Re: [PATCH] mm: backing-dev: use kfree_rcu() instead of synchronize_rcu_expedited()

[Zqiang <qiang.zhang1211@gmail.com>]

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On 2021/10/15 上午10:57, Qiang Zhang wrote:
>
>
> Matthew Wilcox <willy@infradead.org <mailto:willy@infradead.org>> 
> 于2021年10月14日周四 下午7:26写道：
>
>     On Thu, Oct 14, 2021 at 04:24:33PM +0800, Zqiang wrote:
>     > The bdi_remove_from_list() is called in RCU softirq, however the
>     > synchronize_rcu_expedited() will produce sleep action, use
>     kfree_rcu()
>     > instead of it.
>     >
>     > Reported-by: Hao Sun <sunhao.th@gmail.com
>     <mailto:sunhao.th@gmail.com>>
>     > Signed-off-by: Zqiang <qiang.zhang1211@gmail.com
>     <mailto:qiang.zhang1211@gmail.com>>
>     > ---
>     >  include/linux/backing-dev-defs.h | 1 +
>     >  mm/backing-dev.c                 | 4 +---
>     >  2 files changed, 2 insertions(+), 3 deletions(-)
>     >
>     > diff --git a/include/linux/backing-dev-defs.h
>     b/include/linux/backing-dev-defs.h
>     > index 33207004cfde..35a093384518 100644
>     > --- a/include/linux/backing-dev-defs.h
>     > +++ b/include/linux/backing-dev-defs.h
>     > @@ -202,6 +202,7 @@ struct backing_dev_info {
>     >  #ifdef CONFIG_DEBUG_FS
>     >       struct dentry *debug_dir;
>     >  #endif
>     > +     struct rcu_head rcu;
>     >  };
>
>     >Instead of growing struct backing_dev_info, it seems to me this
>     rcu_head
>     >could be placed in a union with rb_node, since it will have been
>     removed
>     >from the bdi_tree by this point and the tree is never walked under
>     >RCU protection?
>
>
> Thanks for your advice, I find this bdi_tree is traversed under the 
> protection of a spin lock, not under the protection of RCU.
> I find this modification does not avoid the problem described in 
> patch, the flush_delayed_work() may be called in release_bdi()
> The same will cause problems.
> may be  we can replace queue_rcu_work() of call_rcu(&inode->i_rcu, 
> i_callback) or do you have any better suggestions?
>
> Thanks
> Zqiang


diff --git a/fs/inode.c b/fs/inode.c
index a49695f57e1e..300beb19aed6 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -219,9 +219,9 @@ void free_inode_nonrcu(struct inode *inode)
  }
  EXPORT_SYMBOL(free_inode_nonrcu);

-static void i_callback(struct rcu_head *head)
+static void i_callback(struct work_struct *work)
  {
-       struct inode *inode = container_of(head, struct inode, i_rcu);
+       struct inode *inode = container_of(to_rcu_work(work), struct 
inode, rwork);
         if (inode->free_inode)
                 inode->free_inode(inode);
         else
@@ -248,7 +248,7 @@ static struct inode *alloc_inode(struct super_block *sb)
                                 return NULL;
                 }
                 inode->free_inode = ops->free_inode;
-               i_callback(&inode->i_rcu);
+               i_callback(&inode->rwork.work);
                 return NULL;
         }

@@ -289,7 +289,8 @@ static void destroy_inode(struct inode *inode)
                         return;
         }
         inode->free_inode = ops->free_inode;
-       call_rcu(&inode->i_rcu, i_callback);
+       INIT_RCU_WORK(&inode->rwork, i_callback);
+       queue_rcu_work(system_wq, &inode->rwork);
  }

  /**
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8903a95611a2..006d769791a8 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -686,7 +686,7 @@ struct inode {
         struct list_head        i_wb_list;      /* backing dev 
writeback list */
         union {
                 struct hlist_head       i_dentry;
-               struct rcu_head         i_rcu;
+               struct rcu_work         rwork;
         };
         atomic64_t              i_version;
         atomic64_t              i_sequence; /* see futex */


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