linux-kernel - Re: [PATCH] lightnvm: pblk: use vfree

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Date:   Tue, 3 Oct 2017 16:49:35 +0200
From:   Javier González <jg@...htnvm.io>
To:     Javier González <jg@...htnvm.io>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Hans Holmberg <hans.ml.holmberg@...tronix.com>,
        Matias Bjørling <mb@...htnvm.io>,
        linux-block@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Hans Holmberg <hans.holmberg@...xlabs.com>
Subject: Re: [PATCH] lightnvm: pblk: use vfree_atomic when freeing line
 metadata

> On 3 Oct 2017, at 16.43, Javier González <jg@...htnvm.io> wrote:
> 
>> On 3 Oct 2017, at 16.20, Andrey Ryabinin <aryabinin@...tuozzo.com> wrote:
>> 
>> 
>> 
>> On 10/03/2017 05:11 PM, Javier González wrote:
>>>> On 3 Oct 2017, at 16.07, Andrey Ryabinin <aryabinin@...tuozzo.com> wrote:
>>>> 
>>>> 
>>>> 
>>>> On 10/03/2017 04:48 PM, Hans Holmberg wrote:
>>>>> From: Hans Holmberg <hans.holmberg@...xlabs.com>
>>>>> 
>>>>> The commit bf22e37a6413 ("mm: add vfree_atomic()") made vfree unsafe to
>>>>> call in atomic context (unless the call came from an interrupt) and
>>>>> introduced vfree_atomic that is safe to call in atomic context.
>>>>> 
>>>>> So, since we're holding locks when freeing line metadata, we need to
>>>>> use the atomic version of vfree.
>>>>> 
>>>>> Fix this by introducing an atomic variant of pblk_mfree and
>>>>> switching to that in pblk_line_meta_free.
>>>>> 
>>>>> Signed-off-by: Hans Holmberg <hans.holmberg@...xlabs.com>
>>>>> ---
>>>>> 
>>>>> The patch is for:
>>>>> https://github.com/OpenChannelSSD/linux branch for-4.15/pblk
>>>>> 
>>>>> drivers/lightnvm/pblk-init.c | 3 ++-
>>>>> drivers/lightnvm/pblk.h      | 8 ++++++++
>>>>> 2 files changed, 10 insertions(+), 1 deletion(-)
>>>>> 
>>>>> diff --git a/drivers/lightnvm/pblk-init.c b/drivers/lightnvm/pblk-init.c
>>>>> index c452478..3a191a6 100644
>>>>> --- a/drivers/lightnvm/pblk-init.c
>>>>> +++ b/drivers/lightnvm/pblk-init.c
>>>>> @@ -396,7 +396,8 @@ static void pblk_line_meta_free(struct pblk *pblk)
>>>>> 	spin_lock(&l_mg->free_lock);
>>>> 
>>>> What's the point in holding ->free_lock here? It seems like it could be just dropped.
>>> 
>>> This lock can indeed be dropped,
>> 
>> So, let's do this. This would be the best way to fix this.
>> 
>>> but the general pblk semaphore, which
>>> serializes initialization and tear down cannot. This is taken on
>>> pblk_exit().
>> 
>> But semaphore is not the problem here. We can sleep under semaphore, so it's fine.
> 
> It seems to me like a false positive, but lockdep complains on the
> mentioned rw_semaphore held by pblk, and on the mutex held by the
> lightnvm subsystem when removing a target (dev->mlock).
> 
> [ 6037.778889] BUG: sleeping function called from invalid context at mm/vmalloc.c:1492
> [ 6037.786579] in_atomic(): 1, irqs_disabled(): 0, pid: 1282, name: nvme
> [ 6037.793050] 3 locks held by nvme/1282:
> [ 6037.793053]  #0:  (&dev->mlock){+.+.+.}, at: [<ffffffff8ddff395>] nvm_ctl_ioctl+0x3c5/0x6a0
> [ 6037.793075]  #1:  (pblk_lock){+.+.+.}, at: [<ffffffff8de0439b>] pblk_exit+0x1b/0x100
> [ 6037.793092]  #2:  (&(&l_mg->free_lock)->rlock){+.+...}, at: [<ffffffff8de040ea>] pblk_line_meta_free+0x8a/0x130
> 
> Any ideas?
> 

Ok. When dropping ->free_lock, lockdep does not complain. It's just a
misleading notification from lockdep, signalling semaphores as "held
locks" when a real non sleeping lock is being taken.

We will just remove ->free_lock then.

Thanks Andrey.

Javier


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