Messages in this thread |  | | | Date | Sun, 23 Nov 2008 20:35:54 +0100 | | From | Ingo Molnar <> | | Subject | Re: [PATCH] tracing/function-return-tracer: don't trace kfree while it frees the return stack |
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* Steven Rostedt <rostedt@goodmis.org> wrote:
> > On Sun, 23 Nov 2008, Ingo Molnar wrote: > > > > > > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c > > > index 90d99fb..ffff7ec 100644 > > > --- a/kernel/trace/ftrace.c > > > +++ b/kernel/trace/ftrace.c > > > @@ -1628,8 +1628,9 @@ void ftrace_retfunc_init_task(struct task_struct *t) > > > > > > void ftrace_retfunc_exit_task(struct task_struct *t) > > > { > > > - kfree(t->ret_stack); > > > + struct ftrace_ret_stack *ret_stack = t->ret_stack; > > > t->ret_stack = NULL; > > > + kfree(ret_stack); > > > > heh, nice one :) > > > > note that we also need to keep gcc from reordering things here (no > > matter how unlikely in this particular case). > > I first thought that too, but thinking about it, if gcc does do that, then > it will break the logic for a correct C program. > > t is passed in as a pointer, then it modifies the contents of t > (which could be a global pointer), then it calls a external > function, that might also reference the global pointer. > > This means that if it were to reorder the two, it would break C, > because the compiler can not assume that the called function will > read the global pointer either. > > In other words, the compiler should not need to worry about SMP or > modifications done by interrupts or other threads. But the compiler > should always preserve the order that is assumed by a single > context.
Correct, but this assumes that kfree is a C function. Which it might not necessarily be: it could be optimized via an inline in certain cases, etc. It's best to document such cases explicitly.
In any case, the real solution is what i suggested in the previous mail, to do the freeing from the task-struct freeing path in kernel/fork.c:free_task() - that has other advantages as well.
Ingo
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