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Re: [dm-devel] [PATCH 8/8] dm-mpath: do not activate failed paths



On 02/28/14 18:52, Hannes Reinecke wrote:
> On 02/28/2014 10:37 AM, Junichi Nomura wrote:
>> On 02/28/14 18:32, Hannes Reinecke wrote:
>>> On 02/28/2014 09:49 AM, Junichi Nomura wrote:
>>>> On 02/27/14 16:30, Hannes Reinecke wrote:
>>>>> activate_path() is run without a lock, so the path might be
>>>>> set to failed before activate_path() had a chance to run.
>>>>>
>>>>> Signed-off-by: Hannes Reinecke <hare suse de>
>>>>> ---
>>>>>  drivers/md/dm-mpath.c | 7 +++++--
>>>>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
>>>>> index 0a40fa9..22e7365 100644
>>>>> --- a/drivers/md/dm-mpath.c
>>>>> +++ b/drivers/md/dm-mpath.c
>>>>> @@ -1196,8 +1196,11 @@ static void activate_path(struct work_struct *work)
>>>>>  	struct pgpath *pgpath =
>>>>>  		container_of(work, struct pgpath, activate_path.work);
>>>>>  
>>>>> -	scsi_dh_activate(bdev_get_queue(pgpath->path.dev->bdev),
>>>>> -				pg_init_done, pgpath);
>>>>> +	if (pgpath->is_active)
>>>>> +		scsi_dh_activate(bdev_get_queue(pgpath->path.dev->bdev),
>>>>> +				 pg_init_done, pgpath);
>>>>> +	else
>>>>> +		pg_init_done(pgpath, SCSI_DH_DEV_OFFLINED);
>>>>
>>>> Hi Hannes,
>>>>
>>>> What problem are you going to fix with this?
>>>> It is still possible that the path is set to failed just after
>>>> "if (pgpath->is_active)" and before "scsi_dh_activate".
>>>>
>>> activate_path() is run from a workqueue, so there is a race window
>>> between the check ->is_active in __pg_init_all_paths() and the time
>>> the scsi_dh_activate() is executed. And as activate_path)() is run
>>> without any locks we don't have any synchronization with fail_path().
>>> So it's well possible that a path gets failed in between
>>> __pg_init_all_paths() and activate_paths().
>>
>> What I pointed is that your patch makes the window very small
>> but doesn't close it.
>> If the race is a problem, this patch doesn't fix the problem.
>>
> True. As said I just want to avoid unnecessary overhead by calling
> functions which are known to be failing.

"to avoid unnecessary overhead by calling functions which are known
 to be failing"

IMO, that should be in the patch description. :)

> This patch actually makes more sense in combination with the 'accept
> failed paths' patch, where it's much more likely to trigger.
> So if you insist I could move it over to there.

If the description is fixed, I have no problem with this patch.

-- 
Jun'ichi Nomura, NEC Corporation


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