Re: [Drbd-dev] [DRBD-8.0 PATCH] Fix deadlock between transfer log and resync

[Lars Ellenberg <lars.ellenberg@linbit.com>]

On Fri, Jan 11, 2008 at 10:29:57AM -0500, Graham, Simon wrote:
> The attached patches fix some deadlocks between the transfer log and
> resync - when the TL is in use (previously only protocols A and B but
> now all protocols), if there is a request on the TL that conflicts with
> a resync region, the code would deadlock with the resync processing
> waiting for the AL area to be clean and new requests that might lead to
> a barrier that would clear out the TL blocked by the resync.
> 
> The attached patches include the following:
> 
> 1. Non-TCQ DRBD Barrier implementation on target now flushes 
>    disk to force cached data to disk.

this is correct, we should do that.

> 2. A deadlock between resync and requests sitting in the TL
>    is fixed - if a resync request is started that conflicts
>    with entries in the TL, a DRBD barrier is initiated - this
>    will clear up the TL when the barrier ack is received and
>    allow the resync to procede.
> 3. When changing role from Primary, it is necessary to clear out
>    transfer log - do this by initiating barrier
> 4. When SyncTarget is also Primary, it is possible for
> drbd_try_rs_begin_io
>    to never make progress due to entries in tl that will not be flushed.
>    Change code to initiate barrier IF conflict with AL is found.

these will be handled differently and more generically,
as recently discussed in the "Crash in lru_cache.c" thread.


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