From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754555AbbIIP1V (ORCPT ); Wed, 9 Sep 2015 11:27:21 -0400 Received: from mx2.suse.de ([195.135.220.15]:41005 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753566AbbIIP1T (ORCPT ); Wed, 9 Sep 2015 11:27:19 -0400 Subject: Re: Multiple potential races on vma->vm_flags To: "Kirill A. Shutemov" , Sasha Levin , Rik van Riel , Andrew Morton References: <55EC9221.4040603@oracle.com> <20150907114048.GA5016@node.dhcp.inet.fi> Cc: Andrey Konovalov , Dmitry Vyukov , linux-mm@kvack.org, linux-kernel@vger.kernel.org From: Vlastimil Babka Message-ID: <55F04FD4.6060308@suse.cz> Date: Wed, 9 Sep 2015 17:27:16 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <20150907114048.GA5016@node.dhcp.inet.fi> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/07/2015 01:40 PM, Kirill A. Shutemov wrote: > On Sun, Sep 06, 2015 at 03:21:05PM -0400, Sasha Levin wrote: >> ================================================================== >> ThreadSanitizer: data-race in munlock_vma_pages_range >> >> Write of size 8 by thread T378 (K2633, CPU3): >> [] munlock_vma_pages_range+0x59/0x3e0 mm/mlock.c:425 >> [] mlock_fixup+0x1c9/0x280 mm/mlock.c:549 >> [] do_mlock+0x14c/0x180 mm/mlock.c:589 >> [< inlined >] SyS_munlock+0x74/0xb0 SYSC_munlock mm/mlock.c:651 >> [] SyS_munlock+0x74/0xb0 mm/mlock.c:643 >> [] entry_SYSCALL_64_fastpath+0x12/0x71 >> arch/x86/entry/entry_64.S:186 > > ... > >> Previous read of size 8 by thread T398 (K2623, CPU2): >> [] try_to_unmap_one+0x78/0x4f0 mm/rmap.c:1208 >> [< inlined >] rmap_walk+0x147/0x450 rmap_walk_file mm/rmap.c:1540 >> [] rmap_walk+0x147/0x450 mm/rmap.c:1559 >> [] try_to_munlock+0xa2/0xc0 mm/rmap.c:1423 >> [] __munlock_isolated_page+0x30/0x60 mm/mlock.c:129 >> [] __munlock_pagevec+0x236/0x3f0 mm/mlock.c:331 >> [] munlock_vma_pages_range+0x380/0x3e0 mm/mlock.c:476 >> [] mlock_fixup+0x1c9/0x280 mm/mlock.c:549 >> [] do_mlock+0x14c/0x180 mm/mlock.c:589 >> [< inlined >] SyS_munlock+0x74/0xb0 SYSC_munlock mm/mlock.c:651 >> [] SyS_munlock+0x74/0xb0 mm/mlock.c:643 >> [] entry_SYSCALL_64_fastpath+0x12/0x71 >> arch/x86/entry/entry_64.S:186 > > Okay, the detected race is mlock/munlock vs. rmap. > > On rmap side we check vma->vm_flags in few places without taking > vma->vm_mm->mmap_sem. The vma cannot be freed since we hold i_mmap_rwsem > or anon_vma_lock, but nothing prevent vma->vm_flags from changing under > us. > > In this particular case, speculative check in beginning of > try_to_unmap_one() is fine, since we re-check it under mmap_sem later in > the function. > > False-negative is fine too here, since we will mlock the page in > __mm_populate() on mlock side after mlock_fixup(). > > BUT. > > We *must* have all speculative vm_flags accesses wrapped READ_ONCE() to > avoid all compiler trickery, like duplication vm_flags access with > inconsistent results. Doesn't taking a semaphore, as in try_to_unmap_one(), already imply a compiler barrier forcing vm_flags to be re-read? > I looked only on VM_LOCKED checks, but there are few other flags checked > in rmap. All of them must be handled carefully. At least READ_ONCE() is > required. > > Other solution would be to introduce per-vma spinlock to protect > vma->vm_flags and probably other vma fields and offload this duty > from mmap_sem. > But that's much bigger project. Sounds like an overkill, unless we find something more serious than this.