Re: Use case for TASKS_RCU

[Paul E. McKenney]

On Mon, May 15, 2017 at 02:48:10PM -0400, Steven Rostedt wrote:
> On Mon, 15 May 2017 11:23:54 -0700
> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > Hello!
> > 
> > The question of the use case for TASKS_RCU came up, and here is my
> > understanding.  Steve will not be shy about correcting any misconceptions
> > I might have.  ;-)
> > 
> > The use case is to support freeing of trampolines used in tracing/probing
> > in CONFIG_PREEMPT=y kernels.  It is necessary to wait until any task
> > executing in the trampoline in question has left it, taking into account
> > that the trampoline's code might be interrupted and preempted.  However,
> > the code in the trampolines is guaranteed never to context switch.
> 
> nit, "never to voluntarily context switch" as it can still be
> preempted. It should never call schedule nor a mutex. And really it
> shouldn't even call any spinlocks. Although, trace_stack does, but it
> does so after checking if in_nmi(), which it bails if that is true.

Good catch, thank you!  And thank you for the checking on the rest.

Ingo, thoughts?

							Thanx, Paul

> > Note that in CONFIG_PREEMPT=n kernels, synchronize_sched() suffices.
> > It is therefore tempting to think in terms of disabling preemption across
> > the trampolines, but there is apparently not enough room to accommodate
> > the needed preempt_disable() and preempt_enable() in the code invoking
> > the trampoline, and putting the preempt_disable() and preempt_enable()
> > in the trampoline itself fails because of the possibility of preemption
> > just before the preempt_disable() and just after the preempt_enable().
> > Similar reasoning rules out use of rcu_read_lock() and rcu_read_unlock().
> 
> Correct, as the jump to the trampoline may be preempted. And preemption
> happens just before the first instruction on the trampoline is being
> executed.
> 
> 
> > 
> > Another possibility would be to place the trampolines in a known region
> > of memory, and check for the task's PC being in that region.  This fails
> > because trampolines can be interrupted, and I vaguely recall something
> > about them calling function as well.  Stack tracing could be added,
> > but stack tracing is not as reliable as it would need to be.
> 
> Correct.
> 
> > 
> > The solution chosen relies on the fact that code in trampolines
> > (and code invoked from trampolines) is not permitted to do voluntary
> > context switches.  Thus, if a trampoline is removed, and a given task
> > later does a voluntary context switch (or has been seen in usermode),
> > that task will never again reference that trampoline.  Once all tasks
> > are accounted for, the trampoline may safely be removed.
> 
> Correct.
> 
> > 
> > TASKS_RCU implements a flavor of RCU that does exactly this.  It has
> > only a single use at the moment, but avoiding memory leaks on
> > production machines being instrumented seems to me to be quite valuable.
> 
> Optimized kprobes can also benefit from this, as it currently is
> disabled on CONFIG_PREEMPT due to exactly the same issue. I'll poke
> Masami about this again. I should be seeing him in a couple of weeks at
> the Open Source Summit in Tokyo.
> 
> 
> > 
> > So, Steve, please correct any misconceptions!
> 
> Nope, all looks good.
> 
> -- Steve
>