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.
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