From mboxrd@z Thu Jan 1 00:00:00 1970 From: Greg Kroah-Hartman Date: Fri, 08 Jan 2016 05:09:04 +0000 Subject: Re: [PATCH 1b/1] serial: sh-sci: Remove cpufreq notifier to fix crash/deadlock Message-Id: <20160108050904.GH30293@kroah.com> List-Id: References: <1452018997-14103-1-git-send-email-geert+renesas@glider.be> <1452018997-14103-3-git-send-email-geert+renesas@glider.be> In-Reply-To: <1452018997-14103-3-git-send-email-geert+renesas@glider.be> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Geert Uytterhoeven Cc: Jiri Slaby , Kuninori Morimoto , Simon Horman , Magnus Damm , Yoshinori Sato , Laurent Pinchart , Michael Turquette , linux-serial@vger.kernel.org, linux-sh@vger.kernel.org, linux-kernel@vger.kernel.org On Tue, Jan 05, 2016 at 07:36:37PM +0100, Geert Uytterhoeven wrote: > The BSP team noticed that there is spin/mutex lock issue on sh-sci when > CPUFREQ is used. The issue is that the notifier function may call > mutex_lock() while the spinlock is held, which can lead to a BUG(). > This may happen if CPUFREQ is changed while another CPU calls > clk_get_rate(). > > Taking the spinlock was added to the notifier function in commit > e552de2413edad1a ("sh-sci: add platform device private data"), to > protect the list of serial ports against modification during traversal. > At that time the Common Clock Framework didn't exist yet, and > clk_get_rate() just returned clk->rate without taking a mutex. > Note that since commit d535a2305facf9b4 ("serial: sh-sci: Require a > device per port mapping."), there's no longer a list of serial ports to > traverse, and taking the spinlock became superfluous. > > To fix the issue, just remove the cpufreq notifier: > 1. The notifier doesn't work correctly: all it does is update stored > clock rates; it does not update the divider in the hardware. > The divider will only be updated when calling sci_set_termios(). > I believe this was broken back in 2004, when the old > drivers/char/sh-sci.c driver (where the notifier did update the > divider) was replaced by drivers/serial/sh-sci.c (where the > notifier just updated port->uartclk). > Cfr. full-history-linux commits 6f8deaef2e9675d9 ("[PATCH] sh: port > sh-sci driver to the new API") and 3f73fe878dc9210a ("[PATCH] > Remove old sh-sci driver"). > 2. On modern SoCs, the sh-sci parent clock rate is no longer related > to the CPU clock rate anyway, so using a cpufreq notifier is > futile. > > Signed-off-by: Geert Uytterhoeven > --- > This version applies against scif-clk-sck-brg-for-v4.5. I took this version. If the 1a needs to go to 4.4-stable, email it, and the git commit id to stable@vger.kernel.org when this ends up in Linus's tree. thanks, greg k-h