[RFC] eal: add destructor to unregister tailq on unload

[Stephen Hemminger <stephen@networkplumber.org>]

Libraries that use EAL_REGISTER_TAILQ insert a pointer to a static
struct rte_tailq_elem into the process-local tailq list via a
constructor, but have no matching destructor. When such a library
is loaded as a dependency of a plugin via dlopen() and later
unloaded via dlclose(), the list retains a dangling pointer to the
now-unmapped static. Reloading the plugin crashes in
rte_eal_tailq_local_register() when it traverses the stale entry.

Add rte_eal_tailq_unregister() and extend the EAL_REGISTER_TAILQ
macro to emit an RTE_FINI destructor alongside the existing
RTE_INIT constructor. Every library that uses the macro
automatically gets both sides; no per-library changes are needed.

Bugzilla ID: 1081
Fixes: 873a61c7526b ("tailq: introduce dynamic register system")
Cc: stable@dpdk.org

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 lib/eal/common/eal_common_tailqs.c |  8 ++++++++
 lib/eal/include/rte_tailq.h        | 17 +++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/lib/eal/common/eal_common_tailqs.c b/lib/eal/common/eal_common_tailqs.c
index c581f43b6f..714f91d0ec 100644
--- a/lib/eal/common/eal_common_tailqs.c
+++ b/lib/eal/common/eal_common_tailqs.c
@@ -148,6 +148,14 @@ rte_eal_tailq_register(struct rte_tailq_elem *t)
 	return -1;
 }
 
+RTE_EXPORT_SYMBOL(rte_eal_tailq_unregister)
+void
+rte_eal_tailq_unregister(struct rte_tailq_elem *t)
+{
+	TAILQ_REMOVE(&rte_tailq_elem_head, t, next);
+	t->head = NULL;
+}
+
 int
 rte_eal_tailqs_init(void)
 {
diff --git a/lib/eal/include/rte_tailq.h b/lib/eal/include/rte_tailq.h
index e7caed6812..c5d5cb782f 100644
--- a/lib/eal/include/rte_tailq.h
+++ b/lib/eal/include/rte_tailq.h
@@ -117,11 +117,28 @@ struct rte_tailq_head *rte_eal_tailq_lookup(const char *name);
  */
 int rte_eal_tailq_register(struct rte_tailq_elem *t);
 
+/**
+ * Remove a tail queue element from the local list.
+ * This function is mainly used for EAL_REGISTER_TAILQ macro which pairs
+ * an RTE_FINI destructor with the existing RTE_INIT constructor.
+ * The destructor calls this function during dlclose() to prevent
+ * dangling pointers to unmapped library data.
+ *
+ * @param t
+ *   The tailq element which contains the name of the tailq you want to
+ *   delete
+ */
+void rte_eal_tailq_unregister(struct rte_tailq_elem *t);
+
 #define EAL_REGISTER_TAILQ(t) \
 RTE_INIT(tailqinitfn_ ##t) \
 { \
 	if (rte_eal_tailq_register(&t) < 0) \
 		rte_panic("Cannot initialize tailq: %s\n", t.name); \
+} \
+RTE_FINI(tailqfinifn_ ##t) \
+{ \
+	rte_eal_tailq_unregister(&t); \
 }
 
 /* This macro permits both remove and free var within the loop safely.*/
-- 
2.53.0