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    SubjectRe: [PATCH v2] epoll: add nsec timeout support
    On Mon, Nov 16, 2020 at 12:11 PM David Laight <David.Laight@aculab.com> wrote:
    >
    > From: Willem de Bruijn
    > > Sent: 16 November 2020 17:01
    > >
    > > On Mon, Nov 16, 2020 at 11:19 AM Matthew Wilcox <willy@infradead.org> wrote:
    > > >
    > > > On Mon, Nov 16, 2020 at 11:10:01AM -0500, Willem de Bruijn wrote:
    > > > > diff --git a/include/uapi/linux/eventpoll.h b/include/uapi/linux/eventpoll.h
    > > > > index 8a3432d0f0dc..f6ef9c9f8ac2 100644
    > > > > --- a/include/uapi/linux/eventpoll.h
    > > > > +++ b/include/uapi/linux/eventpoll.h
    > > > > @@ -21,6 +21,7 @@
    > > > >
    > > > > /* Flags for epoll_create1. */
    > > > > #define EPOLL_CLOEXEC O_CLOEXEC
    > > > > +#define EPOLL_NSTIMEO 0x1
    > > > >
    > > > > /* Valid opcodes to issue to sys_epoll_ctl() */
    > > > > #define EPOLL_CTL_ADD 1
    > > >
    > > > Not a problem with your patch, but this concerns me. O_CLOEXEC is
    > > > defined differently for each architecture, so we need to stay out of
    > > > several different bits when we define new flags for EPOLL_*. Maybe
    > > > this:
    > > >
    > > > /*
    > > > * Flags for epoll_create1. O_CLOEXEC may be different bits, depending
    > > > * on the CPU architecture. Reserve the known ones.
    > > > */
    > > > #define EPOLL_CLOEXEC O_CLOEXEC
    > > > #define EPOLL_RESERVED_FLAGS 0x00680000
    > > > #define EPOLL_NSTIMEO 0x00000001
    > >
    > > Thanks. Good point, I'll add that in v3.
    >
    > You could also add a compile assert to check that the flag is reserved.

    Like this?

    /* Check the EPOLL_* constant for consistency. */
    BUILD_BUG_ON(EPOLL_CLOEXEC != O_CLOEXEC);
    + BUILD_BUG_ON(EPOLL_NSTIMEO & EPOLL_RESERVED_FLAGS);

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