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

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Date:	Mon, 30 Aug 2010 07:22:57 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Michael Kerrisk <mtk.manpages@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-api@...r.kernel.org
Subject: Re: [patch/rfc] eventfd semaphore-like behavior

On Mon, 30 Aug 2010, Michael Kerrisk wrote:

> On Thu, Feb 5, 2009 at 2:02 AM, Davide Libenzi <davidel@...ilserver.org> wrote:
> > On Thu, 5 Feb 2009, Michael Kerrisk wrote:
> >
> >> > 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.
> >>
> >> > If so, it would be sensible to mention this in
> >> > the description somewhere as the approved probing method and to
> >> > maintain it.
> >>
> >> I'll add something to the man page, as this patch progresses.
> >
> > I see we already have stuff like this inside the man pages:
> >
> > O_CLOEXEC (Since Linux 2.6.23)
> >          Enable the close-on-exec flag for the new file descriptor.
> >          ...
> >
> > Maybe a similar note for the new flag?
> 
> It took a while, but here's the new text in the eventfd.2 (will be in
> man-pages-2.36). Could you please ACK, Davide?
> 
>        EFD_SEMAPHORE (since Linux 2.6.30)
>               Provide semaphore-like semantics  for  reads  from
>               the new file descriptor.  See below.
> ...
>               *  If EFD_SEMAPHORE  was  not  specified  and  the
>                  eventfd  counter  has  a  nonzero value, then a
>                  read(2) returns 8 bytes containing that  value,
>                  and the counter's value is reset to zero.
> 
>               *  If  EFD_SEMAPHORE was specified and the eventfd
>                  counter has a nonzero  value,  then  a  read(2)
>                  returns 8 bytes containing the value 1, and the
>                  counter's value is decremented by 1.

Looks fine to me.


- Davide