Bug 1202362

Description of problem:


https://lkml.org/lkml/2015/3/14/163


From	Torsten Kaiser <>
	

On Mon, Mar 9, 2015 at 12:30 AM, NeilBrown <neilb> wrote:
> On Sun, 08 Mar 2015 18:14:39 +0100 Prakash Punnoor <prakash> wrote:
>
>> Hi,
>>
>> I noticed the udisks daemon (version 2.1.4) suddenly started using high
>> cpu (one core at 100%) with linux 4.0 git kernel. I bisected it to:
>>
>> 750f199ee8b578062341e6ddfe36c59ac8ff2dcb

I had the same problem upgrading from 4.0-rc1 to 4.0-rc3.
I have just finished bisecting and "fixing" it.

My bisect points to the same commit.

Looking at udisksd with strace sees a loop of polling and then
accessing several md related sysfs files.
The only file that udisksd monitors and was changes by that commit was
"sync_action".

If I revert this part of the commit, my system works normal again:

 static struct md_sysfs_entry md_scan_mode =
- __ATTR_PREALLOC(sync_action, S_IRUGO|S_IWUSR, action_show, action_store);
+ __ATTR(sync_action, S_IRUGO|S_IWUSR, action_show, action_store);
It seems that polling is broken for peralloc files.

The cause seems to be that kernfs_seq_show() updates ->event, while
the new sysfs_kf_read() does not.
So the polling will always trigger and udisksd goes into an inifinite
loop looking for changes that are not there.

I fixed my local system by copying the line "of->event =
atomic_read(&of->kn->attr.open->event);" from kernfs_seq_show() into
sysfs_kf_read(). (I also needed to move the definition of struct
kernfs_open_node from kernfs/file.c to kefs-internal.h)

udisksd now again behaves normal, but I'm not sending this change as a
patch, because I do not know about the locking and livetime of these
objects to evaluate, if that is really the correct fix.


Thanks,

Torsten

---------
[root@localhost ~]# uname -a
Linux mujpc 4.0.0-0.rc3.git0.1.fc22.x86_64 #1 SMP Mon Mar 9 13:36:59 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

[root@localhost ~]# top

top - 14:18:29 up 8 min,  2 users,  load average: 1.11, 0.91, 0.48
Tasks: 215 total,   2 running, 213 sleeping,   0 stopped,   0 zombie
%Cpu(s): 10.3 us, 15.3 sy,  0.0 ni, 74.4 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem : 16362728 total, 14351344 free,  1204720 used,   806664 buff/cache
KiB Swap: 18890748 total, 18890748 free,        0 used. 14890228 avail Mem 

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND                                                                                                        
 1623 root      20   0  371856   8272   6624 R 100.0  0.1   8:24.98 udisksd