From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753784AbcBBHPF (ORCPT ); Tue, 2 Feb 2016 02:15:05 -0500 Received: from LGEAMRELO13.lge.com ([156.147.23.53]:56446 "EHLO lgeamrelo13.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752354AbcBBHPA (ORCPT ); Tue, 2 Feb 2016 02:15:00 -0500 X-Original-SENDERIP: 156.147.1.125 X-Original-MAILFROM: byungchul.park@lge.com X-Original-SENDERIP: 10.177.222.33 X-Original-MAILFROM: byungchul.park@lge.com From: Byungchul Park To: willy@linux.intel.com, akpm@linux-foundation.org, mingo@kernel.org Cc: linux-kernel@vger.kernel.org, akinobu.mita@gmail.com, jack@suse.cz, sergey.senozhatsky.work@gmail.com, peter@hurleysoftware.com, torvalds@linux-foundation.org Subject: [PATCH] lock/semaphore: Avoid a deadlock within __up() Date: Tue, 2 Feb 2016 16:14:28 +0900 Message-Id: <1454397268-6022-1-git-send-email-byungchul.park@lge.com> X-Mailer: git-send-email 1.9.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Since I faced a infinite recursive printk() bug, I've tried to propose patches the title of which is "lib/spinlock_debug.c: prevent a recursive cycle in the debug code". But I noticed the root problem cannot be fixed by that, through some discussion thanks to Sergey and Peter. So I focused on preventing the DEADLOCK. -----8<----- >>From 94a66990677735459a7790b637179d8600479639 Mon Sep 17 00:00:00 2001 From: Byungchul Park Date: Tue, 2 Feb 2016 15:35:48 +0900 Subject: [PATCH] lock/semaphore: Avoid a deadlock within __up() When the semaphore __up() is called from within printk() with console_sem.lock, a DEADLOCK can happen, since the wake_up_process() can call printk() again, esp. if defined CONFIG_DEBUG_SPINLOCK. And the wake_up_process() don't need to be within a critical section. The scenario the bad thing can happen is, printk console_trylock console_unlock up_console_sem up raw_spin_lock_irqsave(&sem->lock, flags) __up wake_up_process try_to_wake_up raw_spin_lock_irqsave(&p->pi_lock) __spin_lock_debug spin_dump printk console_trylock raw_spin_lock_irqsave(&sem->lock, flags) *** DEADLOCK *** Signed-off-by: Byungchul Park --- kernel/locking/semaphore.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c index b8120ab..d3a28dc 100644 --- a/kernel/locking/semaphore.c +++ b/kernel/locking/semaphore.c @@ -259,5 +259,14 @@ static noinline void __sched __up(struct semaphore *sem) struct semaphore_waiter, list); list_del(&waiter->list); waiter->up = true; + + /* + * Trying to acquire this sem->lock in wake_up_process() leads a + * DEADLOCK unless we unlock it here. For example, it's possile + * in the case that called from within printk() since + * wake_up_process() might call printk(). + */ + raw_spin_unlock_irq(&sem->lock); wake_up_process(waiter->task); + raw_spin_lock_irq(&sem->lock); } -- 1.9.1