Messages in this thread | | | Date | Fri, 4 Jul 2014 10:25:25 -0400 | From | Steven Rostedt <> | Subject | Re: [RFC][PATCH 1/3] ftrace/x86: Add dynamic allocated trampoline for ftrace_ops |
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On Fri, 04 Jul 2014 22:32:44 +0900 Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> wrote:
> (2014/07/04 5:07), Steven Rostedt wrote: > > + > > +void arch_ftrace_update_trampoline(struct ftrace_ops *ops) > > +{ > > + unsigned char *new; > > + unsigned long start_offset; > > + unsigned long call_offset; > > + unsigned long offset; > > + unsigned long ip; > > + int ret; > > + > > + if (ops->trampoline) { > > + /* > > + * The ftrace_ops caller may set up its own trampoline. > > + * In such a case, this code must not modify it. > > + */ > > + if (!(ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP)) > > + return; > > Just a question, what happen if the ftrace_ops caller sets up a trampoline which is > not compatible to the ftrace's trampoline, and the ftrace_ops conflicts on a IP with other > ftrace_ops? I guess in that case ftrace will use the loop callback on the IP, but since > the trampoline is not compatible, the result will not be same, is that right? :)
If the caller sets up a trampoline, it must not set the ALLOC_TRAMP flag. If you look at the comment about that flag it states this:
+ * ALLOC_TRAMP - A dynamic trampoline was allocated by the core code. + * The arch specific code sets this flag when it allocated a + * trampoline. This lets the arch know that it can update the + * trampoline in case the callback function changes. + * The ftrace_ops trampoline can be set by the ftrace users, and + * in such cases the arch must not modify it. Only the arch ftrace + * core code should set this flag.
That last line is important. Only the arch ftrace code (the one that may modify it with arch_ftrace_update_trampoline should set the ALLOC_TRAMP flag. That's how it knows if it can modify it or not.
The function_graph tracer sets up its own trampoline. Although it needs to go through some hoops there because it shares the ftrace_ops with the function tracer. Thus, it has to store the trampoline and this flag before registering ftrace ops, and then it has to restore it when it unregisters.
-- Steve
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