linux-kernel - Re: asterisk hangs with RT priority

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Message-Id: <1230191527.9487.262.camel@twins>
Date:	Thu, 25 Dec 2008 08:52:07 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	dhaval@...ux.vnet.ibm.com, kaber@...sh.net, mingo@...e.hu,
	linux-kernel@...r.kernel.org, bharata@...ux.vnet.ibm.com,
	vatsa@...ux.vnet.ibm.com, balbir@...ibm.com
Subject: Re: asterisk hangs with RT priority

On Thu, 2008-12-25 at 09:07 +1100, Herbert Xu wrote:
> Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> >
> > So you have uid-group scheduling and RT-group scheduling enabled (a
> > feature that's experimental for real and has never been enabled by
> > default), looking at the sys_setuid() code, the real uid change is done
> > by switch_uid() and that doesn't have a failable scheduler hook.
> 
> An experimental marking is no excuse for being broken.

True, and we strive to fix them -- so thanks for finding this one.

> > The thing is, I suspect the uid you switch to doesn't have a RT runtime
> > quota configured, therefore the RT task that gets placed in it by
> > switch_uid() doesn't get to run.
> > 
> > [ Please read Documentation/scheduler/sched-rt-group.txt
> >  when you enable RT group scheduling ]
> 
> This seems broken to me.  The only way for a process to get into
> RR mode is if it had been set by someone with the right privileges
> or if it was inherited from its parent.
> 
> So having a default where such a process stops executing altogether
> after performing a setuid is wrong.

True, therefore the setuid must fail.

> The default should be to give each user a non-zero allotment.

That's sadly impossible. The thing is, you cannot over-commit this time
-- nor change an active configuration. So the only possibility left is a
default of 0.

> > The correct thing would be for switch_uid() (or set_user) to fail with
> > -EINVAL, much like cpu_cgroup_can_attach() currently does for cgroup
> > grouping.
> > 
> > After that it demonstrates a bug in your test program, which fails to
> > check errors ;-)
> 
> Well the program which this was based on, asterisk does check for
> errors on setuid.  However, even if setuid did return an error this
> isn't much better than the status quo since the user will be left
> with the question "why on earth is asterisk failing on setuid?".

Maybe a more descriptive error like -ENOTIME ?

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