Re: [RFC][WIP] DIO simplification and AIO-DIO stability

[Wendy Cheng]

Suparna Bhattacharya wrote:

> > A new DIO flag is added into our distribution (2.6.9 based) to work 
> > around the problem by moving the inode semaphore acquiring within 
> > __blockdev_direct_IO() (patch attached) into GFS code path (so lock 
> > order can be re-arranged). The new lock granularity is not ideal but it 
> > gets us out of this deadlock.
> >    
>
> Could you help me understand in a little more detail why DIO_OWN_LOCKING
> does not work for you ? Is the releasing of i_sem during READ a problem ?
> Doesn't holding i_sem for the entire duration of IO for read slow down
> concurrent DIO reads to different parts of the file ?
>  

We don't mess around with i_sem during read. So that piece of locking 
code is ok (but I have to say it looks very messy). The thing we can't 
work around is #518 in get_more_blocks() (from 2.6.9 base code so line 
offset may be different but you can get the idea):

   491 static int get_more_blocks(struct dio *dio)
   492 {
   493         int ret;
   ......
   516
   517                 create = dio->rw == WRITE;
   518                 if ((dio->lock_type == DIO_LOCKING) || 
(dio->lock_type =        = DIO_CLUSTER_LOCKING)) {
   519                         if (dio->block_in_file < 
(i_size_read(dio->inode        ) >>
   520                                                         
dio->blkbits))
   521                                 create = 0;
   522                 } else if (dio->lock_type == DIO_NO_LOCKING) {
   523                         create = 0;
   524                 }

> One of the things I wanted to achieve in the proposal was to avoid
> the need for these various locking mode flag checks in the DIO code,
> leaving it to the higher level to just select the right entry points,
> i.e. the _nolock or lock versions of generic_file_aio_write et al.
>  
That's a good news - these DIO locking flags are all last-minute 
bandaids anyway.

> Would appreciate your thoughts on this, once you've had a chance to
> go throught it.
>
>  
On the road all next week but will get to it when I'm back to office. On 
the other hand, thank you for looking into this.

-- Wendy
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/