[Devel] [PATCH] fuse kio: Add comment about why we need to wait pending read requests

[Pavel Butsykin]

On 23.10.2018 13:19, Kirill Tkhai wrote:
> On 23.10.2018 13:15, Pavel Butsykin wrote:
>>
>>
>> On 23.10.2018 13:01, Kirill Tkhai wrote:
>>> Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
>>> ---
>>>    fs/fuse/kio/pcs/pcs_fuse_kdirect.c |    7 ++++++-
>>>    1 file changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/fuse/kio/pcs/pcs_fuse_kdirect.c b/fs/fuse/kio/pcs/pcs_fuse_kdirect.c
>>> index 3940d6c255ba..9b45fcbf8941 100644
>>> --- a/fs/fuse/kio/pcs/pcs_fuse_kdirect.c
>>> +++ b/fs/fuse/kio/pcs/pcs_fuse_kdirect.c
>>> @@ -1081,7 +1081,12 @@ static void pcs_kio_setattr_handle(struct fuse_inode *fi, struct fuse_req *req)
>>>    	r->end = req->end;
>>>    	if (di->size.op == PCS_SIZE_SHRINK) {
>>>    		BUG_ON(!mutex_is_locked(&req->io_inode->i_mutex));
>>> -		/* wait for aio reads in flight */
>>> +		/*
>>> +		 * Wait for already submitted aio reads. Further reads
>>> +		 * (including already queued to bg_queue) will be stopped
>>> +		 * by wait_shrink(), and they will be processed from
>>> +		 * _pcs_shrink_end().
>>> +		 */
>>
>> Why do we have to wait already submitted aio reads ?
> 
> Because there could be grow requests. This place is broken, I'm fixing that at the moment.
> One-dimensional size.op is not enough to drive race between grow and shrink, we need to
> convert PCS_SIZE_* into bit fields like it used to be before.
> 

I don't understand. There are no chances to races between grow and
shrink, because the shrink is protected from simultaneous execution with
write requests at the fuse level. No write requests - no grows.

>>>    		inode_dio_wait(req->io_inode);
>>>    		/*
>>>    		 * Writebackcache was flushed already so it is safe to
>>>