Steven Rostedt wrote:

On Thu, 27 Jun 2019 20:58:20 +0530
"Naveen N. Rao" [off-list ref] wrote:

quoted

quoted

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.

<snip>

quoted

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 
synchronization.

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:

	<func>:
	mflr	r0 <-- preempt here
	bl	_mcount

	<func>:
	mflr	r0
	nop

And this is fine regardless.

OK, Reviewed-by: Steven Rostedt (VMware) [off-list ref]

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 
things.


- Naveen