Hi
On Mon, 10 Aug 2015, NeilBrown wrote:
>
> Hi Mikulas,
> I have a customer hitting the deadlock you described over a year ago
> in:
>
> Subject: [PATCH] block: flush queued bios when the process
> blocks
Ask block layer maintainers to accept that patch.
> I notice that patch never went upstream.
>
> Has anything else been done to fix this deadlock?
>
> My thought was that something like the below would be sufficient. Do
> you see any problem with that? It avoids the deadlock by dropping the
> lock while sleeping.
The patch below introduces a bug - if the user is continuously reading the
same chunk (for example, by issuing multiple direct i/o requests to the
same chunk), the function pending_complete never finishes.
We need to hold the lock while waiting to make sure that no new read
requests are submitted.
Mikulas
> Thanks
> NeilBrown
>
> diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
> index 7c82d3ccce87..d29bcd02f9cf 100644
> --- a/drivers/md/dm-snap.c
> +++ b/drivers/md/dm-snap.c
> @@ -1454,6 +1454,7 @@ static void pending_complete(struct dm_snap_pending_exception *pe, int success)
> }
> *e = pe->e;
>
> +retry:
> down_write(&s->lock);
> if (!s->valid) {
> free_completed_exception(e);
> @@ -1462,7 +1463,11 @@ static void pending_complete(struct dm_snap_pending_exception *pe, int success)
> }
>
> /* Check for conflicting reads */
> - __check_for_conflicting_io(s, pe->e.old_chunk);
> + if (__chunk_size_tracked(s, pe->e.old_chunk)) {
> + up_write(&s->lock);
> + msleep(1);
> + goto retry;
> + }
>
> /*
> * Add a proper exception, and remove the
>
--
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