linux-kernel - Re: [patch/rfc] eventfd semaphore-like behavior

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Date:	Thu, 5 Feb 2009 13:26:32 +1300
From:	Michael Kerrisk <mtk.manpages@...glemail.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	davidel@...ilserver.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, linux-api@...r.kernel.org
Subject: Re: [patch/rfc] eventfd semaphore-like behavior

On Thu, Feb 5, 2009 at 1:18 PM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Thu, 5 Feb 2009 12:59:07 +1300
> Michael Kerrisk <mtk.manpages@...glemail.com> wrote:
>
>> >> > > > 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?
>> >>
>> >
>> > Dunno.  Probably try the syscall and see if it returned -EINVAL.  Does
>> > that work in this case?
>>
>> As youll have seen by now, Ulrich and I noted that it works.
>
> I think you means "should work" ;)
>
> We're talking about this, yes?
>
> SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
> {
>        int fd;
>        struct eventfd_ctx *ctx;
>
>        /* Check the EFD_* constants for consistency.  */
>        BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
>        BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);
>
>        if (flags & ~(EFD_CLOEXEC | EFD_NONBLOCK))
>                return -EINVAL;
>
> That looks like it should work to me.

Yes, that's what we're talking about, plus a similar check that Ulrih
added in the case that glibc's eventfd() falls back to sy_event().

Cheers,

Michael

--
Michael Kerrisk
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