Re: Use case for TASKS_RCU

[Paul E. McKenney]

On Tue, May 16, 2017 at 08:22:33AM +0200, Ingo Molnar wrote:
> 
> * 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.
> > 
> > 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().
> 
> So how was this solved before TASKS_RCU? Also, nothing uses call_rcu_tasks() at 
> the moment, so it's hard for me to review its users. What am I missing?

Before TASKS_RCU, the trampolines were just leaked when CONFIG_PREEMPT=y.

Current mainline kernel/trace/ftrace.c uses synchronize_rcu_tasks().
So yes, currently one user.

							Thanx, Paul