Re: [PATCH v4 43/48] drm/ttm: introduce pool_shrink_rwsem

[Daniel Vetter]

On Mon, Aug 07, 2023 at 07:09:31PM +0800, Qi Zheng wrote:
> Currently, the synchronize_shrinkers() is only used by TTM pool. It only
> requires that no shrinkers run in parallel.
> 
> After we use RCU+refcount method to implement the lockless slab shrink,
> we can not use shrinker_rwsem or synchronize_rcu() to guarantee that all
> shrinker invocations have seen an update before freeing memory.
> 
> So we introduce a new pool_shrink_rwsem to implement a private
> synchronize_shrinkers(), so as to achieve the same purpose.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>

On the 5 drm patches (I counted 2 ttm and 3 drivers) for merging through
some other tree (since I'm assuming that's how this will land):

Acked-by: Daniel Vetter <daniel.vetter@xxxxxxxx>

> ---
>  drivers/gpu/drm/ttm/ttm_pool.c | 15 +++++++++++++++
>  include/linux/shrinker.h       |  2 --
>  mm/shrinker.c                  | 15 ---------------
>  3 files changed, 15 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/gpu/drm/ttm/ttm_pool.c b/drivers/gpu/drm/ttm/ttm_pool.c
> index c9c9618c0dce..38b4c280725c 100644
> --- a/drivers/gpu/drm/ttm/ttm_pool.c
> +++ b/drivers/gpu/drm/ttm/ttm_pool.c
> @@ -74,6 +74,7 @@ static struct ttm_pool_type global_dma32_uncached[MAX_ORDER + 1];
>  static spinlock_t shrinker_lock;
>  static struct list_head shrinker_list;
>  static struct shrinker *mm_shrinker;
> +static DECLARE_RWSEM(pool_shrink_rwsem);
>  
>  /* Allocate pages of size 1 << order with the given gfp_flags */
>  static struct page *ttm_pool_alloc_page(struct ttm_pool *pool, gfp_t gfp_flags,
> @@ -317,6 +318,7 @@ static unsigned int ttm_pool_shrink(void)
>  	unsigned int num_pages;
>  	struct page *p;
>  
> +	down_read(&pool_shrink_rwsem);
>  	spin_lock(&shrinker_lock);
>  	pt = list_first_entry(&shrinker_list, typeof(*pt), shrinker_list);
>  	list_move_tail(&pt->shrinker_list, &shrinker_list);
> @@ -329,6 +331,7 @@ static unsigned int ttm_pool_shrink(void)
>  	} else {
>  		num_pages = 0;
>  	}
> +	up_read(&pool_shrink_rwsem);
>  
>  	return num_pages;
>  }
> @@ -572,6 +575,18 @@ void ttm_pool_init(struct ttm_pool *pool, struct device *dev,
>  }
>  EXPORT_SYMBOL(ttm_pool_init);
>  
> +/**
> + * synchronize_shrinkers - Wait for all running shrinkers to complete.
> + *
> + * This is useful to guarantee that all shrinker invocations have seen an
> + * update, before freeing memory, similar to rcu.
> + */
> +static void synchronize_shrinkers(void)
> +{
> +	down_write(&pool_shrink_rwsem);
> +	up_write(&pool_shrink_rwsem);
> +}
> +
>  /**
>   * ttm_pool_fini - Cleanup a pool
>   *
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index c55c07c3f0cb..025c8070dd86 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -103,8 +103,6 @@ struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...);
>  void shrinker_register(struct shrinker *shrinker);
>  void shrinker_free(struct shrinker *shrinker);
>  
> -extern void synchronize_shrinkers(void);
> -
>  #ifdef CONFIG_SHRINKER_DEBUG
>  extern int __printf(2, 3) shrinker_debugfs_rename(struct shrinker *shrinker,
>  						  const char *fmt, ...);
> diff --git a/mm/shrinker.c b/mm/shrinker.c
> index 3ab301ff122d..a27779ed3798 100644
> --- a/mm/shrinker.c
> +++ b/mm/shrinker.c
> @@ -650,18 +650,3 @@ void shrinker_free(struct shrinker *shrinker)
>  	kfree(shrinker);
>  }
>  EXPORT_SYMBOL_GPL(shrinker_free);
> -
> -/**
> - * synchronize_shrinkers - Wait for all running shrinkers to complete.
> - *
> - * This is equivalent to calling unregister_shrink() and register_shrinker(),
> - * but atomically and with less overhead. This is useful to guarantee that all
> - * shrinker invocations have seen an update, before freeing memory, similar to
> - * rcu.
> - */
> -void synchronize_shrinkers(void)
> -{
> -	down_write(&shrinker_rwsem);
> -	up_write(&shrinker_rwsem);
> -}
> -EXPORT_SYMBOL(synchronize_shrinkers);
> -- 
> 2.30.2
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch