Re: [PATCH REPOST 2/2] signal: Don't disable preemption in ptrace_stop() on PREEMPT_RT.

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 06, 2023 at 01:04:48PM +0200, Oleg Nesterov wrote:
> The patch LGTM, but I am a bit confused by the changelog/comments,
> I guess I missed something...
> 
> On 06/06, Sebastian Andrzej Siewior wrote:
> >
> > --- a/kernel/signal.c
> > +++ b/kernel/signal.c
> > @@ -2328,11 +2328,16 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
> >  	 * The preempt-disable section ensures that there will be no preemption
> >  	 * between unlock and schedule() and so improving the performance since
> >  	 * the ptracer has no reason to sleep.
> > +	 *
> > +	 * This optimisation is not doable on PREEMPT_RT due to the spinlock_t
> > +	 * within the preempt-disable section.
> >  	 */
> > -	preempt_disable();
> > +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> > +		preempt_disable();
> 
> Not only we the problems with cgroup_enter_frozen(), afaics (please correct me)
> this optimisation doesn't work on RT anyway?
> 
> IIUC, read_lock() on RT disables migration but not preemption, so it is simply
> too late to do preempt_disable() before unlock/schedule. The tracer can preempt
> the tracee right after do_notify_parent_cldstop().

Correct -- but I think you can disable preemption over what is
effectivly rwsem_up_read(), but you can't over the effective
rtmutex_lock() that cgroup_enter_frozen() will then attempt.

(iow, unlock() doesn't tend to sleep, while lock() does)

But you're correct to point out that the whole preempt_disable() thing
is entirely pointless due to the whole task_lock region being
preemptible before it.