Re: Lockups with btrfs on 3.16-rc1 - bisected

[Josef Bacik <jbacik@fb.com>]

On 06/18/2014 03:47 PM, Waiman Long wrote:
> On 06/18/2014 06:27 PM, Josef Bacik wrote:
>>
>>
>> On 06/18/2014 03:17 PM, Waiman Long wrote:
>>> On 06/18/2014 04:57 PM, Marc Dionne wrote:
>>>> Hi,
>>>>
>>>> I've been seeing very reproducible soft lockups with 3.16-rc1 similar
>>>> to what is reported here:
>>>> https://urldefense.proofpoint.com/v1/url?u=http://marc.info/?l%3Dlinux-btrfs%26m%3D140290088532203%26w%3D2&k=ZVNjlDMF0FElm4dQtryO4A%3D%3D%0A&r=cKCbChRKsMpTX8ybrSkonQ%3D%3D%0A&m=aoagvtZMwVb16gh1HApZZL00I7eP50GurBpuEo3l%2B5g%3D%0A&s=c62558feb60a480bbb52802093de8c97b5e1f23d4100265b6120c8065bd99565
>>>>
>>>> , along with the
>>>> occasional hard lockup, making it impossible to complete a parallel
>>>> build on a btrfs filesystem for the package I work on.  This was
>>>> working fine just a few days before rc1.
>>>>
>>>> Bisecting brought me to the following commit:
>>>>
>>>>    commit bd01ec1a13f9a327950c8e3080096446c7804753
>>>>    Author: Waiman Long<Waiman.Long@hp.com>
>>>>    Date:   Mon Feb 3 13:18:57 2014 +0100
>>>>
>>>>        x86, locking/rwlocks: Enable qrwlocks on x86
>>>>
>>>> And sure enough if I revert that commit on top of current mainline,
>>>> I'm unable to reproduce the soft lockups and hangs.
>>>>
>>>> Marc
>>>
>>> The queue rwlock is fair. As a result, recursive read_lock is not
>>> allowed unless the task is in an interrupt context. Doing recursive
>>> read_lock will hang the process when a write_lock happens somewhere in
>>> between. Are recursive read_lock being done in the btrfs code?
>>>
>>
>> We walk down a tree and read lock each node as we walk down, is that
>> what you mean?  Or do you mean read_lock multiple times on the same
>> lock in the same process, cause we definitely don't do that.  Thanks,
>>
>> Josef
>
> I meant recursively read_lock the same lock in a process.

I take it back, we do actually do this in some cases.  Thanks,

Josef