[PATCH v1 2/4] fbarray: fix incorrect lookbehind behavior

[Anatoly Burakov <anatoly.burakov@intel.com>]

Currently, whenever first bit of current index mask is set (meaning, there is
potentially a run starting at the start of the mask), lookbehind loop is
entered. In that loop, if the last bit of lookbehind mask is not set, the
lookbehind is stopped, and the current lookbehind mask index is assigned to
current index mask. However, because at that point we are inside a while-loop
that decrements current index mask after each iteration, this results in
erroneous mask index decrement.

Fix lookbehind to avoid erroneous decrement, and add corresponding unit test.

Fixes: e1ca5dc86226 ("fbarray: add reverse finding of chunk")
Cc: stable@dpdk.org

Signed-off-by: Vipin P R <vipinp@vmware.com>
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
---
 app/test/test_fbarray.c             | 24 ++++++++++++++++++++++++
 lib/eal/common/eal_common_fbarray.c |  3 ++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/app/test/test_fbarray.c b/app/test/test_fbarray.c
index bf89b99e5b..147d6e2a07 100644
--- a/app/test/test_fbarray.c
+++ b/app/test/test_fbarray.c
@@ -111,6 +111,14 @@ static int lookahead_test_setup(void)
 	return init_array();
 }
 
+static int lookbehind_test_setup(void)
+{
+	/* set index 63 as used */
+	param.start = 63;
+	param.end = 63;
+	return init_array();
+}
+
 static int test_invalid(void)
 {
 	struct rte_fbarray dummy;
@@ -732,6 +740,21 @@ static int test_lookahead(void)
 	return TEST_SUCCESS;
 }
 
+static int test_lookbehind(void)
+{
+	int ret, free_len = 2;
+
+	/* run regular test first */
+	ret = test_find();
+	if (ret != TEST_SUCCESS)
+		return ret;
+
+	/* test if we can find free chunk while crossing mask boundary */
+	TEST_ASSERT_EQUAL(rte_fbarray_find_prev_n_free(&param.arr, param.start + 1, free_len),
+			param.start - free_len, "Free chunk index is wrong\n");
+	return TEST_SUCCESS;
+}
+
 static struct unit_test_suite fbarray_test_suite = {
 	.suite_name = "fbarray autotest",
 	.setup = autotest_setup,
@@ -746,6 +769,7 @@ static struct unit_test_suite fbarray_test_suite = {
 		TEST_CASE_ST(full_msk_test_setup, reset_array, test_find),
 		TEST_CASE_ST(empty_msk_test_setup, reset_array, test_empty),
 		TEST_CASE_ST(lookahead_test_setup, reset_array, test_lookahead),
+		TEST_CASE_ST(lookbehind_test_setup, reset_array, test_lookbehind),
 		TEST_CASES_END()
 	}
 };
diff --git a/lib/eal/common/eal_common_fbarray.c b/lib/eal/common/eal_common_fbarray.c
index 2680b34823..b4f0b0b0c3 100644
--- a/lib/eal/common/eal_common_fbarray.c
+++ b/lib/eal/common/eal_common_fbarray.c
@@ -512,7 +512,8 @@ find_prev_n(const struct rte_fbarray *arr, unsigned int start, unsigned int n,
 				 * as well, so skip that on next iteration.
 				 */
 				ignore_msk = UINT64_MAX << need;
-				msk_idx = lookbehind_idx;
+				/* outer loop will decrement msk_idx so add 1 */
+				msk_idx = lookbehind_idx + 1;
 				break;
 			}
 
-- 
2.43.0