On Fri, Aug 08, 2014 at 06:51:12PM +0200, Peter Zijlstra wrote: > 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.
Not seeing much in the way of __kprobe, so guessing lack of bravery. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/