lib/stackdepot: annotate racy pool_index accesses

Accesses to pool_index are protected by pool_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.

[[e-mail]: s/pool_index/pool_index_cached/ in stack_depot_fetch()]
Link: https://lkml.kernel.org/r/95[e-mail]
Link: https://lkml.kernel.org/r/35[e-mail]
Signed-off-by: Andrey Konovalov <[e-mail]>
Reviewed-by: Alexander Potapenko <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

lib/stackdepot.c

···
 			return NULL;
 		}
 
-		/* Move on to the next pool. */
-		pool_index++;
+		/*
+		 * Move on to the next pool.
+		 * WRITE_ONCE pairs with potential concurrent read in
+		 * stack_depot_fetch().
+		 */
+		WRITE_ONCE(pool_index, pool_index + 1);
 		pool_offset = 0;
 		/*
 		 * If the maximum number of pools is not reached, take note
···
 			       unsigned long **entries)
 {
 	union handle_parts parts = { .handle = handle };
+	/*
+	 * READ_ONCE pairs with potential concurrent write in
+	 * depot_alloc_stack.
+	 */
+	int pool_index_cached = READ_ONCE(pool_index);
 	void *pool;
 	size_t offset = parts.offset << DEPOT_STACK_ALIGN;
 	struct stack_record *stack;
···
 	if (!handle)
 		return 0;
 
-	if (parts.pool_index > pool_index) {
+	if (parts.pool_index > pool_index_cached) {
 		WARN(1, "pool index %d out of bounds (%d) for stack id %08x\n",
-			parts.pool_index, pool_index, handle);
+			parts.pool_index, pool_index_cached, handle);
 		return 0;
 	}
 	pool = stack_pools[parts.pool_index];