Re: Should close() release locks atomically?

On Fri, Jun 23, 2023 at 1:03 PM Konstantin Belousov <kostikbel@gmail.com> wrote:
>
> On Fri, Jun 23, 2023 at 12:00:36PM -0700, Alan Somers wrote:
> > The close() syscall automatically releases locks.  Should it do so
> > atomically or is a delay permitted?  I can't find anything in our man
> > pages or the open group specification that says.
> >
> > The distinction matters when using O_NONBLOCK.  For example:
> >
> > fd = open(..., O_DIRECT | O_EXLOCK | O_NONBLOCK); //succeeds
> > // do some I/O
> > close(fd);
> > fd = open(..., O_DIRECT | O_EXLOCK | O_NONBLOCK); //fails with EAGAIN!
> >
> > I see this error frequently on a heavily loaded system.  It isn't a
> > typical thread race though; ktrace shows that only one thread tries to
> > open the file in question.  From the ktrace, I can see that the final
> > open() comes immediately after the close(), with no intervening
> > syscalls from that thread.  It seems that close() doesn't release the
> > lock right away.  I wouldn't notice if I weren't using O_NONBLOCK.
> >
> > Should this be considered a bug?  If so I could try to come up with a
> > minimal test case.  But it's somewhat academic, since I plan to
> > refactor the code in a way that will eliminate the duplicate open().
> What type of the object is behind fd?  O_NONBLOCK affects open itself.
> We release flock after object close method, but before close(2) returns.

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