Messages in this thread | | | Subject | Re: [PATCH 0/4] RFC: jump label - (tracepoint optimizations) | From | Steven Rostedt <> | Date | Thu, 10 Sep 2009 17:15:59 -0400 |
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On Mon, 2009-09-07 at 13:06 -0400, Mathieu Desnoyers wrote: > * Mathieu Desnoyers (mathieu.desnoyers@polymtl.ca) wrote: > > * Jason Baron (jbaron@redhat.com) 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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