On Wed, Apr 21, 2010 at 1:59 AM, Benny Halevy <bhalevy@xxxxxxxxxxx> wrote:
> On Apr. 16, 2010, 19:04 +0300, "William A. (Andy) Adamson" <androsadamson@xxxxxxxxx> wrote:
>> On Fri, Apr 16, 2010 at 11:52 AM, <andros@xxxxxxxxxx> wrote:
>>> From: Andy Adamson <andros@xxxxxxxxxx>
>>>
>>> A shared RCU device ID cache servicing multiple mounts of a single layout type
>>> per meta data server (struct nfs_client).
>>>
>>> Device IDs of type deviceid4 are required by all layout types, long lived and
>>> read at each I/O. They are added to the deviceid cache at first reference by
>>> a layout via GETDEVICEINFO and (currently) are only removed at umount.
>>>
>>> Reference count the device ID cache for each mounted file system
>>> in the initialize_mountpoint layoutdriver_io_operation.
>>>
>>> Dereference the device id cache on file system in the uninitialize_mountpoint
>>> layoutdriver_io_operation called at umount
>>>
>>> Signed-off-by: Andy Adamson <andros@xxxxxxxxxx>
>>> ---
>>> fs/nfs/pnfs.c | 119 +++++++++++++++++++++++++++++++++++++++++++++
>>> include/linux/nfs4_pnfs.h | 27 ++++++++++
>>> include/linux/nfs_fs_sb.h | 1 +
>>> 3 files changed, 147 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/fs/nfs/pnfs.c b/fs/nfs/pnfs.c
>>> index 91572aa..8492aef 100644
>>> --- a/fs/nfs/pnfs.c
>>> +++ b/fs/nfs/pnfs.c
>>> @@ -45,6 +45,7 @@
>>> #include <linux/nfs4.h>
>>> #include <linux/pnfs_xdr.h>
>>> #include <linux/nfs4_pnfs.h>
>>> +#include <linux/rculist.h>
>>>
>>> #include "internal.h"
>>> #include "nfs4_fs.h"
>>> @@ -2296,3 +2297,121 @@ struct pnfs_client_operations pnfs_ops = {
>>>
>>> EXPORT_SYMBOL(pnfs_unregister_layoutdriver);
>>> EXPORT_SYMBOL(pnfs_register_layoutdriver);
>>> +
>>> +
>>> +/* Device ID cache. Supports one layout type per struct nfs_client */
>>> +int
>>> +nfs4_alloc_init_deviceid_cache(struct nfs_client *clp,
>>> + void (*free_callback)(struct rcu_head *))
>>> +{
>>> + struct nfs4_deviceid_cache *c;
>>> +
>>> + c = kzalloc(sizeof(struct nfs4_deviceid_cache), GFP_KERNEL);
>>> + if (!c)
>>> + return -ENOMEM;
>>> + spin_lock(&clp->cl_lock);
>>> + if (clp->cl_devid_cache != NULL) {
>>> + kref_get(&clp->cl_devid_cache->dc_kref);
>>> + spin_unlock(&clp->cl_lock);
>>> + dprintk("%s [kref [%d]]\n", __func__,
>>> + atomic_read(&clp->cl_devid_cache->dc_kref.refcount));
>>> + kfree(c);
>>> + } else {
>>> + spin_lock_init(&c->dc_lock);
>>> + INIT_LIST_HEAD(&c->dc_deviceids);
>>> + kref_init(&c->dc_kref);
>>> + c->dc_free_callback = free_callback;
>>> + c->dc_clp = clp;
>>> + clp->cl_devid_cache = c;
>>> + spin_unlock(&clp->cl_lock);
>>> + dprintk("%s [new]\n", __func__);
>>> + }
>>> + return 0;
>>> +}
>>> +EXPORT_SYMBOL(nfs4_alloc_init_deviceid_cache);
>>> +
>>> +void
>>> +nfs4_init_deviceid_node(struct nfs4_deviceid *d)
>>> +{
>>> + INIT_LIST_HEAD(&d->de_node);
>>> + INIT_RCU_HEAD(&d->de_rcu);
>>> +}
>>> +EXPORT_SYMBOL(nfs4_init_deviceid_node);
>>> +
>>> +struct nfs4_deviceid *
>>> +nfs4_find_deviceid(struct nfs4_deviceid_cache *c, struct pnfs_deviceid *id)
>>> +{
>>> + struct nfs4_deviceid *d;
>>> +
>>> + rcu_read_lock();
>>> + list_for_each_entry_rcu(d, &c->dc_deviceids, de_node) {
>>> + if (!memcmp(&d->de_id, id, NFS4_PNFS_DEVICEID4_SIZE)) {
>>> + rcu_read_unlock();
>>> + return d;
>>> + }
>>> + }
>>> + rcu_read_unlock();
>
> I hope this is worth the added complexity...
>
> Out of curiosity, do you have a benchmark comparing the cost
> of rcu_read_lock (and unlock) vs. spin_lock(&c->dc_lock)?
The deviceid cache is read at each I/O. If we use a spin_lock to
protect the deviceid cache, this would mean that all I/0 is serialized
behind the spin_lock even though the deviceid cache is changed
infrequently. The RCU allows readers to "run almost naked" and does
not serialize I/O behind reading the deviceid cache.
So I think it is worth it. I have not benchmarked the difference.
>
>>> + return NULL;
>>> +}
>>> +EXPORT_SYMBOL(nfs4_find_deviceid);
>>> +
>>> +/*
>>> + * Add or kref_get a deviceid.
>>> + * GETDEVICEINFOs for same deviceid can race. If deviceid is found, discard new
>>> + */
>>> +void
>>> +nfs4_add_deviceid(struct nfs4_deviceid_cache *c, struct nfs4_deviceid *new)
>>> +{
>>> + struct nfs4_deviceid *d;
>>> +
>>> + spin_lock(&c->dc_lock);
>>> + list_for_each_entry_rcu(d, &c->dc_deviceids, de_node) {
>>> + if (!memcmp(&d->de_id, &new->de_id, NFS4_PNFS_DEVICEID4_SIZE)) {
>>> + spin_unlock(&c->dc_lock);
>>> + dprintk("%s [discard]\n", __func__);
>>> + c->dc_free_callback(&new->de_rcu);
>>> + }
>>> + }
>>> + list_add_rcu(&new->de_node, &c->dc_deviceids);
>>> + spin_unlock(&c->dc_lock);
>>> + dprintk("%s [new]\n", __func__);
>>> +}
>>> +EXPORT_SYMBOL(nfs4_add_deviceid);
>>> +
>>> +static int
>>> +nfs4_remove_deviceid(struct nfs4_deviceid_cache *c)
>>> +{
>>> + struct nfs4_deviceid *d;
>>> +
>>> + spin_lock(&c->dc_lock);
>>> + list_for_each_entry_rcu(d, &c->dc_deviceids, de_node) {
>>> + list_del_rcu(&d->de_node);
>>> + spin_unlock(&c->dc_lock);
>>> + synchronize_rcu();
>>> + c->dc_free_callback(&d->de_rcu);
>>> + return 1;
>>> + }
>>> + spin_unlock(&c->dc_lock);
>>> + return 0;
>>> +}
>>> +
>>> +static void
>>> +nfs4_free_deviceid_cache(struct kref *kref)
>>> +{
>>> + struct nfs4_deviceid_cache *cache =
>>> + container_of(kref, struct nfs4_deviceid_cache, dc_kref);
>>> + int more = 1;
>>> +
>>> + while (more)
>>> + more = nfs4_remove_deviceid(cache);
>>> + cache->dc_clp->cl_devid_cache = NULL;
>>
>> Need to take the cl_lock around this assignment
>>
>> spin_lock(&cache->dc_clp->cl_lock);
>> cache->dc_clp->cl_devid_cache = NULL
>> spin_unlock(&cache->dc_clp->cl_lock);
>>
>>
>
> That must happen atomically before kref_put.
> It's illegal to have cl_devid_cache be referenced by cache->dc_clp
> without a reference count backing it up.
> Otherwise, if accessed concurrently to this piece of code
> someone might call kref_get while the refcount is zero.
>
> Normally, you'd first clear the referencing pointer to prevent any
> new reference to it and only then kref_put it, e.g.:
>
> spin_lock(&cache->dc_clp->cl_lock);
> tmp = cache->dc_clp->cl_devid_cache;
> cache->dc_clp->cl_devid_cache = NULL
> spin_unlock(&cache->dc_clp->cl_lock);
> kref_put(&tmp->dc_kref, nfs4_free_deviceid_cache);
Good point. Thanks for the review. I'll rethink and resend
-->Andy
>
> Benny
>
>>> + kfree(cache);
>>> +}
>>> +
>>> +void
>>> +nfs4_put_deviceid_cache(struct nfs4_deviceid_cache *c)
>>> +{
>>> + dprintk("%s [%d]\n", __func__, atomic_read(&c->dc_kref.refcount));
>>> + kref_put(&c->dc_kref, nfs4_free_deviceid_cache);
>>> +}
>>> +EXPORT_SYMBOL(nfs4_put_deviceid_cache);
>>> diff --git a/include/linux/nfs4_pnfs.h b/include/linux/nfs4_pnfs.h
>>> index 1d521f4..2a88a06 100644
>>> --- a/include/linux/nfs4_pnfs.h
>>> +++ b/include/linux/nfs4_pnfs.h
>>> @@ -281,6 +281,33 @@ struct pnfs_devicelist {
>>> struct pnfs_deviceid dev_id[NFS4_PNFS_GETDEVLIST_MAXNUM];
>>> };
>>>
>>> +/*
>>> + * Device ID RCU cache. A device ID is unique per client ID and layout type.
>>> + */
>>> +struct nfs4_deviceid_cache {
>>> + spinlock_t dc_lock;
>>> + struct list_head dc_deviceids;
>>> + struct kref dc_kref;
>>> + void (*dc_free_callback)(struct rcu_head *);
>>> + struct nfs_client *dc_clp;
>>> +};
>>> +
>>> +/* Device ID cache node */
>>> +struct nfs4_deviceid {
>>> + struct list_head de_node;
>>> + struct rcu_head de_rcu;
>>> + struct pnfs_deviceid de_id;
>>> +};
>>> +
>>> +extern int nfs4_alloc_init_deviceid_cache(struct nfs_client *,
>>> + void (*free_callback)(struct rcu_head *));
>>> +extern void nfs4_put_deviceid_cache(struct nfs4_deviceid_cache *);
>>> +extern void nfs4_init_deviceid_node(struct nfs4_deviceid *);
>>> +extern struct nfs4_deviceid *nfs4_find_deviceid(struct nfs4_deviceid_cache *,
>>> + struct pnfs_deviceid *);
>>> +extern void nfs4_add_deviceid(struct nfs4_deviceid_cache *,
>>> + struct nfs4_deviceid *);
>>> +
>>> /* pNFS client callback functions.
>>> * These operations allow the layout driver to access pNFS client
>>> * specific information or call pNFS client->server operations.
>>> diff --git a/include/linux/nfs_fs_sb.h b/include/linux/nfs_fs_sb.h
>>> index 8522461..ef2e18e 100644
>>> --- a/include/linux/nfs_fs_sb.h
>>> +++ b/include/linux/nfs_fs_sb.h
>>> @@ -87,6 +87,7 @@ struct nfs_client {
>>> u32 cl_exchange_flags;
>>> struct nfs4_session *cl_session; /* sharred session */
>>> struct list_head cl_lo_inodes; /* Inodes having layouts */
>>> + struct nfs4_deviceid_cache *cl_devid_cache; /* pNFS deviceid cache */
>>> #endif /* CONFIG_NFS_V4_1 */
>>>
>>> #ifdef CONFIG_NFS_FSCACHE
>>> --
>>> 1.6.6
>>>
>>> --
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