linux-kernel - Re: [PATCH] NFS: Fix RCU warnings in nfs_inode_return_delegation

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Date:	Wed, 07 Apr 2010 14:22:41 +0100
From:	David Howells <dhowells@...hat.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	dhowells@...hat.com, Eric Dumazet <eric.dumazet@...il.com>,
	Trond.Myklebust@...app.com, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] NFS: Fix RCU warnings in nfs_inode_return_delegation_noreclaim() [ver #2]

Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> +#define rcu_access_pointer(p, c) \

Why is there a need for 'c'?

> +#define rcu_dereference_protect(p, c) \

I'd prefer rcu_dereference_protected(), I think.  This macro doesn't protect
anything.  Also, again, why the need for 'c'?

For instance, in:

	static struct nfs_delegation *nfs_detach_delegation_locked(struct nfs_inode *nfsi, const nfs4_stateid *stateid)
	{
		struct nfs_delegation *delegation =
			rcu_dereference_protected(nfsi->delegation, ????);

what would be the condition?  That the spinlock is held?  That's a condition
for calling the function.

And in:

	void nfs_inode_return_delegation_noreclaim(struct inode *inode)
	{
		struct nfs_client *clp = NFS_SERVER(inode)->nfs_client;
		struct nfs_inode *nfsi = NFS_I(inode);
		struct nfs_delegation *delegation;

		if (rcu_access_pointer(nfsi->delegation, ????) != NULL) {

what would be the condition here?  There's no lock to check - that's the whole
point of the macro.  I also can't give it nfsi->delegation to check as the
value may change between the two accesses.

David
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