linux-kernel - Re: Re: Re: [PATCH] spi: tegra20-slink: Fix runtime PM imbalance on error

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Message-ID: <53722a45.c204d.172414e8ed8.Coremail.dinghao.liu@zju.edu.cn>
Date:   Sat, 23 May 2020 19:32:46 +0800 (GMT+08:00)
From:   dinghao.liu@....edu.cn
To:     "Andy Shevchenko" <andy.shevchenko@...il.com>
Cc:     "Kangjie Lu" <kjlu@....edu>,
        "Laxman Dewangan" <ldewangan@...dia.com>,
        "Mark Brown" <broonie@...nel.org>,
        "Thierry Reding" <thierry.reding@...il.com>,
        "Jonathan Hunter" <jonathanh@...dia.com>,
        linux-spi <linux-spi@...r.kernel.org>,
        linux-tegra@...r.kernel.org,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: Re: Re: [PATCH] spi: tegra20-slink: Fix runtime PM imbalance on
 error

> On Fri, May 22, 2020 at 6:20 PM Andy Shevchenko
> <andy.shevchenko@...il.com> wrote:
> > On Fri, May 22, 2020 at 10:46 AM <dinghao.liu@....edu.cn> wrote:
> 
> ...
> 
> > Moreover, consider below case
> >
> > CPU1: ...somewhere in the code...
> > pm_runtime_get() // with success!
> > ...see below...
> > pm_runtime_put()
> >
> > CPU2: ...on parallel thread...
> > ret = pm_runtime_get_sync() // failed!
> > if (ret)
> >   pm_runtime_put() // oi vei, we put device into sleep
> >
> > So, there is a potential issue.
> 
> ...and even if it's impossible (no bugs in runtime PM core, etc) the
> code with put() looks suspicious.
> 

I may understand what you are worried about. Do you mean that
executing pm_runtime_put() will influence other threads (e.g.,
one parallel thread can put the device into sleep while other
threads are using this device)?

I think this will never happen. Because in this case the PM usage
counter cannot be decreased to zero if there are still some threads
using this device. Otherwise, pm_runtime_put() should never be
used in the case of multithreading, which is strange since this
API is used widely. 

I also checked many other implementation of probe in drivers.
It seems that using pm_runtime_put() is ok. If I misunderstood
your opinion, please point it out, thanks.

Regards,
Dinghao

> -- 
> With Best Regards,
> Andy Shevchenko