[PATCH] selftests/bpf: Remove duplicate copies of the arena spinlock qnodes

[Changwoo Min]

bpf_arena_spin_lock.h defines its 64KB qnodes array in the header, so
every translation unit including it emits a copy. __weak makes them all
resolve to one instance, but bpftool gen object merges only the symbols
and concatenates each input's .addr_space.1 bytes, leaving the surplus
copies unreferenced in the linked object.

libarena links ten such units, so nine copies were dead weight (bytes):

  object                             before       after
  -----------------------------------------------------
  .addr_space.1 in libarena.bpf.o    676200       86376
  libarena.skel.h                   2100123      892371
  libarena_asan.skel.h              2641124     1466477

Declare qnodes in the header and let each program define it once:
libarena in src/common.bpf.c, and the arena_spin_lock test beside the
lock it guards.

Tested with test_progs -t arena_spin_lock and -t libarena.

Signed-off-by: Changwoo Min <changwoo@xxxxxxxxxx>
---
 .../selftests/bpf/libarena/include/bpf_arena_spin_lock.h   | 7 +------
 tools/testing/selftests/bpf/libarena/src/common.bpf.c      | 7 +++++++
 tools/testing/selftests/bpf/progs/arena_spin_lock.c        | 7 +++++++
 3 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/bpf/libarena/include/bpf_arena_spin_lock.h b/tools/testing/selftests/bpf/libarena/include/bpf_arena_spin_lock.h
index ae6b72d15bb6..71d9db610263 100644
--- a/tools/testing/selftests/bpf/libarena/include/bpf_arena_spin_lock.h
+++ b/tools/testing/selftests/bpf/libarena/include/bpf_arena_spin_lock.h
@@ -103,12 +103,7 @@ struct arena_qnode {
 #define _Q_LOCKED_VAL		(1U << _Q_LOCKED_OFFSET)
 #define _Q_PENDING_VAL		(1U << _Q_PENDING_OFFSET)
 
-/*
- * The qnodes are marked __weak so we can define them in the header
- * while still ensuring all compilation units use the same struct
- * instance.
- */
-struct arena_qnode __weak __arena __hidden qnodes[_Q_MAX_CPUS][_Q_MAX_NODES];
+extern struct arena_qnode __arena __hidden qnodes[_Q_MAX_CPUS][_Q_MAX_NODES];
 
 static inline u32 encode_tail(int cpu, int idx)
 {
diff --git a/tools/testing/selftests/bpf/libarena/src/common.bpf.c b/tools/testing/selftests/bpf/libarena/src/common.bpf.c
index 50be57213dfb..06481f2a2892 100644
--- a/tools/testing/selftests/bpf/libarena/src/common.bpf.c
+++ b/tools/testing/selftests/bpf/libarena/src/common.bpf.c
@@ -8,6 +8,13 @@ const volatile u32 zero = 0;
 
 struct buddy __arena buddy;
 
+/*
+ * Storage for the queue nodes declared by bpf_arena_spin_lock.h. Each program
+ * linking the arena spinlock provides exactly one definition, so that the array
+ * is emitted once rather than once per translation unit.
+ */
+struct arena_qnode __arena __hidden qnodes[_Q_MAX_CPUS][_Q_MAX_NODES];
+
 int arena_fls(__u64 word)
 {
 	if (!word)
diff --git a/tools/testing/selftests/bpf/progs/arena_spin_lock.c b/tools/testing/selftests/bpf/progs/arena_spin_lock.c
index cf7cda79c16c..92e75ec3844c 100644
--- a/tools/testing/selftests/bpf/progs/arena_spin_lock.c
+++ b/tools/testing/selftests/bpf/progs/arena_spin_lock.c
@@ -23,6 +23,13 @@ int cs_count;
 #if defined(ENABLE_ATOMICS_TESTS) && defined(__BPF_FEATURE_ADDR_SPACE_CAST)
 arena_spinlock_t __arena lock;
 int test_skip = 1;
+
+/*
+ * Storage for the queue nodes declared by bpf_arena_spin_lock.h. Each program
+ * linking the arena spinlock provides exactly one definition; libarena's lives
+ * in libarena/src/common.bpf.c.
+ */
+struct arena_qnode __arena __hidden qnodes[_Q_MAX_CPUS][_Q_MAX_NODES];
 #else
 int test_skip = 2;
 #endif
-- 
2.55.0