Re: [PATCH] padata: Remove broken queue flushing

[Daniel Jordan]

On Tue, Nov 19, 2019 at 01:17:31PM +0800, Herbert Xu wrote:
> The function padata_flush_queues is fundamentally broken because
> it cannot force padata users to complete the request that is
> underway.  IOW padata has to passively wait for the completion
> of any outstanding work.
> 
> As it stands flushing is used in two places.  Its use in padata_stop
> is simply unnecessary because nothing depends on the queues to
> be flushed afterwards.
> 
> The other use in padata_replace is more substantial as we depend
> on it to free the old pd structure.  This patch instead uses the
> pd->refcnt to dynamically free the pd structure once all requests
> are complete.

__padata_free unconditionally frees pd, so a padata job might choke on it
later.  padata_do_parallel calls seem safe because they use RCU, but it seems
possible that a job could call padata_do_serial after the instance is gone.

Best idea I can think of now is to indicate the instance has been freed in the
pd before dropping the initial pd ref in __padata_free, and use that to bail
out early from places that touch the instance or its data (workqueues say).
Will think more on this.


(By the way, I was on leave longer than anticipated, so thanks for picking up
my slack on this patch.  I plan to repost my other padata fixes soon.)