Re: [PATCH 0/4] RFC: jump label - (tracepoint optimizations)

[Mathieu Desnoyers]

* 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.

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.

Mathieu

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Mathieu Desnoyers
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