linux-kernel - Re: Query regarding synchronize_sched_expedited and resched

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Message-Id: <20170918165527.GN3521@linux.vnet.ibm.com>
Date:   Mon, 18 Sep 2017 09:55:27 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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, Sep 18, 2017 at 12:29:31PM -0400, Steven Rostedt wrote:
> 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 ***
> 
> :-)

Nice!!!

My next step is reverting 12ac1d0f6c3e ("genirq: Make sparse_irq_lock
protect what it should protect") to see if that helps.

							Thanx, Paul