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

    Well, this looks like it may make kprobes a bit more complex, and even
    slow down slightly the optimized probe.

    Also note that if we add call_rcu_tasks(), then perf function tracing
    can be called directly instead of being added to the trampoline that
    disables and enables preemption before calling it.

    -- Steve


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