linux-kernel - Re: [PATCH v5 2/7] arm64: introduce interfaces to hotpatch kernel and module code

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Date:	Wed, 6 Nov 2013 10:41:10 +0000
From:	Will Deacon <will.deacon@....com>
To:	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>
Cc:	Jiang Liu <liuj97@...il.com>, Steven Rostedt <rostedt@...dmis.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Jiang Liu <jiang.liu@...wei.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 2/7] arm64: introduce interfaces to hotpatch kernel
 and module code

On Mon, Nov 04, 2013 at 03:12:56PM +0000, Sandeepa Prabhu wrote:
> On 3 November 2013 23:55, Jiang Liu <liuj97@...il.com> wrote:
> > On 10/30/2013 08:12 AM, Will Deacon wrote:
> >> On Fri, Oct 18, 2013 at 04:19:56PM +0100, Jiang Liu wrote:
> >>> +    atomic_set(&text_patch_id, smp_processor_id());
> >>> +    ret = stop_machine(aarch64_insn_patch_text_cb, &patch, cpu_online_mask);
> >>
> >> Instead of doing this, why not instead call aarch64_insn_patch_text_nosync
> >> inline, then call kick_all_cpus_sync immediately afterwards, without the
> >> need to stop_machine.
> > Sandeepa, who is working on kprobe for ARM64, needs the stop_machine()
> > mechanism to synchronize all online CPUs, so it's a preparation for
> > kprobe.
> 
> I had published kprobes patches for ARM64:
> http://lwn.net/Articles/570648/  and using your patcset (v3) for
> patching support, it works so far.
> I CCed you on my RFC but unfortunately to your huawei email not the gmail.
> 
> I can give a try with kick_all_cpus_sync but wanted to understand this
> a bit  detail on hows different from stop_machine and how this work.

My point was just that for nosync patching, the update to the instruction
stream is atomic with respect to instruction fetch, so stop_machine seems a
bit overkill. kick_all_cpus can be used to ensure visibility of the new
instruction.

Jiang Liu seemed to imply that this isn't suitable for kprobes, but I would
like to know if/why that is the case.

Will
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