Re: virt_blk BUG: sleeping function called from invalid context

[Jens Axboe <axboe@kernel.dk>]

On 06/29/2014 02:47 PM, Michael S. Tsirkin wrote:
> On Sun, Jun 29, 2014 at 09:32:22PM +0200, Christoph Hellwig wrote:
>> On Sun, Jun 29, 2014 at 11:26:37AM +0300, Michael S. Tsirkin wrote:
>>> On Fri, Jun 27, 2014 at 07:57:38AM -0400, Josh Boyer wrote:
>>>> Hi All,
>>>>
>>>> We've had a report[1] of the virt_blk driver causing a lot of spew
>>>> because it's calling a sleeping function from an invalid context.  The
>>>> backtrace is below.  This is with kernel v3.16-rc2-69-gd91d66e88ea9.
>>>
>>> Hi Jens, pls see below - it looks like the call to blk_mq_end_io
>>> from IRQ context is causing the issue.
>>> IIUC you switched virtio to this from __blk_end_request_all in
>>>
>>> commit 1cf7e9c68fe84248174e998922b39e508375e7c1
>>>     virtio_blk: blk-mq support
>>>
>>> Is this always safe?
>>> I note that at least one other driver is doing this:
>>> drivers/block/mtip32xx/mtip32xx.c
>>
>> Just like __blk_end_request_all blk_mq_end_io is supposed to be called
>> from irq context.  The problem is that the MD bio end_io handler is calling
>> a sleeping function.  Not sure if that's a bug in MD though given the
>> kernfs symbols in the all trace and the recent churn in that area.
> 
> My understanding is this:
> 
> bitmap_endwrite -> calls sysfs_notify_dirent_safe under spinlock
>  -> calls kernfs_notify which takes a mutex.
> 
> So I am guessing it is this commit:
> 
> commit d911d98748018f7c8facc035ba39c30f5cce6f9c
> Author: Tejun Heo <tj@kernel.org>
> Date:   Wed Apr 9 11:07:31 2014 -0400
> 
>     kernfs: make kernfs_notify() trigger inotify events too
> 
> Tejun, what do you think?
> 
> Josh, Brian, could you try reverting that commit to see if it helps?

That definitely be a bug. If you need to block off ->bi_end_io(), just
must punt to a worker thread.

-- 
Jens Axboe