Re: [patch] Real-Time Preemption, -RT-2.6.12-rc1-V0.7.41-07

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 23 Mar 2005, Ingo Molnar wrote:

>
> * Ingo Molnar <mingo@elte.hu> wrote:
>
> > That callback will be queued on CPU#2 - while the task still keeps
> > current->rcu_data of CPU#1. It also means that CPU#2's read counter
> > did _not_ get increased - and a too short grace period may occur.
> >
> > it seems to me that that only safe method is to pick an 'RCU CPU' when
> > first entering the read section, and then sticking to it, no matter
> > where the task gets migrated to. Or to 'migrate' the +1 read count
> > from one CPU to the other, within the scheduler.
>
> i think the 'migrate read-count' method is not adequate either, because
> all callbacks queued within an RCU read section must be called after the
> lock has been dropped - while with the migration method CPU#1 would be
> free to process callbacks queued in the RCU read section still active on
> CPU#2.
>

Hi Ingo,

Although you can't disable preemption for the duration of the
rcu_readlock, what about pinning the process to a CPU while it has the
lock.  Would this help solve the migration issue?

-- Steve