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    SubjectRe: [patch 05/10] Linux Kernel Markers - i386 optimized version
    On Fri, May 11, 2007 at 10:27:29AM +0530, Ananth N Mavinakayanahalli wrote:
    > On Thu, May 10, 2007 at 12:59:18PM -0400, Mathieu Desnoyers wrote:
    > > * Alan Cox (alan@lxorguk.ukuu.org.uk) wrote:
    >
    > ...
    > > > > * Third issue : Scalability. Changing code will stop every CPU on the
    > > > > system for a while. Compared to this, the int3-based approach will run
    > > > > through the breakpoint handler "if" one of the CPU happens to execute
    > > > > this code at the wrong time. The standard case is just an IPI (to
    > > >
    > > > If I read the errata right then patching in an int3 will itself trigger
    > > > the errata so anything could happen.
    > > >
    > > > I believe there are other safe sequences for doing code patching - perhaps
    > > > one of the Intel folk can advise ?
    >
    > IIRC, when the first implementation of what exists now as kprobes was
    > done (as part of the dprobes framework), this question did come up. I
    > think the conclusion was that the errata applies only to multi-byte
    > modifications and single-byte changes are guaranteed to be atomic.
    > Given int3 on Intel is just 1-byte, we are safe.
    >
    > > I'll let the Intel guys confirm this, I don't have the reference nearby
    > > (I got this information by talking with the kprobe team members, and
    > > they got this information directly from Intel developers) but the
    > > int3 is the one special case to which the errata does not apply.
    > > Otherwise, kprobes and gdb would have a big, big issue.
    >
    > Perhaps Richard/Suparna can confirm.

    I just tried digging up past discussions on this from Richard, about int3
    being safe

    http://sourceware.org/ml/systemtap/2005-q3/msg00208.html
    http://lkml.org/lkml/2006/9/20/30

    Regards
    Suparna

    >
    > Ananth

    --
    Suparna Bhattacharya (suparna@in.ibm.com)
    Linux Technology Center
    IBM Software Lab, India

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