Re: [cocci] patch idea: convert trivial call_rcu users to kfree_rcu

[Uladzislau Rezki <urezki@gmail.com>]

Hello.

> 
> 
> On Mon, 27 May 2024, Paul E. McKenney wrote:
> 
> > On Mon, May 27, 2024 at 10:13:40AM +0200, Julia Lawall wrote:
> > >
> > >
> > > On Mon, 27 May 2024, Vlastimil Babka wrote:
> > >
> > > > Hi,
> > > >
> > > > one bit from LSF/MM discussions is that there might be call_rcu users with a
> > > > callback that only does a kmem_cache_free() to a specific cache. Since SLOB
> > > > was removed, it's always ok to use kfree() and thus also kfree_rcu() on
> > > > allocations from kmem_cache_alloc() in addition to kmalloc(). Thus, such
> > > > call_rcu() users might be simplified to kfree_rcu(). I found some cases
> > > > semi-manually, but I'd expect coccinelle could help here so if anyone wants
> > > > to take this task, feel free to.
> > >
> > > Thanks for the suggestion!  I will try to look into it.
> >
> > Thank you both!
> 
> I found the following functions.  Do you want some other information, such
> as where they are called from?
> 
> Ignore the -s at the beginning of some lines.  Those are for emphasis. not
> to suggest to remove the code.
> 
> I checked that the functions are only used in calls to call_rcu.
> 
> Without more effort, Coccinelle only looks for functions defined in the
> same file.  Here are the functions that are passed to call_rcu where the
> function is not defined in the same file:
> 
> need definition for audit_free_rule_rcu
> need definition for __i915_gem_free_object_rcu
> need definition for io_eventfd_ops
> need definition for ip_vs_dest_dst_rcu_free
> need definition for __put_task_struct_rcu_cb
> need definition for radix_tree_node_rcu_free
> 
> They all do something more, although radix_tree_node_rcu_free doesn't do
> much more (some memsets).
> 
> julia
> 
> diff -u -p /home/jll/linux/drivers/net/wireguard/allowedips.c /tmp/nothing/drivers/net/wireguard/allowedips.c
> --- /home/jll/linux/drivers/net/wireguard/allowedips.c
> +++ /tmp/nothing/drivers/net/wireguard/allowedips.c
> @@ -50,7 +50,6 @@ static void push_rcu(struct allowedips_n
> 
>  static void node_free_rcu(struct rcu_head *rcu)
>  {
> -	kmem_cache_free(node_cache, container_of(rcu, struct allowedips_node, rcu));
>  }
> 
>  static void root_free_rcu(struct rcu_head *rcu)
> diff -u -p /home/jll/linux/fs/ecryptfs/dentry.c /tmp/nothing/fs/ecryptfs/dentry.c
> --- /home/jll/linux/fs/ecryptfs/dentry.c
> +++ /tmp/nothing/fs/ecryptfs/dentry.c
> @@ -53,8 +53,6 @@ struct kmem_cache *ecryptfs_dentry_info_
> 
>  static void ecryptfs_dentry_free_rcu(struct rcu_head *head)
>  {
> -	kmem_cache_free(ecryptfs_dentry_info_cache,
> -		container_of(head, struct ecryptfs_dentry_info, rcu));
>  }
> 
>  /**
> diff -u -p /home/jll/linux/kernel/fork.c /tmp/nothing/kernel/fork.c
> --- /home/jll/linux/kernel/fork.c
> +++ /tmp/nothing/kernel/fork.c
> @@ -378,7 +378,6 @@ static struct kmem_cache *thread_stack_c
> 
>  static void thread_stack_free_rcu(struct rcu_head *rh)
>  {
> -	kmem_cache_free(thread_stack_cache, rh);
>  }
> 
>  static void thread_stack_delayed_free(struct task_struct *tsk)
> diff -u -p /home/jll/linux/kernel/workqueue.c /tmp/nothing/kernel/workqueue.c
> --- /home/jll/linux/kernel/workqueue.c
> +++ /tmp/nothing/kernel/workqueue.c
> @@ -5024,8 +5024,6 @@ fail:
> 
>  static void rcu_free_pwq(struct rcu_head *rcu)
>  {
> -	kmem_cache_free(pwq_cache,
> -			container_of(rcu, struct pool_workqueue, rcu));
>  }
> 
>  /*
> diff -u -p /home/jll/linux/net/ipv4/inetpeer.c /tmp/nothing/net/ipv4/inetpeer.c
> --- /home/jll/linux/net/ipv4/inetpeer.c
> +++ /tmp/nothing/net/ipv4/inetpeer.c
> @@ -130,7 +130,6 @@ static struct inet_peer *lookup(const st
> 
>  static void inetpeer_free_rcu(struct rcu_head *head)
>  {
> -	kmem_cache_free(peer_cachep, container_of(head, struct inet_peer, rcu));
>  }
> 
>  /* perform garbage collect on all items stacked during a lookup */
> diff -u -p /home/jll/linux/net/ipv6/xfrm6_tunnel.c /tmp/nothing/net/ipv6/xfrm6_tunnel.c
> --- /home/jll/linux/net/ipv6/xfrm6_tunnel.c
> +++ /tmp/nothing/net/ipv6/xfrm6_tunnel.c
> @@ -180,8 +180,6 @@ EXPORT_SYMBOL(xfrm6_tunnel_alloc_spi);
> 
>  static void x6spi_destroy_rcu(struct rcu_head *head)
>  {
> -	kmem_cache_free(xfrm6_tunnel_spi_kmem,
> -			container_of(head, struct xfrm6_tunnel_spi, rcu_head));
>  }
> 
>  static void xfrm6_tunnel_free_spi(struct net *net, xfrm_address_t *saddr)
> diff -u -p /home/jll/linux/security/security.c /tmp/nothing/security/security.c
> --- /home/jll/linux/security/security.c
> +++ /tmp/nothing/security/security.c
> @@ -1599,7 +1599,6 @@ static void inode_free_by_rcu(struct rcu
>  	/*
>  	 * The rcu head is at the start of the inode blob
>  	 */
> -	kmem_cache_free(lsm_inode_cache, head);
>  }
> 
>  /**
> 
See below some extra functions which can be eliminated. How i found them:

find ./ -name "*.c" -o -name "*.h" | xargs grep -rn call_rcu -B 10 | grep kmem_cache_free

<snip>
 static void nfsd4_free_file_rcu(struct rcu_head *rcu);
 static void lima_fence_release(struct dma_fence *fence);
 void intel_context_free(struct intel_context *ce);
 static void ipmr_cache_free(struct mfc_cache *c);
 static inline void alias_free_mem_rcu(struct fib_alias *fa);
 dsthash_free(struct xt_hashlimit_htable *ht, struct dsthash_ent *ent);
 void nf_ct_expect_put(struct nf_conntrack_expect *exp);
 static void node_free(struct net *net, struct fib6_node *fn);
 static inline void ip6mr_cache_free(struct mfc6_cache *c);
 static void posix_timer_free(struct k_itimer *tmr);
 static void thread_stack_delayed_free(struct task_struct *tsk);
<snip>

Those are can be replaced by directly calling of kfree_rcu() instead of call_rcu().

--
Uladzislau Rezki