Re: Use case for TASKS_RCU

[Ingo Molnar <mingo@kernel.org>]

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> 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?

Thanks,

	Ingo