[PATCH v2 4/7] powerpc/ftrace: Additionally nop out the preceding mflr with -mprofile-kernel
Naveen N. Rao
naveen.n.rao at linux.vnet.ibm.com
Mon Jul 1 18:51:17 AEST 2019
Steven Rostedt wrote:
> On Thu, 27 Jun 2019 20:58:20 +0530
> "Naveen N. Rao" <naveen.n.rao at linux.vnet.ibm.com> wrote:
>> > But interesting, I don't see a synchronize_rcu_tasks() call
>> > there.
>> We felt we don't need it in this case. We patch the branch to ftrace
>> with a nop first. Other cpus should see that first. But, now that I
>> think about it, should we add a memory barrier to ensure the writes get
>> ordered properly?
> Do you send an ipi to the other CPUs. I would just to be safe.
>> We are handling this through ftrace_replace_code() and
>> __ftrace_make_call_prep() below. For FTRACE_UPDATE_MAKE_CALL, we patch
>> in the mflr, followed by smp_call_function(isync) and
>> synchronize_rcu_tasks() before we proceed to patch the branch to ftrace.
>> I don't see any other scenario where we end up in
>> __ftrace_make_nop_kernel() without going through ftrace_replace_code().
>> For kernel modules, this can happen during module load/init and so, I
>> patch out both instructions in __ftrace_make_call() above without any
>> Am I missing anything?
> No, I think I got confused ;-), it's the patch out that I was worried
> about, but when I was going through the scenario, I somehow turned it
> into the patching in (which I already audited :-p). I was going to
> reply with just the top part of this email, but then the confusion
> started :-/
> OK, yes, patching out should be fine, and you already covered the
> patching in. Sorry for the noise.
> Just to confirm and totally remove the confusion, the patch does:
> mflr r0 <-- preempt here
> bl _mcount
> mflr r0
> And this is fine regardless.
> OK, Reviewed-by: Steven Rostedt (VMware) <rostedt at goodmis.org>
Thanks for confirming! We do need an IPI to be sure, as you pointed out
above. I will have the patching out take the same path to simplify
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