Re: thoughts about exception-handling requirements for kprobes

[Richard J Moore <richardj_moore@uk.ibm.com>]

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"


Richard

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> Prasanna S Panchamukhi
> Linux Technology Center
> India Software Labs, IBM Bangalore
> Email: prasanna@in.ibm.com
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