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SubjectRe: [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@google.com>
>Reported-by: Dmitry Vyukov <dvyukov@google.com>
>Signed-off-by: Akinobu Mita <akinobu.mita@gmail.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


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