Re: [linux-pm] [PATCH] PERF(kernel): Cleanup power events V2

From: Alan Stern <stern@rowland.harvard.edu>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Greg Kroah-Hartman <greg@kroah.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Pierre Tardy <tardyp@gmail.com>,
	Arjan van de Ven <arjan@linux.intel.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Jean Pihet <jean.pihet@newoldbits.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	linux-trace-users@vger.kernel.org, Frank Eigler <fche@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-pm@lists.linux-foundation.org,
	Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
	Tejun Heo <tj@kernel.org>, Ingo Molnar <mingo@elte.hu>,
	linux-omap@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [linux-pm] [PATCH] PERF(kernel): Cleanup power events V2
Date: Tue, 26 Oct 2010 14:50:06 -0400 (EDT)	[thread overview]
Message-ID: <Pine.LNX.4.44L0.1010261447090.1634-100000@iolanthe.rowland.org> (raw)
In-Reply-To: <20101026181421.GA30090@Krystal>

On Tue, 26 Oct 2010, Mathieu Desnoyers wrote:

> * Peter Zijlstra (peterz@infradead.org) wrote:
> > On Tue, 2010-10-26 at 11:56 -0500, Pierre Tardy wrote:
> > > 
> > > +       trace_runtime_pm_usage(dev, atomic_read(&dev->power.usage_count)+1);
> > >         atomic_inc(&dev->power.usage_count); 
> > 
> > That's terribly racy..
> 
> Looking at the original code, it looks racy even without considering the
> tracepoint:
> 
> int __pm_runtime_get(struct device *dev, bool sync)
>  {
>         int retval;
> 
> +       trace_runtime_pm_usage(dev, atomic_read(&dev->power.usage_count)+1);
>         atomic_inc(&dev->power.usage_count);
>         retval = sync ? pm_runtime_resume(dev) : pm_request_resume(dev);
> 
> There is no implied memory barrier after "atomic_inc". So either all these
> inc/dec are protected with mutexes or spinlocks, in which case one might wonder
> why atomic operations are used at all, or it's a racy mess. (I vote for the
> second option)

I don't understand.  What's the problem?  The inc/dec are atomic 
because they are not protected by spinlocks, but everything else is 
(aside from the tracepoint, which is new).

> kref should certainly be used there.

What for?

Alan Stern