Messages in this thread | | | Date | Tue, 16 May 2017 05:23:54 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Use case for TASKS_RCU |
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On Tue, May 16, 2017 at 08:22:33AM +0200, Ingo Molnar wrote: > > * 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?
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
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