Re: [for-next][PATCH 3/4] ftrace/x86: Allow !CONFIG_PREEMPT dynamic ops to use allocated trampolines

[Borislav Petkov <bp@alien8.de>]

On Mon, Oct 27, 2014 at 02:27:05PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
> 
> When the static ftrace_ops (like function tracer) enables tracing, and it
> is the only callback that is referencing a function, a trampoline is
> dynamically allocated to the function that calls the callback directly
> instead of calling a loop function that iterates over all the registered
> ftrace ops (if more than one ops is registered).
> 
> But when it comes to dynamically allocated ftrace_ops, where they may be
> freed, on a CONFIG_PREEMPT kernel there's no way to know when it is safe
> to free the trampoline. If a task was preempted while executing on the
> trampoline, there's currently no way to know when it will be off that
> trampoline.
> 
> But this is not true when it comes to !CONFIG_PREEMPT. The current method
> of calling schedule_on_each_cpu() will force tasks off the trampoline,
> becaues they can not schedule while on it (kernel preemption is not
> configured). That means it is safe to free a dynamically allocated
> ftrace ops trampoline when CONFIG_PREEMPT is not configured.
> 
> Cc: H. Peter Anvin <hpa@linux.intel.com>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Acked-by: Borislav Petkov <bp@suse.de>

-- 
Regards/Gruss,
    Boris.

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