From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH-next] kvm: don't try to take mmu_lock while holding the main raw kvm_lock Date: Thu, 27 Jun 2013 13:38:29 +0200 Message-ID: <51CC2435.7080204@redhat.com> References: <1372199643-3936-1-git-send-email-paul.gortmaker@windriver.com> <20130627110911.GH18508@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Paul Gortmaker , linux-rt-users@vger.kernel.org, kvm@vger.kernel.org, Jan Kiszka To: Gleb Natapov Return-path: Received: from mx1.redhat.com ([209.132.183.28]:64673 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750978Ab3F0Lii (ORCPT ); Thu, 27 Jun 2013 07:38:38 -0400 In-Reply-To: <20130627110911.GH18508@redhat.com> Sender: linux-rt-users-owner@vger.kernel.org List-ID: Il 27/06/2013 13:09, Gleb Natapov ha scritto: > On Tue, Jun 25, 2013 at 06:34:03PM -0400, Paul Gortmaker wrote: >> In commit e935b8372cf8 ("KVM: Convert kvm_lock to raw_spinlock"), > I am copying Jan, the author of the patch. Commit message says: > "Code under this lock requires non-preemptibility", but which code > exactly is this? Is this still true? hardware_enable_nolock/hardware_disable_nolock does. Paolo >> the kvm_lock was made a raw lock. However, the kvm mmu_shrink() >> function tries to grab the (non-raw) mmu_lock within the scope of >> the raw locked kvm_lock being held. This leads to the following: >> >> BUG: sleeping function called from invalid context at kernel/rtmutex.c:659 >> in_atomic(): 1, irqs_disabled(): 0, pid: 55, name: kswapd0 >> Preemption disabled at:[] mmu_shrink+0x5c/0x1b0 [kvm] >> >> Pid: 55, comm: kswapd0 Not tainted 3.4.34_preempt-rt >> Call Trace: >> [] __might_sleep+0xfd/0x160 >> [] rt_spin_lock+0x24/0x50 >> [] mmu_shrink+0xec/0x1b0 [kvm] >> [] shrink_slab+0x17d/0x3a0 >> [] ? mem_cgroup_iter+0x130/0x260 >> [] balance_pgdat+0x54a/0x730 >> [] ? set_pgdat_percpu_threshold+0xa7/0xd0 >> [] kswapd+0x18f/0x490 >> [] ? get_parent_ip+0x11/0x50 >> [] ? __init_waitqueue_head+0x50/0x50 >> [] ? balance_pgdat+0x730/0x730 >> [] kthread+0xdb/0xe0 >> [] ? finish_task_switch+0x52/0x100 >> [] kernel_thread_helper+0x4/0x10 >> [] ? __init_kthread_worker+0x >> >> Since we only use the lock for protecting the vm_list, once we've >> found the instance we want, we can shuffle it to the end of the >> list and then drop the kvm_lock before taking the mmu_lock. We >> can do this because after the mmu operations are completed, we >> break -- i.e. we don't continue list processing, so it doesn't >> matter if the list changed around us. >> >> Signed-off-by: Paul Gortmaker >> --- >> >> [Note1: do double check that this solution makes sense for the >> mainline kernel; consider this an RFC patch that does want a >> review from people in the know.] >> >> [Note2: you'll need to be running a preempt-rt kernel to actually >> see this. Also note that the above patch is against linux-next. >> Alternate solutions welcome ; this seemed to me the obvious fix.] >> >> arch/x86/kvm/mmu.c | 12 ++++++++++-- >> 1 file changed, 10 insertions(+), 2 deletions(-) >> >> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c >> index 748e0d8..db93a70 100644 >> --- a/arch/x86/kvm/mmu.c >> +++ b/arch/x86/kvm/mmu.c >> @@ -4322,6 +4322,7 @@ mmu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc) >> { >> struct kvm *kvm; >> int nr_to_scan = sc->nr_to_scan; >> + int found = 0; >> unsigned long freed = 0; >> >> raw_spin_lock(&kvm_lock); >> @@ -4349,6 +4350,12 @@ mmu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc) >> continue; >> >> idx = srcu_read_lock(&kvm->srcu); >> + >> + list_move_tail(&kvm->vm_list, &vm_list); >> + found = 1; >> + /* We can't be holding a raw lock and take non-raw mmu_lock */ >> + raw_spin_unlock(&kvm_lock); >> + >> spin_lock(&kvm->mmu_lock); >> >> if (kvm_has_zapped_obsolete_pages(kvm)) { >> @@ -4370,11 +4377,12 @@ unlock: >> * per-vm shrinkers cry out >> * sadness comes quickly >> */ >> - list_move_tail(&kvm->vm_list, &vm_list); >> break; >> } >> >> - raw_spin_unlock(&kvm_lock); >> + if (!found) >> + raw_spin_unlock(&kvm_lock); >> + >> return freed; >> >> } >> -- >> 1.8.1.2 > > -- > Gleb. >