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Date:   Mon, 11 May 2020 15:48:24 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Lai Jiangshan <jiangshanlai@gmail.com>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        "Paul E. McKenney" <paulmck@kernel.org>, rcu <rcu@vger.kernel.org>,
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        Andrew Morton <akpm@linux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
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        Frederic Weisbecker <fweisbec@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Masami Hiramatsu <mhiramat@kernel.org>
Subject: Re: [PATCH RFC tip/core/rcu 09/16] rcu-tasks: Add an RCU-tasks rude
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On Sun, 10 May 2020 17:59:27 +0800
Lai Jiangshan <jiangshanlai@gmail.com> wrote:

> Hello
> 
> I think adding a small number of instructions to preempt_schedule_irq()
> is sufficient to create the needed protected region between the start
> of a function and the trampoline body.
> 
> preempt_schedule_irq() {
> +  if (unlikely(is_trampoline_page(page_of(interrupted_ip)))) {
> +      return; // don't do preempt schedule
> +
> +  }
>   preempt_schedule_irq() original body
> }
> 
> // generated on trampoline pages
> trace_trampoline() {
>    preempt_disable();
>    trace_trampoline body
>    jmp preempt_enable_traced(clobbers)
> }
> 
> asm(kernel text):
> preempt_enable_traced:
>    preempt_enable_notrace();
>    restore cobblers
>    return(the return ip on the stack is traced_function_start_code)
> 
> 
> If the number of instructions added in preempt_schedule_irq() and
> the complexity to make trampoline ip detectable(is_trampoline_page(),
> or is_trampoline_range()) are small, and tasks_rcu is rendered useless,
> I think it will be win-win.

To make this even more complex, with ftrace direct callers (used by bpf to
define their own non ftrace trampoline), if a direct call is on the same
location as a ftrace caller, we have something like this:

ftrace_caller:
	save_regs
	call ftrace_ops_list_func
	cmp ORIG_RAX
	jnz do_direct
	restore_regs
	ret

do_direct:
	mov ORIG_RAX to return
	restore_regs
	ret

What the above is basically doing, is that the ftrace_ops_list_func() will
call the ftrace callbacks, but also a special callback to handle the direct
that is also registered to that same location. The direct callback will
place the address of the direct trampoline into ORIG_RAX. Then on return
from ftrace_ops_list_func(), it will jump directly to the direct caller.

To implement what you are proposing, you have to have a way to keep
preemption off between the setting of ORIG_RAX and the jump to the direct
caller (which would require its own preempt_disable() section). But if we
preempt between the two, the direct trampoline may disappear and then this
code will jump to it.

-- Steve