Re: [PATCH 4/5] drm/ttm: introduce pool_shrink_rwsem

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Hi Christian,

On 2023/8/16 17:14, Christian König wrote:
Am 16.08.23 um 10:34 schrieb Qi Zheng:
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>
---
  drivers/gpu/drm/ttm/ttm_pool.c | 15 +++++++++++++++
  include/linux/shrinker.h       |  1 -
  mm/shrinker.c                  | 15 ---------------
  3 files changed, 15 insertions(+), 16 deletions(-)

diff --git a/drivers/gpu/drm/ttm/ttm_pool.c b/drivers/gpu/drm/ttm/ttm_pool.c
index cddb9151d20f..713b1c0a70e1 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)

Please rename that function to ttm_pool_synchronize_shrinkers().

OK, will do.


With that done feel free to add Reviewed-by: Christian König <christian.koenig@xxxxxxx>


Thanks,
Qi

Regards,
Christian.

+{
+    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 8dc15aa37410..6b5843c3b827 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -103,7 +103,6 @@ extern int __printf(2, 3) register_shrinker(struct shrinker *shrinker,
                          const char *fmt, ...);
  extern void unregister_shrinker(struct shrinker *shrinker);
  extern void free_prealloced_shrinker(struct shrinker *shrinker);
-extern void synchronize_shrinkers(void);
  #ifdef CONFIG_SHRINKER_DEBUG
  extern int __printf(2, 3) shrinker_debugfs_rename(struct shrinker *shrinker,
diff --git a/mm/shrinker.c b/mm/shrinker.c
index 043c87ccfab4..a16cd448b924 100644
--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -692,18 +692,3 @@ void unregister_shrinker(struct shrinker *shrinker)
      shrinker->nr_deferred = NULL;
  }
  EXPORT_SYMBOL(unregister_shrinker);
-
-/**
- * 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);




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