Messages in this thread | | | Date | Mon, 11 May 2020 15:48:24 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH RFC tip/core/rcu 09/16] rcu-tasks: Add an RCU-tasks rude variant |
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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
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