Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757735AbZFKGTA (ORCPT ); Thu, 11 Jun 2009 02:19:00 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754252AbZFKGSx (ORCPT ); Thu, 11 Jun 2009 02:18:53 -0400 Received: from casper.infradead.org ([85.118.1.10]:47908 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753918AbZFKGSw (ORCPT ); Thu, 11 Jun 2009 02:18:52 -0400 Subject: Re: Bug: fio traps into kernel without exiting because futex has a deadloop From: Peter Zijlstra To: "Zhang, Yanmin" Cc: Darren Hart , Rusty Russell , LKML , Thomas Gleixner In-Reply-To: <1244699756.6691.4.camel@laptop> References: <1244689688.2560.268.camel@ymzhang> <1244699756.6691.4.camel@laptop> Content-Type: text/plain; charset="UTF-8" Date: Thu, 11 Jun 2009 08:18:48 +0200 Message-Id: <1244701128.6691.5.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.26.1 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2590 Lines: 57 On Thu, 2009-06-11 at 07:55 +0200, Peter Zijlstra wrote: > On Thu, 2009-06-11 at 11:08 +0800, Zhang, Yanmin wrote: > > I investigate a fio hang issue. When I run fio multi-process > > testing on many disks, fio traps into kernel and doesn't exit > > (mostly hit once after runing sub test cases for hundreds of times). > > > > Oprofile data shows kernel consumes time with some futex functions. > > Command kill couldn't kill the process and machine reboot also hangs. > > > > Eventually, I locate the root cause as a bug of futex. Kernel enters > > a deadloop between 'retry' and 'goto retry' in function futex_wake_op. > > By unknown reason (might be an issue of fio or glibc), parameter uaddr2 > > points to an area which is READONLY. So futex_atomic_op_inuser returns > > -EFAULT when trying to changing the data at uaddr2, but later get_user > > still succeeds becasue the area is READONLY. Then go back to retry. > > > > I create a simple test case to trigger it, which just shmat an READONLY > > area for address uaddr2. > > > > It could be used as a DOS attack. /me has morning juice and notices he sent the wrong commit... commit 64d1304a64477629cb16b75491a77bafe6f86963 Author: Thomas Gleixner Date: Mon May 18 21:20:10 2009 +0200 futex: setup writeable mapping for futex ops which modify user space data The futex code installs a read only mapping via get_user_pages_fast() even if the futex op function has to modify user space data. The eventual fault was fixed up by futex_handle_fault() which walked the VMA with mmap_sem held. After the cleanup patches which removed the mmap_sem dependency of the futex code commit 4dc5b7a36a49eff97050894cf1b3a9a02523717 (futex: clean up fault logic) removed the private VMA walk logic from the futex code. This change results in a stale RO mapping which is not fixed up. Instead of reintroducing the previous fault logic we set up the mapping in get_user_pages_fast() read/write for all operations which modify user space data. Also handle private futexes in the same way and make the current unconditional access_ok(VERIFY_WRITE) depend on the futex op. Reported-by: Andreas Schwab Signed-off-by: Thomas Gleixner CC: stable@kernel.org -- 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/