On Mon, Jan 14, 2013 at 10:40 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > On Fri, Jan 11, 2013 at 09:01:01AM -0800, David Daney wrote: >> >> I thought all CPUs were in stop_machine() when the modifications >> were done, so that there is no issue with multi-word instruction >> patching. >> >> Am I wrong about this? >> >> So really I think you can do two NOP just as easily. > > The problem with double NOPs is that it can only work if there's no > problem executing one nop and a non NOP. Which I think is an issue here. > > > If you have something like: > > bl _mcount > addiu sp,sp,-8 > > And you convert that to: > > nop > nop > > Now if you convert that back to: > > bl ftrace_caller > addiu sp,sp,-8 > > then you can have an issue if the task was preempted after that first > nop. Because stop_machine() doesn't wait for tasks to exit kernel space. > If you have a CONFIG_PREEMPT kernel, a task can be sleeping anywhere. > Thus you have a task execute the first nop, get preempted. You update > the code to be: > > bl ftrace_caller > addiu sp,sp,-8 > > When that task gets scheduled back in, it will act like it just > executed: > > nop > addiu sp,sp,-8 > > Which is the problem you're trying to solve in the first place. > > Now that said, There's no reason we need that addiu sp,sp,-8 there. > That's just what the mips defined mcount requires. But as you can see > above, with dynamic ftrace, the defined mcount is only called at boot > up, and never again. That means at boot up you can convert to: > > nop > nop > > and then when you enable tracing just convert it to: > > bl ftrace_caller > nop > > There's nothing that states what the ftrace caller must be. We can have > it do a proper stack update. That is, only at boot up do we need to > handle the defined mcount. After that, those instructions are just place > holders for our own algorithms. If the addiu was needed for the defined > mcount, there's no reason to keep it for our own ftrace_caller. > > Would that work? > > -- Steve >
