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    SubjectRe: [patch/rfc] eventfd semaphore-like behavior
    On Wed, 4 Feb 2009, Andrew Morton wrote:

    > On Wed, 4 Feb 2009 15:18:43 -0800 (PST)
    > Davide Libenzi <davidel@xmailserver.org> wrote:
    >
    > > > > Simple test here:
    > > > >
    > > > > http://www.xmailserver.org/eventfd-sem.c
    > > > >
    > > > >
    > > > > Signed-off-by: Davide Libenzi <davidel@xmailserver.org>
    > > > >
    > > > > +/*
    > > > > + * CAREFUL: Check include/asm-generic/fcntl.h when defining
    > > > > + * new flags, since they might collide with O_* ones. We want
    > > > > + * to re-use O_* flags that couldn't possibly have a meaning
    > > > > + * from eventfd, in order to leave a free define-space for
    > > > > + * shared O_* flags.
    > > > > + */
    > > > > +#define EFD_SEMAPHORE (1 << 0)
    > > > > #define EFD_CLOEXEC O_CLOEXEC
    > > > > #define EFD_NONBLOCK O_NONBLOCK
    > > > >
    > > > > +#define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
    > > > > +#define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)
    > > >
    > > > How would you recommend that userspace determine whether its kernel
    > > > supports this feature, bearing in mind that someone might backport this
    > > > patch into arbitrarily earlier kernel versions?
    > > >
    > > > What should be userspace's fallback strategy if that support is not
    > > > present?
    > >
    > > #ifdef EFD_SEMAPHORE, maybe?
    >
    > That's compile-time. People who ship binaries will probably want
    > to find a runtime thing for back-compatibility.

    I dunno. How do they actually do when we add new flags, like the O_ ones?



    - Davide




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