[Ocfs2-devel] [PATCH] ocfs2/cluster: unlock the o2hb_live_lock before the o2nm_depend_item()

[alex chen <alex.chen@huawei.com>]

Hi Joseph and Changwei,

It's our basic principle that the function in which may sleep can't be called
within spinlock hold.

On 2017/11/1 9:03, Joseph Qi wrote:
> Hi Alex,
> 
> On 17/10/31 20:41, alex chen wrote:
>> In the following situation, the down_write() will be called under
>> the spin_lock(), which may lead a soft lockup:
>> o2hb_region_inc_user
>>  spin_lock(&o2hb_live_lock)
>>   o2hb_region_pin
>>    o2nm_depend_item
>>     configfs_depend_item
>>      inode_lock
>>       down_write
>>       -->here may sleep and reschedule
>>
>> So we should unlock the o2hb_live_lock before the o2nm_depend_item(), and
>> get item reference in advance to prevent the region to be released.
>>
>> Signed-off-by: Alex Chen <alex.chen@huawei.com>
>> Reviewed-by: Yiwen Jiang <jiangyiwen@huawei.com>
>> Reviewed-by: Jun Piao <piaojun@huawei.com>
>> ---
>>  fs/ocfs2/cluster/heartbeat.c | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>>
>> diff --git a/fs/ocfs2/cluster/heartbeat.c b/fs/ocfs2/cluster/heartbeat.c
>> index d020604..f1142a9 100644
>> --- a/fs/ocfs2/cluster/heartbeat.c
>> +++ b/fs/ocfs2/cluster/heartbeat.c
>> @@ -2399,6 +2399,9 @@ static int o2hb_region_pin(const char *region_uuid)
>>  		if (reg->hr_item_pinned || reg->hr_item_dropped)
>>  			goto skip_pin;
>>
>> +		config_item_get(&reg->hr_item);
>> +		spin_unlock(&o2hb_live_lock);
>> +
> If unlock here, the iteration of o2hb_all_regions is no longer safe.
> 
> Thanks,
> Joseph
> 

In local heartbeat mode, here we already found the region and will break the loop after
depending item, we get the item reference before spin_unlock(), that means the region will
never be released by the o2hb_region_release() until we put the item reference after
spin_lock(&o2hb_live_lock), so we can safely iterate over the list.
In global heartbeat mode, it doesn't matter that some regions may be deleted after
spin_unlock(), because we just pin all the active regions.

Thanks,
Alex

>>  		/* Ignore ENOENT only for local hb (userdlm domain) */
>>  		ret = o2nm_depend_item(&reg->hr_item);
>>  		if (!ret) {
>> @@ -2410,9 +2413,14 @@ static int o2hb_region_pin(const char *region_uuid)
>>  			else {
>>  				mlog(ML_ERROR, "Pin region %s fails with %d\n",
>>  				     uuid, ret);
>> +				config_item_put(&reg->hr_item);
>> +				spin_lock(&o2hb_live_lock);
>>  				break;
>>  			}
>>  		}
>> +
>> +		config_item_put(&reg->hr_item);
>> +		spin_lock(&o2hb_live_lock);
>>  skip_pin:
>>  		if (found)
>>  			break;
>> -- 1.9.5.msysgit.1
>>
>>
> 
> .
>