Re: [PATCH RFC tip/core/rcu 09/16] rcu-tasks: Add an RCU-tasks rude variant

[Paul E. McKenney]

On Mon, Mar 16, 2020 at 06:03:52PM -0400, Steven Rostedt wrote:
> On Mon, 16 Mar 2020 17:45:40 -0400
> Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> 
> > >
> > > Same for the function side (if not even more so). This would require adding
> > > a srcu_read_lock() to all functions that can be traced! That would be a huge
> > > kill in performance. Probably to the point no one would bother even using
> > > function tracer.  
> > 
> > Point well taken! Thanks,
> 
> Actually, it's worse than that. (We talked about this on IRC but I wanted
> it documented here too).
> 
> You can't use any type of locking, unless you insert it around all the
> callers of the nops (which is unreasonable).
> 
> That is, we have gcc -pg -mfentry that creates at the start of all traced
> functions:
> 
>  <some_func>:
>     call __fentry__
>     [code for function here]
> 
> At boot up (or even by the compiler itself) we convert that to:
> 
>  <some_func>:
>     nop
>     [code for function here]
> 
> 
> When we want to trace this function we use text_poke (with current kernels)
> and convert it to this:
> 
>  <some_func>:
>     call trace_trampoline
>     [code for function here]
> 
> 
> That trace_trampoline can be allocated, which means when its no longer
> needed, it must be freed. But when do we know it's safe to free it? Here's
> the issue.
> 
> 
>  <some_func>:
>     call trace_trampoline  <- interrupt happens just after the jump
>     [code for function here]
> 
> Now the task has just executed the call to the trace_trampoline. Which
> means the instruction pointer is set to the start of the trampoline. But it
> has yet executed that trampoline.
> 
> Now if the task is preempted, and a real time hog is keeping it from
> running for minutes at a time (which is possible!). And in the mean time,
> we are done with that trampoline and free it. What happens when that task
> is scheduled back? There's no more trampoline to execute even though its
> instruction pointer is to execute the first operand on the trampoline!
> 
> I used the analogy of jumping off the cliff expecting a magic carpet to be
> there to catch you, and just before you land, it disappears. That would be
> a very bad day indeed!

I never have thought of an analogy between Tasks RCU and magic carpets
before.  Maybe time to go watch Aladdin or something.  ;-)

							Thanx, Paul

> We have no way to add a grace period between the start of a function (can
> be *any* function) and the start of the trampoline. Since the problem is
> that the task was non-voluntarily preempted before it could execute the
> trampoline, and that trampolines are not allowed (suppose) to call
> schedule, then we have our quiescent state to track (voluntary scheduling).
> When all tasks have either voluntarily scheduled, or entered user space
> after disconnecting a trampoline from a function, we know that it is safe to
> free the trampoline.
> 
> -- Steve