Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751438AbaK0RKb (ORCPT ); Thu, 27 Nov 2014 12:10:31 -0500 Received: from e06smtp14.uk.ibm.com ([195.75.94.110]:54099 "EHLO e06smtp14.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751173AbaK0RK3 (ORCPT ); Thu, 27 Nov 2014 12:10:29 -0500 From: David Hildenbrand To: linuxppc-dev@lists.ozlabs.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org Cc: benh@kernel.crashing.org, paulus@samba.org, akpm@linux-foundation.org, heiko.carstens@de.ibm.com, dahi@linux.vnet.ibm.com, schwidefsky@de.ibm.com, borntraeger@de.ibm.com, mst@redhat.com, tglx@linutronix.de, David.Laight@ACULAB.COM Subject: [PATCH RFC 2/2] mm, sched: trigger might_sleep() in might_fault() when pagefaults are disabled Date: Thu, 27 Nov 2014 18:10:17 +0100 Message-Id: <1417108217-42687-3-git-send-email-dahi@linux.vnet.ibm.com> X-Mailer: git-send-email 1.8.5.5 In-Reply-To: <1417108217-42687-1-git-send-email-dahi@linux.vnet.ibm.com> References: <1416915806-24757-1-git-send-email-dahi@linux.vnet.ibm.com> <1417108217-42687-1-git-send-email-dahi@linux.vnet.ibm.com> X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14112717-0017-0000-0000-00000201700D Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Commit 662bbcb2747c2422cf98d3d97619509379eee466 removed might_sleep() checks for all user access code (that uses might_fault()). The reason was to disable wrong "sleep in atomic" warnings in the following scenario: pagefault_disable(); rc = copy_to_user(...); pagefault_enable(); Which is valid, as pagefault_disable() increments the preempt counter and therefore disables the pagefault handler. copy_to_user() will not sleep and return an invalid return code if a page is not available. However, as all might_sleep() checks are removed, CONFIG_DEBUG_ATOMIC_SLEEP would no longer detect the following scenario: spin_lock(&lock); rc = copy_to_user(...); spin_unlock(&lock); If the kernel is compiled with preemption turned on, the preempt counter would be incremented and copy_to_user() would never sleep. However, with preemption turned off, the preempt counter will not be touched, we will therefore sleep in atomic context. We really want to enable CONFIG_DEBUG_ATOMIC_SLEEP checks for user access functions again, otherwise horrible deadlocks might be hard to debug. Root of all evil is that pagefault_disable() acted almost as preempt_disable(), depending on preemption being turned on/off. As we now have a fixed pagefault_disable() implementation in place, that uses own bits in the preempt counter, we can reenable might_sleep() checks. This patch reverts commit 662bbcb2747c2422cf98d3d97619509379eee466 taking care of the !MMU optimization and the new pagefault_disabled() check. Signed-off-by: David Hildenbrand --- include/linux/kernel.h | 9 +++++++-- mm/memory.c | 15 ++++----------- 2 files changed, 11 insertions(+), 13 deletions(-) diff --git a/include/linux/kernel.h b/include/linux/kernel.h index 3d770f55..64b5f93 100644 --- a/include/linux/kernel.h +++ b/include/linux/kernel.h @@ -225,9 +225,14 @@ static inline u32 reciprocal_scale(u32 val, u32 ep_ro) return (u32)(((u64) val * ep_ro) >> 32); } -#if defined(CONFIG_MMU) && \ - (defined(CONFIG_PROVE_LOCKING) || defined(CONFIG_DEBUG_ATOMIC_SLEEP)) +#if defined(CONFIG_MMU) && defined(CONFIG_PROVE_LOCKING) void might_fault(void); +#elif defined(CONFIG_MMU) && defined(CONFIG_DEBUG_ATOMIC_SLEEP) +static inline void might_fault(void) +{ + if (unlikely(!pagefault_disabled())) + __might_sleep(__FILE__, __LINE__, 0); +} #else static inline void might_fault(void) { } #endif diff --git a/mm/memory.c b/mm/memory.c index 3e50383..0e59db9 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3699,7 +3699,7 @@ void print_vma_addr(char *prefix, unsigned long ip) up_read(&mm->mmap_sem); } -#if defined(CONFIG_PROVE_LOCKING) || defined(CONFIG_DEBUG_ATOMIC_SLEEP) +#ifdef CONFIG_PROVE_LOCKING void might_fault(void) { /* @@ -3711,17 +3711,10 @@ void might_fault(void) if (segment_eq(get_fs(), KERNEL_DS)) return; - /* - * it would be nicer only to annotate paths which are not under - * pagefault_disable, however that requires a larger audit and - * providing helpers like get_user_atomic. - */ - if (in_atomic()) - return; - - __might_sleep(__FILE__, __LINE__, 0); + if (unlikely(!pagefault_disabled())) + __might_sleep(__FILE__, __LINE__, 0); - if (current->mm) + if (!in_atomic() && current->mm) might_lock_read(¤t->mm->mmap_sem); } EXPORT_SYMBOL(might_fault); -- 1.8.5.5 -- 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/