[lttng-dev] [RFC] wfcqueue: allow defining CDS_WFCQ_WAIT_SLEEP to override `poll'

[Eric Wong]

Users may want to use alternative sleeping behavior instead of
`poll'.  Make CDS_WFCQ_WAIT_SLEEP a macro which may be defined
before including wfcqueue.h.

This alternative behavior could include logging, performing
low-priority cleanup work, sleeping a shorter/longer interval
or any combination of that.

This will also make integration into glibc easier, as `poll'
linkage causes conformance test failures even when relegated
to an impossible code path:
https://public-inbox.org/libc-alpha/20180801092626.jrwyrojfye4avcis@whir/

Signed-off-by: Eric Wong <normalperson at yhbt.net>
---
 include/urcu/static/wfcqueue.h | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/urcu/static/wfcqueue.h b/include/urcu/static/wfcqueue.h
index 67ac05f..3f56493 100644
--- a/include/urcu/static/wfcqueue.h
+++ b/include/urcu/static/wfcqueue.h
@@ -223,6 +223,23 @@ static inline bool _cds_wfcq_enqueue(cds_wfcq_head_ptr_t head,
 	return ___cds_wfcq_append(head, tail, new_tail, new_tail);
 }
 
+/*
+ * CDS_WFCQ_WAIT_SLEEP:
+ *
+ * By default, this sleeps for the given @msec milliseconds.
+ * This is a macro which LGPL users may #define themselves before
+ * including wfcqueue.h to override the default behavior (e.g.
+ * to log a warning or perform other background work).
+ */
+#ifndef CDS_WFCQ_WAIT_SLEEP
+#define CDS_WFCQ_WAIT_SLEEP(msec) ___cds_wfcq_wait_sleep((msec))
+#endif
+
+static inline void ___cds_wfcq_wait_sleep(int msec)
+{
+	(void) poll(NULL, 0, msec);
+}
+
 /*
  * ___cds_wfcq_busy_wait: adaptative busy-wait.
  *
@@ -234,7 +251,7 @@ ___cds_wfcq_busy_wait(int *attempt, int blocking)
 	if (!blocking)
 		return 1;
 	if (++(*attempt) >= WFCQ_ADAPT_ATTEMPTS) {
-		(void) poll(NULL, 0, WFCQ_WAIT);	/* Wait for 10ms */
+		CDS_WFCQ_WAIT_SLEEP(WFCQ_WAIT);		/* Wait for 10ms */
 		*attempt = 0;
 	} else {
 		caa_cpu_relax();
-- 
EW