Re: [PATCH 0/4] RFC: jump label - (tracepoint optimizations)
From: Steven Rostedt
Date: Thu Sep 10 2009 - 17:16:05 EST
On Mon, 2009年09月07日 at 13:06 -0400, Mathieu Desnoyers wrote:
>
* Mathieu Desnoyers (mathieu.desnoyers@xxxxxxxxxx) wrote:
>
> * Jason Baron (jbaron@xxxxxxxxxx) wrote:
>
[...]
>
> > Solution:
>
> >
>
> > In discussing this problem with Roland McGrath and Richard Henderson, we came
>
> > up with a new 'asm goto' statement that allows branching to a label. Thus, this
>
> > patch set introdues a 'STATIC_JUMP_IF()' macro as follows:
>
> >
>
> > #ifdef HAVE_STATIC_JUMP
>
> >
>
> > #define STATIC_JUMP_IF(tag, label, cond) \
>
> > asm goto ("1:" /* 5-byte insn */ \
>
> > P6_NOP5 \
>
>
>
> Hrm, be careful there. P6_NOP5 is not always a single instruction. If
>
> you are preempted in the middle of it, bad things could happen, even
>
> with stop_machine, if you iret in the middle the of the new jump
>
> instruction. It could cause an illegal instruction fault. You should use
>
> an atomic nop5. I think the function tracer already does, since I
>
> told Steven about this exact issue.
>
>
>
>
Just to clarify this statement:
>
>
P6_NOP5 happens to be an atomic nop, but nothing states this requirement
>
in arch/x86/include/asm/nops.h. Other 5-bytes nops are defined as
>
multiple instructions (e.g. 2 bytes + 3 bytes nops). So I recommend to
>
create a family of ATOMIC_P6_NOP5 (and other ATOMIC_*_NOP5 defines) to
>
document this atomicity requirement.
Although I agree that we probably should place a comment in that file, I
highly doubt anyone will change that. But who knows?
>
>
Ftrace could probably handle this more gracefully than it does at the
>
moment. It basically assumes that P6_NOP5 is atomic, and falls back on a
>
5-bytes jmp if it detects that P6_NOP5 faults.
>
>
That's coherent with the
>
"TODO: check the cpuid to determine the best nop."
>
>
present in x86 ftrace.c.
>
>
So, at the very least, if we rely on nops.h having a single-instruction
>
P6_NOP5 5 bytes nop, a comment to that effect should be added to nops.h.
I might as well go add one.
-- Steve
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