linux-kernel - Re: [PATCH RFC v1 2/5] clk: add missing lock when call clk_core_enable in clk_set

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Message-ID: <20150513092105.GB554@shlinux1.ap.freescale.net>
Date:	Wed, 13 May 2015 17:21:06 +0800
From:	Dong Aisheng <b29396@...escale.com>
To:	Stephen Boyd <sboyd@...eaurora.org>
CC:	Dong Aisheng <aisheng.dong@...escale.com>,
	<linux-clk@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<mturquette@...aro.org>, <shawn.guo@...aro.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<Ranjani.Vaidyanathan@...escale.com>, <b20596@...escale.com>,
	<r64343@...escale.com>, <b20788@...escale.com>
Subject: Re: [PATCH RFC v1 2/5] clk: add missing lock when call
 clk_core_enable in clk_set_parent

On Wed, May 06, 2015 at 05:01:54PM -0700, Stephen Boyd wrote:
> On 05/04, Dong Aisheng wrote:
> > On Thu, Apr 30, 2015 at 12:07:47PM -0700, Stephen Boyd wrote:
> > > On 04/15/15 07:26, Dong Aisheng wrote:
> > > > clk_core_enable is executed without &enable_clock in clk_set_parent function.
> > > > Adding it to avoid potential race condition issue.
> > > >
> > > > Fixes: 035a61c314eb ("clk: Make clk API return per-user struct clk instances")
> > > > Cc: Mike Turquette <mturquette@...aro.org>
> > > > Cc: Stephen Boyd <sboyd@...eaurora.org>
> > > > Signed-off-by: Dong Aisheng <aisheng.dong@...escale.com>
> > > > ---
> > > 
> > > Can you please describe the race condition? From what I can tell there
> > > is not a race condition here and we've gone around on this part of the
> > > code before to fix any race conditions.
> > > 
> > 
> > Do you mean we do not need to acquire enable lock when execute clk_core_enable
> > in set_parent function? Can you help explain a bit more why?
> > 
> > The clk doc looks to me says the enable lock should be held across calls to
> > the .enable, .disable and .is_enabled operations.
> > 
> > And before the commit
> > 035a61c314eb ("clk: Make clk API return per-user struct clk instances"),
> > all the clk_enable/disable in set_parent() is executed with lock.
> > 
> > A rough thinking of race condition is assuming Thread A calls
> > clk_set_parent(x, y) while Thread B calls clk_enable(x), clock x is disabled
> > but prepared initially, due to clk_core_enable in set_parent() is not
> > executed with enable clock, the clk_core_enable may be reentrant during
> > the locking time executed by B.
> > Won't this be a race condition?
> > 
> 
> Ah I see now. The commit text could say something like this:
> 
> Before commit 035a61c314eb ("clk: Make clk API return per-user
> struct clk instances") we acquired the enable_lock in
> __clk_set_parent_{before,after}() by means of calling
> clk_enable(). After commit 035a61c314eb we use clk_core_enable()
> in place of the clk_enable(), and clk_core_enable() doesn't
> acquire the enable_lock. This opens up a race condition between
> clk_set_parent() and clk_enable().
> 
> I've replaced the commit text and applied it to clk-fixes.
> 

Got it.
Thanks for the change.

Regards
Dong Aisheng

> -- 
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> a Linux Foundation Collaborative Project
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