Re: [PATCH 0/2] sunrpc: more reliable detection of running gssd

[NeilBrown <neilb@suse.de>]

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On Tue, 12 Nov 2013 10:21:40 -0500 Steve Dickson <SteveD@redhat.com> wrote:

> On 12/11/13 08:00, Jeff Layton wrote:
> > We've gotten a lot of complaints recently about the 15s delay when
> > doing a sec=sys mount without gssd running.
> > 
> > A large part of the problem is that the kernel isn't able to reliably
> > detect when rpc.gssd is running. What we currently have is a
> > gssd_running flag that is initially set to 1. When an upcall times out,
> > that gets set to 0, and subsequent upcalls get a much shorter timeout
> > (1/4s instead of 15s). It's reset back to '1' when a pipe is reopened.
> > 
> > The approach of using a flag like this is pretty inadequate. First, it
> > doesn't eliminate the long delay on the initial upcall attempt. Also,
> > if gssd spontaneously dies, then the flag will still be set to 1 until
> > the next upcall attempt times out. Finally, it currently requires that
> > the pipe be reopened in order to reset the flag back to true.
> > 
> > This patchset replaces that flag with a more reliable mechanism for
> > detecting when gssd is running. When rpc_pipefs is mounted, it creates a
> > new "dummy" pipe that gssd will naturally find and hold open. We'll
> > never send an upcall down this pipe, and writing to it always fails.
> > But, since we can detect when something is holding it open, we can use
> > that to determine whether gssd is running.
> > 
> > The current patch just uses this mechanism to replace the gssd_running
> > flag with this new mechanism. This shortens the long delay when mounting
> > without gssd running, but does not silence these warnings:
> > 
> >     RPC: AUTH_GSS upcall timed out.
> >     Please check user daemon is running.
> > 
> > I'm willing to add a patch to do that, but I'm a little unclear on the
> > best way to do so. Those messages are generated by the auth_gss code. We
> > probably do want to print them if someone mounted with sec=krb5, but
> > suppress them when mounting with sec=sys.
> > 
> > Do we need to somehow pass down that intent to auth_gss? Another idea
> > would be to call gssd_running() from the nfs mount code and use that to
> > determine whether to try and use krb5 at all...
> > 
> > Discuss!
> I've just verified that a mount, with these patches, takes about 
> 1.2 seconds when rpc.gssd is not running.... With rpc.gssd it 
> take about .2 seconds.
> 
> Tested-by: Steve Dickson <steved@redhat.com>
>

Still sounds like about one second too long.

In that patch I see:

 	timeout = 15 * HZ;
-	if (!sn->gssd_running)
+	if (!gssd_running(sn))
 		timeout = HZ >> 2;

Given that "!gssd_running(sn)" is now certain knowledge rather than a hint,
can't we just skip the upcall and any timeout?
i.e.
 	timeout = 15 * HZ;
-	if (!sn->gssd_running)
+	if (!gssd_running(sn))
- 		timeout = HZ >> 2;
+		return -EACCES;

??

NeilBrown


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