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

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 8 Aug 2014 18:27:14 +0200
Peter Zijlstra <peterz@infradead.org> 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.
> 
> With the per-cpu preempt count stuff we have on x86 that is 4
> instructions for the preempt_*() stuff -- they're 'big' instructions
> though, since 3 have memops and 2 have a segment prefix.
> 
> 

Well, this looks like it may make kprobes a bit more complex, and even
slow down slightly the optimized probe.

Also note that if we add call_rcu_tasks(), then perf function tracing
can be called directly instead of being added to the trampoline that
disables and enables preemption before calling it.

-- Steve