linux-kernel - Re: [PATCH -mm] fault-inject: avoid unwanted data race to task->fail

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Date:   Tue, 1 Aug 2017 21:09:07 +0800
From:   Lu Fengqi <lufq.fnst@...fujitsu.com>
To:     Akinobu Mita <akinobu.mita@...il.com>
CC:     <akpm@...ux-foundation.org>, <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH -mm] fault-inject: avoid unwanted data race to
 task->fail_nth

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 <dvyukov@...gle.com>
>Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
>Signed-off-by: Akinobu Mita <akinobu.mita@...il.com>
>---
> 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/<dd pid>/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?

A response would be very much appreciated. Thank you for your time.

-- 
Thanks,
Lu