Re: [PATCH v3 tip/core/rcu 3/9] rcu: Add synchronous grace-period waiting for RCU-tasks

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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.

							Thanx, Paul