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

Steven Rostedt
2014-08-08T16:43:43Z

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.
>
>

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