Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752041AbdHANpo (ORCPT ); Tue, 1 Aug 2017 09:45:44 -0400 Received: from mail-oi0-f54.google.com ([209.85.218.54]:33773 "EHLO mail-oi0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751794AbdHANp3 (ORCPT ); Tue, 1 Aug 2017 09:45:29 -0400 MIME-Version: 1.0 In-Reply-To: <20170801130907.GB3359@fnst> References: <1499962492-8931-1-git-send-email-akinobu.mita@gmail.com> <20170801130907.GB3359@fnst> From: Dmitry Vyukov Date: Tue, 1 Aug 2017 15:45:08 +0200 Message-ID: Subject: Re: [PATCH -mm] fault-inject: avoid unwanted data race to task->fail_nth To: Lu Fengqi Cc: Akinobu Mita , Andrew Morton , LKML Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3796 Lines: 95 On Tue, Aug 1, 2017 at 3:09 PM, Lu Fengqi wrote: > On Fri, Jul 14, 2017 at 01:14:52AM +0900, Akinobu Mita wrote: >>The fault-inject-make-fail-nth-read-write-interface-symmetric.patch in >>-mm tree allows users to set task->fail_nth for non current task by procfs. >>On the other hand, the current task's fail_nth is decreased to zero in >>fault-injection path without any specific locks. >> >>So we need to prevent the task->fail_nth from being unexpected value by >>data races (for example, setting task->fail_nth to zero while decreasing >>the current->fail_nth). In this fix, we use READ_ONCE() and WRITE_ONCE() >>to prevent the compiler from creating unsolicited accesses. >> >>Cc: Dmitry Vyukov >>Reported-by: Dmitry Vyukov >>Signed-off-by: Akinobu Mita >>--- >> fs/proc/base.c | 5 +++-- >> lib/fault-inject.c | 7 +++++-- >> 2 files changed, 8 insertions(+), 4 deletions(-) >> >>diff --git a/fs/proc/base.c b/fs/proc/base.c >>index ecc8a25..719c2e9 100644 >>--- a/fs/proc/base.c >>+++ b/fs/proc/base.c >>@@ -1370,7 +1370,7 @@ static ssize_t proc_fail_nth_write(struct file *file, const char __user *buf, >> task = get_proc_task(file_inode(file)); >> if (!task) >> return -ESRCH; >>- task->fail_nth = n; >>+ WRITE_ONCE(task->fail_nth, n); >> put_task_struct(task); >> >> return count; >>@@ -1386,7 +1386,8 @@ static ssize_t proc_fail_nth_read(struct file *file, char __user *buf, >> task = get_proc_task(file_inode(file)); >> if (!task) >> return -ESRCH; >>- len = snprintf(numbuf, sizeof(numbuf), "%u\n", task->fail_nth); >>+ len = snprintf(numbuf, sizeof(numbuf), "%u\n", >>+ READ_ONCE(task->fail_nth)); >> len = simple_read_from_buffer(buf, count, ppos, numbuf, len); >> put_task_struct(task); >> >>diff --git a/lib/fault-inject.c b/lib/fault-inject.c >>index 09ac73c1..7d315fd 100644 >>--- a/lib/fault-inject.c >>+++ b/lib/fault-inject.c >>@@ -107,9 +107,12 @@ static inline bool fail_stacktrace(struct fault_attr *attr) >> >> bool should_fail(struct fault_attr *attr, ssize_t size) >> { >>- if (in_task() && current->fail_nth) { >>- if (--current->fail_nth == 0) >>+ if (in_task()) { >>+ unsigned int fail_nth = READ_ONCE(current->fail_nth); >>+ >>+ if (fail_nth && !WRITE_ONCE(current->fail_nth, fail_nth - 1)) >> goto fail; >>+ >> return false; >> } >> >>-- >>2.7.4 >> >> >> > hi > > I'm a btrfs developer. I found that fail_make_request didn't produce the > expected IO ERROR when running xfstests on linux 4.13-rc1. > > That testcase enable fail_make_request by the following commands: > # echo 100 > /sys/kernel/debug/fail_make_request/probability > # echo 2 > /sys/kernel/debug/fail_make_request/times > # echo 0 > /sys/kernel/debug/fail_make_request/verbose > # echo 1 > /sys/block/sda/sda1/make-it-fail > # dd if=/dev/zero of=/dev/sda1 bs=128K count=1 oflag=direct > > As I understand it, after applying this patch, I have to write > /proc/
/file-nth firstly so that dd process can catch the IO ERROR. > However, the dd process is so fast that I can't write file-nth. > > So, could you tell me how to produce IO ERROR under these circumstances? Hi, fail-nth is orthogonal to the existing mechanisms, so if you have a setup that fails all sites with certain probability, that should continue to work. If you are writing a new facility and want to use fail-nth, then the test process itself needs to cooperate and write fail-nth accordingly. See the original patch for an example of how to do it: https://groups.google.com/d/msg/syzkaller/DbB4rjYd82s/3MHDwtcqCAAJ