Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751805AbaK0RZV (ORCPT ); Thu, 27 Nov 2014 12:25:21 -0500 Received: from mx1.redhat.com ([209.132.183.28]:41593 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751111AbaK0RZS (ORCPT ); Thu, 27 Nov 2014 12:25:18 -0500 Date: Thu, 27 Nov 2014 19:24:49 +0200 From: "Michael S. Tsirkin" To: David Hildenbrand Cc: linuxppc-dev@lists.ozlabs.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, benh@kernel.crashing.org, paulus@samba.org, akpm@linux-foundation.org, heiko.carstens@de.ibm.com, schwidefsky@de.ibm.com, borntraeger@de.ibm.com, tglx@linutronix.de, David.Laight@ACULAB.COM Subject: Re: [PATCH RFC 2/2] mm, sched: trigger might_sleep() in might_fault() when pagefaults are disabled Message-ID: <20141127172449.GA30380@redhat.com> References: <1416915806-24757-1-git-send-email-dahi@linux.vnet.ibm.com> <1417108217-42687-1-git-send-email-dahi@linux.vnet.ibm.com> <1417108217-42687-3-git-send-email-dahi@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1417108217-42687-3-git-send-email-dahi@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 27, 2014 at 06:10:17PM +0100, David Hildenbrand wrote: > 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); This __FILE__/__FILE__ will always point at kernel.h You want a macro to wrap this up. > +} > #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/