Re: thoughts about exception-handling requirements for kprobes

[Prasanna S Panchamukhi <prasanna at in dot ibm dot com>]

On Mon, Mar 20, 2006 at 08:44:00AM +0000, Richard J Moore wrote:
> 
> 
> 
> 
> systemtap-owner@sourceware.org wrote on 19/03/2006 17:24:54:
> 
> > On Fri, Mar 17, 2006 at 01:50:57PM -0800, Keshavamurthy Anil S wrote:
> > > On Thu, Mar 09, 2006 at 07:57:18AM -0800, Richard J Moore wrote:
> > > >
> > > >    I've been thinking about the need for exception-handling and how
> the
> > > >    current implementation has become a little muddled.
> > >
> > > Here is my thinking on this kprobe fault handling...
> > > Ideally we want the ability to recover from all
> > > the page faults happening from either pre-handler
> > > or happening from post-handler transparently in the
> > > same way as the normal kernel would recover from
> > > do_page_fault() function. In order for this to happen,
> > > I think we should not be calling pre-handler/post-handler
> > > by disabling preempt which is a major design change.
> > > Also in the current code if fixup_exception() fails to
> > > fixup the exception then falling back on the normal
> > > do_page_fault() is a bad thing with preempt disabled.
> > >
> > > I was thinking on this issue for the past several days
> > > and I believe that currently we are disabling preempt
> > > before calling pre/post handler, because we don;t
> > > want the process to get migrated to different CPU
> > > and we don't want another process to be scheduled
> > > while we are servicing kprobe as the newly scheduled
> > > process might trigger another probe and we don;t
> > > have space to save the kprobe control block(kprobe_ctlbk)
> > > info, because we save kprobe_ctlbk in the per cpu structure.
> > >
> > > If we move this saving kprobe_ctlbk to task struct then
> > > I think we will have the ability to call pre/post-handler
> > > without having to disable preempt and their by any faults
> > > happening from either pre/post handler can recover transparently
> > > in the same way as the normal kernel would recover.
> > >
> >
> > Kprobes user-specified pre/post handler are called within
> > the interrupt context and if we allow page faults while within
> 
> Clarify what you mean by "allow"
> 
> > user-specified pre/post handler, then it might sleep.
> 
> Clarify what you mean by "it"
> 

In simple words, system do_page_fault() can sleep and if we
allow page faults while within user-specified kprobes pre/post
handler, then the pre/post handler might sleep. So can the
user-specifed kprobes per/post handler sleep while within
interrupt context?

Thanks
Prasanna
-- 
Prasanna S Panchamukhi
Linux Technology Center
India Software Labs, IBM Bangalore
Email: prasanna@in.ibm.com
Ph: 91-80-51776329