Re: [PATCH] xen-blkback: prevent premature module unload

["Jürgen Groß" <jgross@suse.com>]

On 11.12.19 14:55, Roger Pau Monné wrote:
> On Wed, Dec 11, 2019 at 01:27:42PM +0000, Durrant, Paul wrote:
>>> -----Original Message-----
>>> From: Roger Pau Monné <roger.pau@citrix.com>
>>> Sent: 11 December 2019 11:29
>>> To: Durrant, Paul <pdurrant@amazon.com>
>>> Cc: xen-devel@lists.xenproject.org; linux-block@vger.kernel.org; linux-
>>> kernel@vger.kernel.org; Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>;
>>> Jens Axboe <axboe@kernel.dk>
>>> Subject: Re: [PATCH] xen-blkback: prevent premature module unload
>>>
>>> On Tue, Dec 10, 2019 at 02:53:05PM +0000, Paul Durrant wrote:
>>>> Objects allocated by xen_blkif_alloc come from the 'blkif_cache' kmem
>>>> cache. This cache is destoyed when xen-blkif is unloaded so it is
>>>> necessary to wait for the deferred free routine used for such objects to
>>>> complete. This necessity was missed in commit 14855954f636 "xen-blkback:
>>>> allow module to be cleanly unloaded". This patch fixes the problem by
>>>> taking/releasing extra module references in xen_blkif_alloc/free()
>>>> respectively.
>>>>
>>>> Signed-off-by: Paul Durrant <pdurrant@amazon.com>
>>>
>>> Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
>>>
>>> One nit below.
>>>
>>>> ---
>>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>>> Cc: "Roger Pau Monné" <roger.pau@citrix.com>
>>>> Cc: Jens Axboe <axboe@kernel.dk>
>>>> ---
>>>>   drivers/block/xen-blkback/xenbus.c | 10 ++++++++++
>>>>   1 file changed, 10 insertions(+)
>>>>
>>>> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-
>>> blkback/xenbus.c
>>>> index e8c5c54e1d26..59d576d27ca7 100644
>>>> --- a/drivers/block/xen-blkback/xenbus.c
>>>> +++ b/drivers/block/xen-blkback/xenbus.c
>>>> @@ -171,6 +171,15 @@ static struct xen_blkif *xen_blkif_alloc(domid_t
>>> domid)
>>>>   	blkif->domid = domid;
>>>>   	atomic_set(&blkif->refcnt, 1);
>>>>   	init_completion(&blkif->drain_complete);
>>>> +
>>>> +	/*
>>>> +	 * Because freeing back to the cache may be deferred, it is not
>>>> +	 * safe to unload the module (and hence destroy the cache) until
>>>> +	 * this has completed. To prevent premature unloading, take an
>>>> +	 * extra module reference here and release only when the object
>>>> +	 * has been free back to the cache.
>>>                      ^ freed
>>
>> Oh yes. Can this be done on commit, or would you like me to send a v2?
> 
> Adjusting on commit would be fine for me, but it's up to Juergen since
> he is the one that will pick this up. IIRC the module unload patches
> didn't go through the block subsystem.

Oh, right. Yes, will fix this when committing.


Juergen