  USA
+ */
+
+#ifndef _SEQLOCK_H
+#define _SEQLOCK_H
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <pthread.h>
+#include "frratomic.h"
+
+/*
+ * this locking primitive is intended to use in a 1:N setup.
+ *
+ * - one "counter" seqlock issuing increasing numbers
+ * - multiple seqlock users hold references on these numbers
+ *
+ * this is intended for implementing RCU reference-holding.  There is one
+ * global counter, with threads locking a seqlock whenever they take a
+ * reference.  A seqlock can also be idle/unlocked.
+ *
+ * The "counter" seqlock will always stay locked;  the RCU cleanup thread
+ * continuously counts it up, waiting for threads to release or progress to a
+ * sequence number further ahead.  If all threads are > N, references dropped
+ * in N can be free'd.
+ *
+ * generally, the lock function is:
+ *
+ * Thread-A                  Thread-B
+ *
+ * seqlock_acquire(a)
+ *    | running              seqlock_wait(b)      -- a <= b
+ * seqlock_release()            | blocked
+ * OR: seqlock_acquire(a')      |                 -- a' > b
+ *                           (resumes)
+ */
+
+/* use sequentially increasing "ticket numbers".  lowest bit will always
+ * be 1 to have a 'cleared' indication (i.e., counts 1,3,5,7,etc. )
+ */
+typedef _Atomic uint32_t	seqlock_ctr_t;
+typedef uint32_t		seqlock_val_t;
+#define seqlock_assert_valid(val) assert(val & 1)
+
+
+struct seqlock {
+/* always used */
+	seqlock_ctr_t pos;
+/* used when futexes not available: (i.e. non-linux) */
+	pthread_mutex_t lock;
+	pthread_cond_t  wake;
+};
+
+
+/* sqlo = 0 - init state: not held */
+extern void seqlock_init(struct seqlock *sqlo);
+
+
+/* while (sqlo <= val) - wait until seqlock->pos > val, or seqlock unheld */
+extern void seqlock_wait(struct seqlock *sqlo, seqlock_val_t val);
+extern bool seqlock_check(struct seqlock *sqlo, seqlock_val_t val);
+
+static inline bool seqlock_held(struct seqlock *sqlo)
+{
+	return !!atomic_load_explicit(&sqlo->pos, memory_order_relaxed);
+}
+
+/* sqlo - get seqlock position -- for the "counter" seqlock */
+extern seqlock_val_t seqlock_cur(struct seqlock *sqlo);
+/* sqlo++ - note: like x++, returns previous value, before bumping */
+extern seqlock_val_t seqlock_bump(struct seqlock *sqlo);
+
+
+/* sqlo = val - can be used on held seqlock. */
+extern void seqlock_acquire_val(struct seqlock *sqlo, seqlock_val_t val);
+/* sqlo = ref - standard pattern: acquire relative to other seqlock */
+static inline void seqlock_acquire(struct seqlock *sqlo, struct seqlock *ref)
+{
+	seqlock_acquire_val(sqlo, seqlock_cur(ref));
+}
+
+/* sqlo = 0 - set seqlock position to 0, marking as non-held */
+extern void seqlock_release(struct seqlock *sqlo);
+/* release should normally be followed by a bump on the "counter", if
+ * anything other than reading RCU items was done
+ */
+
+#endif /* _SEQLOCK_H */