linux-kernel - Re: udiskd high CPU usage with 4.0 git

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Date:	Sat, 14 Mar 2015 21:16:51 +0100
From:	Torsten Kaiser <just.for.lkml@...glemail.com>
To:	NeilBrown <neilb@...e.de>
Cc:	Prakash Punnoor <prakash@...noor.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: udiskd high CPU usage with 4.0 git

On Mon, Mar 9, 2015 at 12:30 AM, NeilBrown <neilb@...e.de> wrote:
> On Sun, 08 Mar 2015 18:14:39 +0100 Prakash Punnoor <prakash@...noor.de> 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
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