Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752253Ab3EAEmX (ORCPT ); Wed, 1 May 2013 00:42:23 -0400 Received: from wolverine02.qualcomm.com ([199.106.114.251]:39745 "EHLO wolverine02.qualcomm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751010Ab3EAEmQ (ORCPT ); Wed, 1 May 2013 00:42:16 -0400 X-IronPort-AV: E=Sophos;i="4.87,587,1363158000"; d="scan'208";a="43180309" From: Saravana Kannan To: linux-arm-kernel@lists.infradead.org, Mike Turquette , Paul Walmsley , Shawn Guo , Sascha Hauer , Rob Herring , Mark Brown , Russell King , Ulf Hansson Cc: linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, Andrew Lunn , Jeremy Kerr , Thomas Gleixner , Arnd Bergman , Jamie Iles , Richard Zhao , Magnus Damm , Linus Walleij , Stephen Boyd , Amit Kucheria , Deepak Saxena , Grant Likely Subject: [PATCH] clk: Fix race condition between clk_set_parent and clk_enable() Date: Tue, 30 Apr 2013 21:42:08 -0700 Message-Id: <1367383328-25700-1-git-send-email-skannan@codeaurora.org> X-Mailer: git-send-email 1.7.8.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5479 Lines: 165 Without this patch, the following race conditions are possible. Race condition 1: * clk-A has two parents - clk-X and clk-Y. * All three are disabled and clk-X is current parent. * Thread A: clk_set_parent(clk-A, clk-Y). * Thread A: * Thread A: Grabs enable lock. * Thread A: Sees enable count of clk-A is 0, so doesn't enable clk-Y. * Thread A: Updates clk-A SW parent to clk-Y * Thread A: Releases enable lock. * Thread B: clk_enable(clk-A). * Thread B: clk_enable() enables clk-Y, then enabled clk-A and returns. clk-A is now enabled in software, but not clocking in hardware since the hardware parent is still clk-X. The only way to avoid race conditions between clk_set_parent() and clk_enable/disable() is to ensure that clk_enable/disable() calls don't require changes to hardware enable state between changes to software clock topology and hardware clock topology. There are options to achieve the above: 1. Grab the enable lock before changing software/hardware topology and release it afterwards. 2. Keep the clock enabled for the duration of software/hardware topology change so that any additional enable/disable calls don't try to change the hardware state. Once the topology change is complete, the clock can be put back in its original enable state. Option (1) is not an acceptable solution since the set_parent() ops might need to sleep. Therefore, this patch implements option (2). This patch doesn't violate any API semantics. clk_disable() doesn't guarantee that the clock is actually disabled. So, no clients of a clock can assume that a clock is disabled after their last call to clk_disable(). So, enabling the clock during a parent change is not a violation of any API semantics. This also has the nice side effect of simplifying the error handling code. Signed-off-by: Saravana Kannan --- It's been a while since I submitted a patch. So, apologies if I'm cc'ing people who no longer care about the state of the common clock framework. drivers/clk/clk.c | 72 +++++++++++++++++++++++----------------------------- 1 files changed, 32 insertions(+), 40 deletions(-) diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c index 934cfd1..fe4055f 100644 --- a/drivers/clk/clk.c +++ b/drivers/clk/clk.c @@ -1377,67 +1377,59 @@ static int __clk_set_parent(struct clk *clk, struct clk *parent, u8 p_index) unsigned long flags; int ret = 0; struct clk *old_parent = clk->parent; - bool migrated_enable = false; - /* migrate prepare */ - if (clk->prepare_count) + /* + * Migrate prepare state between parents and prevent race with + * clk_enable(). + * + * If the clock is not prepared, then a race with + * clk_enable/disable() is impossible since we already have the + * prepare lock (future calls to clk_enable() need to be preceded by + * a clk_prepare()). + * + * If the clock is prepared, migrate the prepared state to the new + * parent and also protect against a race with clk_enable() by + * forcing the clock and the new parent on. This ensures that all + * future calls to clk_enable() are practically NOPs with respect to + * hardware and software states. + */ + if (clk->prepare_count) { __clk_prepare(parent); - - flags = clk_enable_lock(); - - /* migrate enable */ - if (clk->enable_count) { - __clk_enable(parent); - migrated_enable = true; + clk_enable(parent); + clk_enable(clk); } /* update the clk tree topology */ + flags = clk_enable_lock(); clk_reparent(clk, parent); - clk_enable_unlock(flags); /* change clock input source */ if (parent && clk->ops->set_parent) ret = clk->ops->set_parent(clk->hw, p_index); - if (ret) { - /* - * The error handling is tricky due to that we need to release - * the spinlock while issuing the .set_parent callback. This - * means the new parent might have been enabled/disabled in - * between, which must be considered when doing rollback. - */ - flags = clk_enable_lock(); + flags = clk_enable_lock(); clk_reparent(clk, old_parent); - - if (migrated_enable && clk->enable_count) { - __clk_disable(parent); - } else if (migrated_enable && (clk->enable_count == 0)) { - __clk_disable(old_parent); - } else if (!migrated_enable && clk->enable_count) { - __clk_disable(parent); - __clk_enable(old_parent); - } - clk_enable_unlock(flags); - if (clk->prepare_count) + if (clk->prepare_count) { + clk_disable(clk); + clk_disable(parent); __clk_unprepare(parent); - + } return ret; } - /* clean up enable for old parent if migration was done */ - if (migrated_enable) { - flags = clk_enable_lock(); - __clk_disable(old_parent); - clk_enable_unlock(flags); - } - - /* clean up prepare for old parent if migration was done */ - if (clk->prepare_count) + /* + * Finish the migration of prepare state and undo the changes done + * for preventing a race with clk_enable(). + */ + if (clk->prepare_count) { + clk_disable(clk); + clk_disable(old_parent); __clk_unprepare(old_parent); + } /* update debugfs with new clk tree topology */ clk_debug_reparent(clk, parent); -- 1.7.8.3 The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/