On Fri, 8 Aug 2014 18:27:14 +0200 Peter Zijlstra <pet...@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. > > Now the question is, how do you do that atomically? And safely. Currently, all we replace at the call sites is a nop that is added by gcc -pg and us replacing the call mcount with it. That looks much more complex than our current solution. -- Steve -- 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/
