Re: [PATCH] NFS: Fix RCU warnings in nfs_inode_return_delegation_noreclaim() [ver #2]

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Mar 18, 2010 at 01:33:02PM +0000, David Howells wrote:
> Fix a number of RCU warnings in nfs_inode_return_delegation_noreclaim().
> nfs_inode_return_delegation_noreclaim() and nfs_inode_return_delegation() don't
> need to use rcu_dereference() outside the spinlocked region as they merely
> examin the pointer and don't follow it, thus rendering unnecessary the need to
> impose a partial ordering over the one item of interest.
> 
> nfs_detach_delegation_locked() doesn't need rcu_derefence() because it can only
> be called if nfs_client::cl_lock is held, and that guards against anyone
> changing nfsi->delegation under it.  Furthermore, the barrier in
> rcu_derefence() is superfluous, given that the spin_lock() is also a barrier.
> 
> nfs_free_delegation() should be using rcu_dereference_check() to validate the
> state that the data is in (the delegation inode must have been cleared).  By
> this point, the delegation is being released, so no one else should be
> attempting to use the saved credentials, and they can be cleared.  However,
> rcu_assign_pointer() should be used to clear them, and the delegation itself
> must still use call_rcu() as the list of delegations could be being traversed.

Thank you for fixing these up!

This looks good at the moment, however, the sparse changes that Arnd
Bergmann is working on will invalidate a couple of the changes below.
Of course, better a future problem than a here-and-now problem, but
is there an easy way to fix both?

							Thanx, Paul

> [ INFO: suspicious rcu_dereference_check() usage. ]
> ---------------------------------------------------
> fs/nfs/delegation.c:332 invoked rcu_dereference_check() without protection!
> 
> other info that might help us debug this:
> 
> 
> rcu_scheduler_active = 1, debug_locks = 0
> 2 locks held by mount.nfs4/2281:
>  #0:  (&type->s_umount_key#34){+.+...}, at: [<ffffffff810b25b4>] deactivate_super+0x60/0x80
>  #1:  (iprune_sem){+.+...}, at: [<ffffffff810c332a>] invalidate_inodes+0x39/0x13a
> 
> stack backtrace:
> Pid: 2281, comm: mount.nfs4 Not tainted 2.6.34-rc1-cachefs #110
> Call Trace:
>  [<ffffffff8105149f>] lockdep_rcu_dereference+0xaa/0xb2
>  [<ffffffffa00b4591>] nfs_inode_return_delegation_noreclaim+0x5b/0xa0 [nfs]
>  [<ffffffffa0095d63>] nfs4_clear_inode+0x11/0x1e [nfs]
>  [<ffffffff810c2d92>] clear_inode+0x9e/0xf8
>  [<ffffffff810c3028>] dispose_list+0x67/0x10e
>  [<ffffffff810c340d>] invalidate_inodes+0x11c/0x13a
>  [<ffffffff810b1dc1>] generic_shutdown_super+0x42/0xf4
>  [<ffffffff810b1ebe>] kill_anon_super+0x11/0x4f
>  [<ffffffffa009893c>] nfs4_kill_super+0x3f/0x72 [nfs]
>  [<ffffffff810b25bc>] deactivate_super+0x68/0x80
>  [<ffffffff810c6744>] mntput_no_expire+0xbb/0xf8
>  [<ffffffff810c681b>] release_mounts+0x9a/0xb0
>  [<ffffffff810c689b>] put_mnt_ns+0x6a/0x79
>  [<ffffffffa00983a1>] nfs_follow_remote_path+0x5a/0x146 [nfs]
>  [<ffffffffa0098334>] ? nfs_do_root_mount+0x82/0x95 [nfs]
>  [<ffffffffa00985a9>] nfs4_try_mount+0x75/0xaf [nfs]
>  [<ffffffffa0098874>] nfs4_get_sb+0x291/0x31a [nfs]
>  [<ffffffff810b2059>] vfs_kern_mount+0xb8/0x177
>  [<ffffffff810b2176>] do_kern_mount+0x48/0xe8
>  [<ffffffff810c810b>] do_mount+0x782/0x7f9
>  [<ffffffff810c8205>] sys_mount+0x83/0xbe
>  [<ffffffff81001eeb>] system_call_fastpath+0x16/0x1b
> 
> Also on:
> 
> fs/nfs/delegation.c:215 invoked rcu_dereference_check() without protection!
>  [<ffffffff8105149f>] lockdep_rcu_dereference+0xaa/0xb2
>  [<ffffffffa00b4223>] nfs_inode_set_delegation+0xfe/0x219 [nfs]
>  [<ffffffffa00a9c6f>] nfs4_opendata_to_nfs4_state+0x2c2/0x30d [nfs]
>  [<ffffffffa00aa15d>] nfs4_do_open+0x2a6/0x3a6 [nfs]
>  ...
> 
> And:
> 
> fs/nfs/delegation.c:40 invoked rcu_dereference_check() without protection!
>  [<ffffffff8105149f>] lockdep_rcu_dereference+0xaa/0xb2
>  [<ffffffffa00b3bef>] nfs_free_delegation+0x3d/0x6e [nfs]
>  [<ffffffffa00b3e71>] nfs_do_return_delegation+0x26/0x30 [nfs]
>  [<ffffffffa00b406a>] __nfs_inode_return_delegation+0x1ef/0x1fe [nfs]
>  [<ffffffffa00b448a>] nfs_client_return_marked_delegations+0xc9/0x124 [nfs]
>  ...
> 
> 
> Signed-off-by: David Howells <dhowells@redhat.com>
> ---
> 
>  fs/nfs/delegation.c |   11 ++++++-----
>  1 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/nfs/delegation.c b/fs/nfs/delegation.c
> index 2563beb..fa9b7c5 100644
> --- a/fs/nfs/delegation.c
> +++ b/fs/nfs/delegation.c
> @@ -37,7 +37,8 @@ static void nfs_free_delegation(struct nfs_delegation *delegation)
>  {
>  	struct rpc_cred *cred;
> 
> -	cred = rcu_dereference(delegation->cred);
> +	cred = rcu_dereference_check(delegation->cred,
> +				     delegation->inode == NULL);
>  	rcu_assign_pointer(delegation->cred, NULL);
>  	call_rcu(&delegation->rcu, nfs_free_delegation_callback);
>  	if (cred)
> @@ -167,7 +168,7 @@ static struct inode *nfs_delegation_grab_inode(struct nfs_delegation *delegation
> 
>  static struct nfs_delegation *nfs_detach_delegation_locked(struct nfs_inode *nfsi, const nfs4_stateid *stateid)
>  {
> -	struct nfs_delegation *delegation = rcu_dereference(nfsi->delegation);
> +	struct nfs_delegation *delegation = nfsi->delegation;

Arnd's work will flag this one.

> 
>  	if (delegation == NULL)
>  		goto nomatch;
> @@ -212,7 +213,7 @@ int nfs_inode_set_delegation(struct inode *inode, struct rpc_cred *cred, struct
>  	spin_lock_init(&delegation->lock);
> 
>  	spin_lock(&clp->cl_lock);
> -	if (rcu_dereference(nfsi->delegation) != NULL) {
> +	if (nfsi->delegation != NULL) {

And this one.  I thought that Trond said that clp->cl_lock protects
this one, in which case this should work:

	if (rcu_dereference_check(nfsi->delegation,
				  lockdep_is_held(&clp->cl_lock)) != NULL) {

>  		if (memcmp(&delegation->stateid, &nfsi->delegation->stateid,
>  					sizeof(delegation->stateid)) == 0 &&
>  				delegation->type == nfsi->delegation->type) {
> @@ -329,7 +330,7 @@ void nfs_inode_return_delegation_noreclaim(struct inode *inode)
>  	struct nfs_inode *nfsi = NFS_I(inode);
>  	struct nfs_delegation *delegation;
> 
> -	if (rcu_dereference(nfsi->delegation) != NULL) {
> +	if (nfsi->delegation != NULL) {

And this one, although the check for cp->cl_lock obviously won't work here.

>  		spin_lock(&clp->cl_lock);
>  		delegation = nfs_detach_delegation_locked(nfsi, NULL);
>  		spin_unlock(&clp->cl_lock);
> @@ -345,7 +346,7 @@ int nfs_inode_return_delegation(struct inode *inode)
>  	struct nfs_delegation *delegation;
>  	int err = 0;
> 
> -	if (rcu_dereference(nfsi->delegation) != NULL) {
> +	if (nfsi->delegation != NULL) {

Ditto...

>  		spin_lock(&clp->cl_lock);
>  		delegation = nfs_detach_delegation_locked(nfsi, NULL);
>  		spin_unlock(&clp->cl_lock);
>