Re: [RFC][PATCH 03/14 v2] function_graph: Allow multiple users to attach to function graph

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 20, 2019 at 10:20:04AM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> Allow for multiple users to attach to function graph tracer at the same
> time. Only 16 simultaneous users can attach to the tracer. This is because
> there's an array that stores the pointers to the attached fgraph_ops. When a
> a function being traced is entered, each of the ftrace_ops entryfunc is
> called and if it returns non zero, its index into the array will be added to
> the shadow stack.
> 
> On exit of the function being traced, the shadow stack will contain the
> indexes of the ftrace_ops on the array that want their retfunc to be called.
> 
> Because a function may sleep for a long time (if a task sleeps itself), the
> return of the function may be literally days later. If the ftrace_ops is
> removed, its place on the array is replaced with a ftrace_ops that contains
> the stub functions and that will be called when the function finally
> returns.

But but but but.. why not add all the required bits to the shadow stack
in the first place and do away with the array entirely?

So on ret, just keep POP'ing until either the stack is empty or the
entry is for another function.