linux-kernel - Re: [PATCH] nfs: delegation: Replace rcu_assign_pointer() with RCU_INIT

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Message-ID: <CAHQdGtQGWBkH23eTF-1=dGcz18QK8atHEom0vy9eWhPXPtCsfg@mail.gmail.com>
Date:	Fri, 22 Aug 2014 10:49:09 -0400
From:	Trond Myklebust <trond.myklebust@...marydata.com>
To:	Andreea-Cristina Bernat <bernat.ada@...il.com>
Cc:	Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
	Linux Kernel mailing list <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] nfs: delegation: Replace rcu_assign_pointer() with RCU_INIT_POINTER()

On Fri, Aug 22, 2014 at 10:07 AM, Andreea-Cristina Bernat
<bernat.ada@...il.com> wrote:
> The use of "rcu_assign_pointer()" is NULLing out the pointer.
> According to RCU_INIT_POINTER()'s block comment:
> "1.   This use of RCU_INIT_POINTER() is NULLing out the pointer"
> it is better to use it instead of rcu_assign_pointer() because it has a
> smaller overhead.
>
> The following Coccinelle semantic patch was used:
> @@
> @@
>
> - rcu_assign_pointer
> + RCU_INIT_POINTER
>   (..., NULL)
>
> Signed-off-by: Andreea-Cristina Bernat <bernat.ada@...il.com>
> ---
>  fs/nfs/delegation.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/nfs/delegation.c b/fs/nfs/delegation.c
> index 5d8ccec..349e63b 100644
> --- a/fs/nfs/delegation.c
> +++ b/fs/nfs/delegation.c
> @@ -249,7 +249,7 @@ nfs_detach_delegation_locked(struct nfs_inode *nfsi,
>         list_del_rcu(&delegation->super_list);
>         delegation->inode = NULL;
>         nfsi->delegation_state = 0;
> -       rcu_assign_pointer(nfsi->delegation, NULL);
> +       RCU_INIT_POINTER(nfsi->delegation, NULL);
>         spin_unlock(&delegation->lock);
>         return delegation;
>  }
> --
> 1.9.1

Sorry, but NACK.

We do need the memory barriers here, as the nfs_inode is globally
visible, and other threads may be checking nfsi->delegation_state
and/or delegation->inode in order to conclude whether or not there is
still a delegation associated with this inode.

Cheers
  Trond
-- 
Trond Myklebust

Linux NFS client maintainer, PrimaryData

trond.myklebust@...marydata.com
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