Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752040AbdHAPy3 (ORCPT ); Tue, 1 Aug 2017 11:54:29 -0400 Received: from mail-pg0-f67.google.com ([74.125.83.67]:34062 "EHLO mail-pg0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751788AbdHAPy2 (ORCPT ); Tue, 1 Aug 2017 11:54:28 -0400 MIME-Version: 1.0 In-Reply-To: References: <1499962492-8931-1-git-send-email-akinobu.mita@gmail.com> <20170801130907.GB3359@fnst> From: Akinobu Mita Date: Wed, 2 Aug 2017 00:54:07 +0900 Message-ID: Subject: Re: [PATCH -mm] fault-inject: avoid unwanted data race to task->fail_nth To: Dmitry Vyukov Cc: Lu Fengqi , 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: 4243 Lines: 102 2017-08-01 22:45 GMT+09:00 Dmitry Vyukov : > 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. Lu's setting for fail_make_request is fine before introducing systematic fault injection and they want to inject fail_make_request only. So I think we need a global parameter to turn on/off the systematic fault injection. (e.g. /sys/kernel/debug/systematic-fault-inject/enable) > 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