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Date: Thu, 1 May 2014 16:27:46 -0500
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Andi Kleen <andi@firstfloor.org>
Cc: Seth Jennings <sjenning@redhat.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Ingo Molnar <mingo@redhat.com>, Jiri Slaby <jslaby@suse.cz>,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 0/2] kpatch: dynamic kernel patching
Message-ID: <20140501212746.GD3907@treble.redhat.com>
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On Thu, May 01, 2014 at 11:06:01PM +0200, Andi Kleen wrote:
> > When bar returns, would it skip foo and go straight back to foo's
> > caller?  If so, then it should be safe to patch foo after it jumps to
> > bar.
> 
> foo is no problem, you see it in the backtrace.
> But you don't see bar.

Sorry, I missed your point the first time.  Good question.

stop_machine schedules a high priority thread on each CPU, which means
every other task will be waiting in a schedule() call (assuming a
non-preemptible kernel).  In my local kernel, a quick grep of the
disassembly doesn't show any jumps to schedule:

  $ egrep 'j.*<.*>' vmlinux.asm |grep -v '\+' |grep schedule
  ffffffff816b89b5:	e9 e2 fe ff ff       	jmpq   ffffffff816b889c <retint_with_reschedule>
  ffffffff816b8cec:	75 1e                	jne    ffffffff816b8d0c <paranoid_schedule>

But yes, that would be a problem if any tail call jumps to schedule()
ever showed up.  We may need to detect that case in our patch generation
tooling and fail to create the patch module binary if the patch affects
a function which does this.

-- 
Josh