Date: Fri, 23 Jun 2023 13:53:20 -0700 From: Alan Somers <asomers@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: Should close() release locks atomically? Message-ID: <CAOtMX2iqaC3YUAPtxjLHPjujJUYuYX98YyhhFv7Jy5cb-QfvBg@mail.gmail.com> In-Reply-To: <ZJYFGa6oOVQxOqEk@kib.kiev.ua> References: <CAOtMX2jjKyj5JNkEXh7_UsEQLkuhpfmybht7gDwQR64BQzAXrQ@mail.gmail.com> <ZJX6c1LcDU97E7z8@kib.kiev.ua> <CAOtMX2jRkyv%2Bs21%2Bdcx16GjiEuVrF_c_X=%2B5r02hMLTrwxZ=Pw@mail.gmail.com> <ZJYFGa6oOVQxOqEk@kib.kiev.ua>
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On Fri, Jun 23, 2023 at 1:48 PM Konstantin Belousov <kostikbel@gmail.com> wrote: > > On Fri, Jun 23, 2023 at 01:11:34PM -0700, Alan Somers wrote: > > 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. > > > > This is a plain file on ZFS. > > Can you write a self-contained example, and check the same issue e.g. on > tmpfs? I just reproduced it on tmpfs. A minimal test case will take some more time...
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