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Date:   Thu, 9 Aug 2018 09:14:18 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
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Subject: Re: [PATCH RFC v2 02/10] mm: Make shrink_slab() lockless
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On Wed 08-08-18 16:20:54, Kirill Tkhai wrote:
> [Added two more places needed srcu_dereference(). All ->shrinker_map
>  dereferences must be under SRCU, and this v2 adds missed in previous]
> 
> The patch makes shrinker list and shrinker_idr SRCU-safe
> for readers. This requires synchronize_srcu() on finalize
> stage unregistering stage, which waits till all parallel
> shrink_slab() are finished
> 
> Note, that patch removes rwsem_is_contended() checks from
> the code, and this does not result in delays during
> registration, since there is no waiting at all. Unregistration
> case may be optimized by splitting unregister_shrinker()
> in tho stages, and this is made in next patches.
>     
> Also, keep in mind, that in case of SRCU is not allowed
> to make unconditional (which is done in previous patch),
> it is possible to use percpu_rw_semaphore instead of it.
> percpu_down_read() will be used in shrink_slab_memcg()
> and in shrink_slab(), and consecutive calls
> 
>         percpu_down_write(percpu_rwsem);
>         percpu_up_write(percpu_rwsem);
> 
> will be used instead of synchronize_srcu().

An obvious question. Why didn't you go that way? What are pros/cons of
both approaches?
-- 
Michal Hocko
SUSE Labs