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    SubjectRe: sched: serial port lockdep warning when offlining CPUs
    On Thu, Sep 28, 2017 at 08:13:26PM +0900, Sergey Senozhatsky wrote:
    > On (09/28/17 11:04), Levin, Alexander (Sasha Levin) wrote:
    > > On Thu, Sep 28, 2017 at 11:38:47AM +0200, Peter Zijlstra wrote:
    > > >On Thu, Sep 28, 2017 at 02:19:46AM -0700, Sasha Levin wrote:
    > > >> Hi all,
    > > >>
    > > >> I seem to be hitting the following warning when offlining CPUs on the
    > > >> latest -next kernel:
    > > >>
    > > >> [289683102.607076] Unregister pv shared memory for cpu 8
    > > >> [289683102.622922] select_fallback_rq: 3 callbacks suppressed
    > > >
    > > >This is because you hit a printk() from the scheduler, that's known
    > > >broken (along with the rest of printk).
    > > >
    > > >You forgot to actually include that printk() though I suspect it reads
    > > >like:
    > > >
    > > > "process %d (%s) no longer affine to cpu%d\n"
    > > >
    > > >Now that uses printk_deferred() which _should_ work lots better, but
    > > >clearly the printk() stuff went wobbly again.
    > >
    > > Hm, so is this an actual possible lockup, or just a false positive?
    >
    > a possible one.

    Yeah, printk() can deadlock when used from scheduler locks / NMI context
    and a few other crucial places. printk_deferred() used to be 'safe'
    though.

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