On Fri, Aug 08, 2014 at 09:39:05AM -0700, Paul E. McKenney wrote:
> On Fri, Aug 08, 2014 at 06:27:14PM +0200, Peter Zijlstra wrote:
> > On Fri, Aug 08, 2014 at 10:58:58AM -0400, Steven Rostedt wrote:
> > 
> > > > > No, they are also used by optimized kprobes. This is why optimized
> > > > > kprobes depend on !CONFIG_PREEMPT. [ added Masami to the discussion ].
> > > > 
> > > > How do those work? Is that one where the INT3 relocates the instruction
> > > > stream into an alternative 'text' and that JMPs back into the original
> > > > stream at the end?
> > > 
> > > No, it's where we replace the 'int3' with a jump to a trampoline that
> > > simulates an INT3. Speeds things up quite a bit.
> > 
> > OK, so the trivial 'fix' for that is to patch the probe site like:
> > 
> > 	preempt_disable();		INC	GS:%__preempt_count
> > 	call trampoline;		CALL	0xDEADBEEF
> > 	preempt_enable();		DEC	GS:%__preempt_count
> > 					JNZ	1f
> > 					CALL	___preempt_schedule
> > 				1f:
> > 
> > At which point the preempt_disable/enable() are the read side primitives
> > and call_rcu_sched/synchronize_sched are sufficient to release it.
> 
> Unless this is done in idle, at which point RCU-sched is studiously
> ignoring any preempt_disable() sections.

Well, given that kprobes is already using it, it 'must' be good ;-) I
suspect much of the idle loop is marked with __kprobe or so, or nobody
has been brave enough to try.