Re: [PATCH V3] md: move bitmap_destroy before __md_stop

[NeilBrown <neilb@suse.com>]

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On Thu, Mar 09 2017, Shaohua Li wrote:

> On Fri, Mar 10, 2017 at 11:51:09AM +1100, Neil Brown wrote:
>> On Thu, Mar 09 2017, Shaohua Li wrote:
....
>> > I think it's ok to add this part into bitmap_destroy, as we need to call
>> > bitmap_destroy before mddev_detach. Look at the usage of mddev_detach, at in
>> > one place (level_store()), we wait for the IO without bitmap_destroy. I think
>> > we should keep this part code in mddev_detach. Maybe create a small function,
>> > let both mddev_detach and bitmap_destroy call it.
>> 
>> I don't think level_store() needs to explicitly wait for behind io.
>> It calls mddev_suspend(), which calls the ->quiesce function in the
>> personality, which is responsible for waiting for all pending IO,
>> including behind.  raid1.c does this correctly.
>
> Can you elaborate Where >quiesce waits for behind IO in raid1? It's not
> obvious. It really should though.

Ahh - you are correct.
allow_barrier() is called by call_bio_endio(), which can be called
before behind-writes complete.

My bi_phys_segments series actually fixed that, by counting every write
request in nr_pending so that nr_pending wouldn't drop to zero until all
the writes had completed.

We should probably come up with a simple fix for -stable.

Unfortunately it doesn't look at all easy.
I'll revisit my bi_phys_segments series next week and try to sort out
the best way to fix this issue.

NeilBrown

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