Messages in this thread | | | Subject | Re: [PATCH] Linux Kernel Markers | From | Richard J Moore <> | Date | Wed, 20 Sep 2006 09:18:30 +0100 |
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systemtap-owner@sourceware.org wrote on 20/09/2006 02:08:52:
> Hi Alan, > > On Wed, Sep 20, 2006 at 01:08:45AM +0100, Alan Cox wrote: > > Ar Maw, 2006-09-19 am 13:54 -0400, ysgrifennodd Mathieu Desnoyers: > > > Very good idea.. However, overwriting the second instruction > with a jump could > > > be dangerous on preemptible and SMP kernels, because we never > know if a thread > > > has an IP in any of its contexts that would return exactly at > the middle of the > > > jump. > > > > No: on x86 it is the *same* case for all of these even writing an int3. > > One byte or a megabyte, > > > > You MUST ensure that every CPU executes a serializing instruction before > > it hits code that was modified by another processor. Otherwise you get > > CPU errata and the CPU produces results which vendors like to describe > > as "undefined". > > Are you referring to Intel erratum "unsynchronized cross-modifying code" > - where it refers to the practice of modifying code on one processor > where another has prefetched the unmodified version of the code. > > Thanks > Prasanna
In the special case of replacing an opcode with int3 that erratum doesn't apply. I know that's not in the manuals but it has been confirmed by the Intel microarchitecture group. And it's not reasonable to it to be any other way.
- - Richard J Moore IBM Advanced Linux Response Team - Linux Technology Centre MOBEX: 264807; Mobile (+44) (0)7739-875237 Office: (+44) (0)1962-817072
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