linux-kernel - Re: Query regarding synchronize_sched_expedited and resched

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Date:   Mon, 18 Sep 2017 12:29:31 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Neeraj Upadhyay <neeraju@...eaurora.org>, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        linux-kernel@...r.kernel.org, sramana@...eaurora.org,
        prsood@...eaurora.org, pkondeti@...eaurora.org,
        markivx@...eaurora.org, peterz@...radead.org
Subject: Re: Query regarding synchronize_sched_expedited and resched_cpu

On Mon, 18 Sep 2017 09:24:12 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:


> As soon as I work through the backlog of lockdep complaints that
> appeared in the last merge window...  :-(
> 
> sparse_irq_lock, I am looking at you!!!  ;-)

I just hit one too, and decided to write a patch to show a chain of 3
when applicable.

For example:

 Chain exists of:
   cpu_hotplug_lock.rw_sem --> smpboot_threads_lock --> (complete)&self->parked
 
  Possible unsafe locking scenario by crosslock:
 
        CPU0                    CPU1                    CPU2
        ----                    ----                    ----
   lock(smpboot_threads_lock);
   lock((complete)&self->parked);
                                lock(cpu_hotplug_lock.rw_sem);
                                lock(smpboot_threads_lock);
                                                       lock(cpu_hotplug_lock.rw_sem);
                                                       unlock((complete)&self->parked);
 
  *** DEADLOCK ***

:-)

-- Steve