[PATCH 16/18] lib/stackdepot: annotate racy slab_index accesses

[andrey.konovalov@linux.dev]

From: Andrey Konovalov <andreyknvl@google.com>

Accesses to slab_index are protected by slab_lock everywhere except
in a sanity check in stack_depot_fetch. The read access there can race
with the write access in depot_alloc_stack.

Use WRITE/READ_ONCE() to annotate the racy accesses.

As the sanity check is only used to print a warning in case of a
violation of the stack depot interface usage, it does not make a lot
of sense to use proper synchronization.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 lib/stackdepot.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index f291ad6a4e72..cc2fe8563af4 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -269,8 +269,11 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
 			return NULL;
 		}
 
-		/* Move on to the next slab. */
-		slab_index++;
+		/*
+		 * Move on to the next slab.
+		 * WRITE_ONCE annotates a race with stack_depot_fetch.
+		 */
+		WRITE_ONCE(slab_index, slab_index + 1);
 		slab_offset = 0;
 		/*
 		 * smp_store_release() here pairs with smp_load_acquire() in
@@ -492,6 +495,8 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
 			       unsigned long **entries)
 {
 	union handle_parts parts = { .handle = handle };
+	/* READ_ONCE annotates a race with depot_alloc_stack. */
+	int slab_index_cached = READ_ONCE(slab_index);
 	void *slab;
 	size_t offset = parts.offset << DEPOT_STACK_ALIGN;
 	struct stack_record *stack;
@@ -500,9 +505,9 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
 	if (!handle)
 		return 0;
 
-	if (parts.slab_index > slab_index) {
+	if (parts.slab_index > slab_index_cached) {
 		WARN(1, "slab index %d out of bounds (%d) for stack id %08x\n",
-			parts.slab_index, slab_index, handle);
+			parts.slab_index, slab_index_cached, handle);
 		return 0;
 	}
 	slab = stack_slabs[parts.slab_index];
-- 
2.25.1