linux-kernel - Re: [PATCH 1/1] PM / Domains: Avoid a potential deadlock

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Date:   Fri, 22 Mar 2019 00:27:33 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Jiada Wang <jiada_wang@...tor.com>,
        Kevin Hilman <khilman@...nel.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Morimoto <kuninori.morimoto.gx@...esas.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH 1/1] PM / Domains: Avoid a potential deadlock

On Wednesday, March 13, 2019 8:35:02 AM CET Ulf Hansson wrote:
> On Tue, 12 Mar 2019 at 07:51, Jiada Wang <jiada_wang@...tor.com> wrote:
> >
> > Lockdep warns that prepare_lock and genpd->mlock can cause a deadlock
> > the deadlock scenario is like following:
> > First thread is probing cs2000
> > cs2000_probe()
> >   clk_register()
> >     __clk_core_init()
> >       clk_prepare_lock()                            ----> acquires prepare_lock
> >         cs2000_recalc_rate()
> >           i2c_smbus_read_byte_data()
> >             rcar_i2c_master_xfer()
> >               dma_request_chan()
> >                 rcar_dmac_of_xlate()
> >                   rcar_dmac_alloc_chan_resources()
> >                     pm_runtime_get_sync()
> >                       __pm_runtime_resume()
> >                         rpm_resume()
> >                           rpm_callback()
> >                             genpd_runtime_resume()   ----> acquires genpd->mlock
> >
> > Second thread is attaching any device to the same PM domain
> > genpd_add_device()
> >   genpd_lock()                                       ----> acquires genpd->mlock
> >     cpg_mssr_attach_dev()
> >       of_clk_get_from_provider()
> >         __of_clk_get_from_provider()
> >           __clk_create_clk()
> >             clk_prepare_lock()                       ----> acquires prepare_lock
> >
> > Since currently no PM provider access genpd's critical section
> > in .attach_dev, and .detach_dev callbacks, so there is no need to protect
> > these two callbacks with genpd->mlock.
> > This patch avoids a potential deadlock by moving out .attach_dev and .detach_dev
> > from genpd->mlock, so that genpd->mlock won't be held when prepare_lock is acquired
> > in .attach_dev and .detach_dev
> 
> Thanks for the detailed description, this seems like a reasonable change to me!
> 
> >
> > Signed-off-by: Jiada Wang <jiada_wang@...tor.com>
> 
> Reviewed-by: Ulf Hansson <ulf.hansson@...aro.org>

Patch applied, thanks!