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    SubjectRe: [PATCH] generic-ipi: fix deadlock in __smp_call_function_single
    On Tue, Sep 14, 2010 at 10:03:47AM +0200, Peter Zijlstra wrote:
    > On Mon, 2010-09-13 at 11:02 -0700, Suresh Siddha wrote:
    > > On Sat, 2010-09-11 at 09:42 -0700, Venkatesh Pallipadi wrote:
    > > > Also, as we don't have rq lock around this point, it seems possible
    > > > that the CPU that was busy and wants to kick idle load balance on
    > > > remote CPU, could have become idle and nominated itself as idle load
    > > > balancer.
    > >
    > > A busy cpu (currently running something -- one task on the rq atleast)
    > > can't become idle in the middle of trigger_load_balance().
    > >
    > > What might be happening is similar what you said but the opposite of it.
    > >
    > > cpu-x is idle which is also ilb_cpu
    > > got a scheduler tick during idle
    > > and the nohz_kick_needed() in trigger_load_balance() checks for
    > > rq_x->nr_running which might not be zero (because of someone waking a
    > > task on this rq etc) and this leads to the situation of the cpu-x
    > > sending a kick to itself.
    > So what patches are we going to merge?
    > I share Heiko's opinion on that its somewhat surprising to have
    > __smp_call_function_single() differ in this detail from
    > smp_call_function_single() and think that merging his patch would be
    > good in that respect. But Andrew seemed to have reservations.
    > We can also merge either my or Suresh's patch (which I think makes
    > sense, but is kinda subtle) to avoid the needless self kick.

    I would prefer to see your's or Suresh's scheduler patch to be merged to
    fix the bug.
    My patch could be merged for 2.6.37 or be dropped in favour of a WARN_ON
    in __smp_call_function_single() if remote cpu == current cpu.
    However I think it would be better if smp_call_function_single() and
    __smp_call_function_single() wouldn't differ here.

     \ /
      Last update: 2010-09-14 13:23    [W:0.021 / U:24.136 seconds]
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