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
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