stack: allow lock-free only on relevant architectures

Since commit 7911ba0473e0 ("stack: enable lock-free implementation for
aarch64"), lock-free stack is supported on arm64 but this description was
missing from the doxygen for the flag.

Currently it is impossible to detect programmatically whether lock-free
implementation of rte_stack is supported. One could check whether the
header guard for lock-free stubs is defined (_RTE_STACK_LF_STUBS_H_) but
that's an unstable implementation detail. Because of that currently all
lock-free ring creations silently succeed (as long as the stack header
is 16B long) which later leads to push and pop operations being NOPs.
The observable effect is that stack_lf_autotest fails on platforms not
supporting the lock-free. Instead it should just skip the lock-free test
altogether.

This commit adds a new errno value (ENOTSUP) that may be returned by
rte_stack_create() to indicate that a given combination of flags is not
supported on a current platform.
This is detected by checking a compile-time flag in the include logic in
rte_stack_lf.h which may be used by applications to check the lock-free
support at compile time.

Use the added RTE_STACK_LF_SUPPORTED flag to disable the lock-free stack
tests at the compile time.
Perf test doesn't fail because rte_ring_create() succeeds, however
marking this test as skipped gives a better indication of what actually
was tested.

Fixes: 7911ba0473e0 ("stack: enable lock-free implementation for aarch64")

Signed-off-by: Stanislaw Kardach <kda@semihalf.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>

app/test/test_stack.c

@@ -373,7 +373,11 @@ test_stack(void)
 static int
 test_lf_stack(void)
 {
+#if defined(RTE_STACK_LF_SUPPORTED)
 	return __test_stack(RTE_STACK_F_LF);
+#else
+	return TEST_SKIPPED;
+#endif
 }
 
 REGISTER_TEST_COMMAND(stack_autotest, test_stack);

app/test/test_stack_perf.c

@@ -349,7 +349,11 @@ test_stack_perf(void)
 static int
 test_lf_stack_perf(void)
 {
+#if defined(RTE_STACK_LF_SUPPORTED)
 	return __test_stack_perf(RTE_STACK_F_LF);
+#else
+	return TEST_SKIPPED;
+#endif
 }
 
 REGISTER_TEST_COMMAND(stack_perf_autotest, test_stack_perf);

doc/guides/rel_notes/release_21_05.rst

@@ -329,6 +329,10 @@ API Changes
   ``policer_action_recolor_supported`` and ``policer_action_drop_supported``
   have been removed.
 
+* stack: Lock-free ``rte_stack`` no longer silently ignores push and pop when
+  it's not supported on the current platform. Instead ``rte_stack_create()``
+  fails and ``rte_errno`` is set to ``ENOTSUP``.
+
 
 ABI Changes
 -----------

lib/stack/rte_stack.c

@@ -64,9 +64,11 @@ rte_stack_create(const char *name, unsigned int count, int socket_id,
 
 #ifdef RTE_ARCH_64
 	RTE_BUILD_BUG_ON(sizeof(struct rte_stack_lf_head) != 16);
-#else
+#endif
+#if !defined(RTE_STACK_LF_SUPPORTED)
 	if (flags & RTE_STACK_F_LF) {
 		STACK_LOG_ERR("Lock-free stack is not supported on your platform\n");
+		rte_errno = ENOTSUP;
 		return NULL;
 	}
 #endif

lib/stack/rte_stack.h

@@ -89,7 +89,7 @@ struct rte_stack {
 
 /**
  * The stack uses lock-free push and pop functions. This flag is only
- * supported on x86_64 platforms, currently.
+ * supported on x86_64 or arm64 platforms, currently.
  */
 #define RTE_STACK_F_LF 0x0001
 
@@ -205,6 +205,7 @@ rte_stack_free_count(struct rte_stack *s)
  *    - EEXIST - a stack with the same name already exists
  *    - ENOMEM - insufficient memory to create the stack
  *    - ENAMETOOLONG - name size exceeds RTE_STACK_NAMESIZE
+ *    - ENOTSUP - platform does not support given flags combination.
  */
 struct rte_stack *
 rte_stack_create(const char *name, unsigned int count, int socket_id,

lib/stack/rte_stack_lf.h

@@ -13,6 +13,11 @@
 #else
 #include "rte_stack_lf_generic.h"
 #endif
+
+/**
+ * Indicates that RTE_STACK_F_LF is supported.
+ */
+#define RTE_STACK_LF_SUPPORTED
 #endif
 
 /**