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

[Akinobu Mita]

2017-08-01 22:45 GMT+09:00 Dmitry Vyukov <dvyukov@xxxxxxxxxx>:
> On Tue, Aug 1, 2017 at 3:09 PM, Lu Fengqi <lufq.fnst@xxxxxxxxxxxxxx> 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 <dvyukov@xxxxxxxxxx>
>>>Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>>>Signed-off-by: Akinobu Mita <akinobu.mita@xxxxxxxxx>
>>>---
>>> 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?
>
> 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