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 <mturque...@linaro.org>
> > > > Cc: Stephen Boyd <sb...@codeaurora.org>
> > > > Signed-off-by: Dong Aisheng <aisheng.d...@freescale.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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