[PATCH] posix: Add p{read,write}v2 RWF_NOWAIT flag (BZ#21738)

[Zack Weinberg]

On Mon, Jul 10, 2017 at 3:24 PM, Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
> Linux 4.12 (b745fafaf70c0a98a2e1e7ac8cb14542889ceb0e) adds a new
> p{read,write}v2 flag RWF_NOWAIT.  This patch adds it for linux
> uio-ext.h header along with RWF_SUPPORTED (a mask with all supported
> flags).

LGTM with a couple of notes on the documentation:

> +@item RWF_NOWAIT
> +Set @code{preadv2} to return -EAGAIN if operation would block.

This is awkward grammar.  Also, this function sets errno, doesn't it?
"Return -EAGAIN" is an intra-kernel convention, not what user space
sees, which is what the manual should document.

Suggest instead "Use nonblocking mode for this operation; that is,
this call to @code{preadv2} will fail and set @code{errno} to
@code{EAGAIN} if the operation would block."

> +@item RWF_SUPPORTED
> +Mask with all supported flags.

It's important to be clear that this does not necessarily reflect the
running kernel:  "Bitmask of all @samp{RWF_*} flags that have so far
been defined.  This is a compile-time constant; the running kernel
does not necessarily support all of the flags defined in this header.
New flags were most recently added in Linux 4.12."

> +#define RWF_NOWAIT     0x00000008 /* per-IO, return -EAGAIN if operation
> +                                     would block */

Same as above: "per-IO nonblocking mode" would be sufficient here, I think.

zw