Re: [PATCH v3 10/15] futex: Add sys_futex_requeue()

On Fri, Sep 22, 2023 at 11:35:03AM +0200, Ingo Molnar wrote:

* peterz@infradead.org [off-list ref] wrote:

quoted

--- linux-2.6.orig/kernel/futex/syscalls.c
+++ linux-2.6/kernel/futex/syscalls.c

@@ -396,6 +396,44 @@ SYSCALL_DEFINE6(futex_wait,
 	return ret;
 }
 
+/*
+ * sys_futex_requeue - Requeue a waiter from one futex to another
+ * @waiters:	array describing the source and destination futex
+ * @flags:	unused
+ * @nr_wake:	number of futexes to wake
+ * @nr_requeue:	number of futexes to requeue
+ *
+ * Identical to the traditional FUTEX_CMP_REQUEUE op, except it is part of the
+ * futex2 family of calls.
+ */
+
+SYSCALL_DEFINE4(futex_requeue,
+		struct futex_waitv __user *, waiters,
+		unsigned int, flags,
+		int, nr_wake,
+		int, nr_requeue)
+{
+	struct futex_vector futexes[2];
+	u32 cmpval;
+	int ret;
+
+	if (flags)
+		return -EINVAL;

Small detail, but isn't -ENOSYS the canonical error code for functionality 
not yet implemented - which the unused 'flags' ABI is arguably?

-EINVAL is for recognized but incorrect parameters, such as:

IIUC 'unknown flag' falls into the -EINVAL return category. Here we
happen to have no known flags, but that should not matter.