Re: [PATCH v3 2/6] futex2: Implement vectorized wait

Hi Gabriel, thanks for the feedback! A few clarifications:

Às 22:03 de 13/09/21, Gabriel Krisman Bertazi escreveu:

André Almeida [off-list ref] writes:

quoted

Add support to wait on multiple futexes. This is the interface
implemented by this syscall:

[...]

quoted

 
+/*
+ * Flags to specify the bit length of the futex word for futex2 syscalls.
+ * Currently, only 32 is supported.
+ */
+#define FUTEX_32		2

Why start at 2?

I was planning to do:

FUTEX_8		0
FUTEX_16	1
FUTEX_32	2
FUTEX_64	3

quoted

+
+/*
+ * Max numbers of elements in a futex_waitv array
+ */
+#define FUTEX_WAITV_MAX		128
+
+/**
+ * struct futex_waitv - A waiter for vectorized wait
+ * @val:	Expected value at uaddr
+ * @uaddr:	User address to wait on
+ * @flags:	Flags for this waiter
+ * @__reserved:	Reserved member to preserve data alignment. Should be 0.
+ */
+struct futex_waitv {
+	__u64 val;
+	__u64 uaddr;
+	__u32 flags;
+	__u32 __reserved;
+};

why force uaddr  to be __u64, even for 32-bit?  uaddr could be a (void*) for
all we care, no?  Also, by adding a reserved field, you are wasting 32
bits even on 32-bit architectures.

We do that to make the structure layout compatible with both entry
points, remove the need for special cast and duplicated code, as
suggested by Thomas and Arnd:

https://lore.kernel.org/lkml/87v94310gm.ffs@tglx/ (local)

https://lore.kernel.org/lkml/CAK8P3a0MO1qJLRkCH8KrZ3+=L66KOsMRmcbrUvYdMoKykdKoyQ@mail.gmail.com/ (local)