On Thu, Feb 5, 2009 at 12:27 PM, Davide Libenzi <davidel@xxxxxxxxxxxxxxx> wrote: > On Wed, 4 Feb 2009, Andrew Morton wrote: > >> On Wed, 4 Feb 2009 15:18:43 -0800 (PST) >> Davide Libenzi <davidel@xxxxxxxxxxxxxxx> wrote: >> >> > > > Simple test here: >> > > > >> > > > http://www.xmailserver.org/eventfd-sem.c >> > > > >> > > > >> > > > Signed-off-by: Davide Libenzi <davidel@xxxxxxxxxxxxxxx> >> > > > >> > > > +/* >> > > > + * 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? Maybe I missed something, but I think we're okay, aren't we? Viz: a) The glibc eventfd() wrapper invokes sys_eventfd2() (which allows a flags arg) if it is available, and otherwise fall back to sys_eventfd() (which does not support a flags arg). b) If glibc falls back to sys_eventfd(), then it knows to reject non-zero flags. (The glibc wrapper already does this.) c) If the old sys_eventfd2() is given a flag that it doesn't recognize, then it fails with EINVAL. (That check is already in the code.) So, userspace can determine whether EFD_SEMAPHORE is supported by not getting an EINVAL error. (Okay, this falls down for binaries that bypass glibc's eventfd() wrapper, but there are unlikely to be such binaries.) Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
