[CRIU] [PATCH 3/8] cr-restore: set cr_error to EEXIST if such pid already exists, v2

[Ruslan Kuprieiev]

On 12/02/2014 03:56 PM, Ruslan Kuprieiev wrote:
> On 12/02/2014 03:20 PM, Pavel Emelyanov wrote:
>> On 12/02/2014 03:29 PM, Ruslan Kuprieiev wrote:
>>> This is a very common error when using criu.
>>>
>>> The problem here is that we need to somehow transfer cr_errno
>>> from one process to another. I suggest using pipe to give
>>> one end to children and read cr_errno on other after restore
>>> is finished.
>>>
>>> v2, Pavel suggested putting errno into shared task_entries.
>>>
>>> Signed-off-by: Ruslan Kuprieiev <kupruser at gmail.com>
>>> ---
>>>   cr-restore.c       | 7 ++++++-
>>>   include/cr-errno.h | 1 +
>>>   include/rst_info.h | 1 +
>>>   3 files changed, 8 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/cr-restore.c b/cr-restore.c
>>> index 93a6ca6..60641d9 100644
>>> --- a/cr-restore.c
>>> +++ b/cr-restore.c
>>> @@ -85,6 +85,8 @@
>>>
>>>   #include "asm/restore.h"
>>>
>>> +#include "cr-errno.h"
>>> +
>>>   static struct pstree_item *current;
>>>
>>>   static int restore_task_with_children(void *);
>>> @@ -1412,6 +1414,7 @@ static int restore_task_with_children(void *_arg)
>>>       pid = getpid();
>>>       if (current->pid.virt != pid) {
>>>           pr_err("Pid %d do not match expected %d\n", pid,
>>> current->pid.virt);
>>> +        task_entries->cr_err = EEXIST;
>>
>> This is racy. Presumably we want to get the very first error from
>> children, thus we need the cmpxchg() here (wrapped in a helper).

Well, implementing cmpxchg() would require implementing futex_cmpxchg,
which would require atomic_cmpxchg, which would require some assembly.
But we currently have __xchg_op only for x86.

How about we use here futex_t cr_err and futex_set() for now?
I mean, we don't care which child reported cr_errno=EEXIST.

>>
>
> Ah, i see.
> Thanks.
>
>>>           goto err;
>>>       }
>>>
>>> @@ -1535,8 +1538,10 @@ static int restore_wait_inprogress_tasks()
>>>
>>>       futex_wait_while_gt(np, 0);
>>>       ret = (int)futex_get(np);
>>> -    if (ret < 0)
>>> +    if (ret < 0) {
>>> +        cr_errno = task_entries->cr_err;
>>>           return ret;
>>> +    }
>>>
>>>       return 0;
>>>   }
>>> diff --git a/include/cr-errno.h b/include/cr-errno.h
>>> index bec72a8..f86bba8 100644
>>> --- a/include/cr-errno.h
>>> +++ b/include/cr-errno.h
>>> @@ -4,6 +4,7 @@ extern int cr_errno;
>>>   /*
>>>    * List of symbolic error names:
>>>    * ESRCH    - no process can be found corresponding to that
>>> specified by pid
>>> + * EEXIST    - process with such pid already exists
>>>    */
>>>
>>>   #endif /* __CR_ERRNO_H__ */
>>> diff --git a/include/rst_info.h b/include/rst_info.h
>>> index 6b00c50..cec0144 100644
>>> --- a/include/rst_info.h
>>> +++ b/include/rst_info.h
>>> @@ -10,6 +10,7 @@ struct task_entries {
>>>       futex_t nr_in_progress;
>>>       futex_t start;
>>>       mutex_t    zombie_lock;
>>> +    int cr_err;
>>>   };
>>>
>>>   struct fdt {
>>>
>>
>