Re: sharing rescuer threads when WQ_MEM_RECLAIM needed? [was: Re: dm verity: don't use WQ_MEM_RECLAIM]

[Tejun Heo <tj@kernel.org>]

Hello,

On Thu, Sep 05, 2024 at 07:35:41PM -0400, Mike Snitzer wrote:
...
> > I wonder if there's any way to safely share the rescuer threads.
> 
> Oh, I like that idea, yes please! (would be surprised if it exists,
> but I love being surprised!).  Like Mikulas pointed out, we have had
> to deal with fundamental deadlocks due to resource sharing in DM.
> Hence the need for guaranteed forward progress that only
> WQ_MEM_RECLAIM can provide.

The most straightforward way to do this would be simply sharing the
workqueue across the entities that wanna be in the same forward progress
guarantee domain. It shouldn't be that difficult to make workqueues share a
rescuer either but may be a bit of an overkill.

Taking a step back tho, how would you determine which ones can share a
rescuer? Things which stack on top of each other can't share the rescuer cuz
higher layer occupying the rescuer and stall lower layers and thus deadlock.
The rescuers can be shared across independent stacks of dm devices but that
sounds like that will probably involve some graph walking. Also, is this a
real problem?

Thanks.

-- 
tejun