On Wed, Aug 29, 2018 at 01:41:14AM +0200, Marc-André Lureau wrote:
> Hi
>
> On Tue, Aug 28, 2018 at 6:01 PM Daniel P. Berrangé <berra...@redhat.com>
> wrote:
> >
> > On Fri, Jul 13, 2018 at 03:09:08PM +0200, Marc-André Lureau wrote:
> > > According to Daniel Berrange, fcntl() locks have better portable
> > > semantics than lockf().
> >
> > Specifically I was referring to this from 'man lockf':
> >
> > On Linux, lockf() is just an interface on top of fcntl(2) locking.
> > Many other systems implement lockf() in this way, but note that POSIX.1
> > leaves the relationship between lockf() and fcntl(2) locks unspecified.
> > A portable application should probably avoid mixing calls to these
> > interfaces.
> >
> > IOW, if its just a shim around fcntl() on many systems, it is clearer
> > if we just use fcntl() directly, as we then know how fcntl() locks will
> > behave if they're on a network filesystem like NFS.
> >
> > > Use an exclusive lock on the first byte with fcntl().
> > >
> > > Signed-off-by: Marc-André Lureau <marcandre.lur...@redhat.com>
> > > ---
> > > util/oslib-posix.c | 8 +++++++-
> > > 1 file changed, 7 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> > > index da1d4a3201..26b11490b9 100644
> > > --- a/util/oslib-posix.c
> > > +++ b/util/oslib-posix.c
> > > @@ -92,6 +92,11 @@ bool qemu_write_pidfile(const char *pidfile, Error
> > > **errp)
> > > {
> > > int pidfd;
> > > char pidstr[32];
> > > + struct flock lock = {
> > > + .l_type = F_WRLCK,
> > > + .l_whence = SEEK_SET,
> > > + .l_len = 1,
> > > + };
> >
> > For the same semantics as lockf we should use len == 0 (ie infinity)
>
> I went to look at how libvirt implements it. Interestingly, it uses
> start=0, len=1, and you added it in:
libvirt never used lockf though. Both are valid, I was just saying that
to be identical to what lockf did, it would use len == 0.
Regards,
Daniel
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