From mboxrd@z Thu Jan 1 00:00:00 1970 From: Geert Uytterhoeven Date: Tue, 05 Jan 2016 18:36:35 +0000 Subject: [PATCH 0/1] serial: sh-sci: Remove cpufreq notifier to fix crash/deadlock Message-Id: <1452018997-14103-1-git-send-email-geert+renesas@glider.be> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Greg Kroah-Hartman , Jiri Slaby Cc: 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, Geert Uytterhoeven Hi Greg, Jiri, 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 the stored clock rate; 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. Both patches fix the same issue: - Patch 1a applies against tty/tty-next and v4.4-rc1..v4.4-rc8, and is intended for current/stable. It needs some rework to apply to earlier kernel versions, as functions were moved around in v4.4.-rc1. - Patch 1b applies against tty/tty-next with scif-clk-sck-brg-for-v4.5 (pending pull request for v4.5) appplied, and is intended for next. Thanks for your comments! Geert Uytterhoeven (1): serial: sh-sci: Remove cpufreq notifier to fix crash/deadlock serial: sh-sci: Remove cpufreq notifier to fix crash/deadlock drivers/tty/serial/sh-sci.c | 39/42 --------------------------------------- 1 file changed, 39/42 deletions(-) Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds