From f682a398b2e24ae0a775ddf37cced83b897198ee Mon Sep 17 00:00:00 2001 From: NeilBrown Date: Mon, 14 Jul 2014 11:28:20 +1000 Subject: NFS: allow lockless access to access_cache The access cache is used during RCU-walk path lookups, so it is best to avoid locking if possible as taking a lock kills concurrency. The rbtree is not rcu-safe and cannot easily be made so. Instead we simply check the last (i.e. most recent) entry on the LRU list. If this doesn't match, then we return -ECHILD and retry in lock/refcount mode. This requires freeing the nfs_access_entry struct with rcu, and requires using rcu access primatives when adding entries to the lru, and when examining the last entry. Calling put_rpccred before kfree_rcu looks a bit odd, but as put_rpccred already provides rcu protection, we know that the cred will not actually be freed until the next grace period, so any concurrent access will be safe. This patch provides about 5% performance improvement on a stat-heavy synthetic work load with 4 threads on a 2-core CPU. Signed-off-by: NeilBrown Signed-off-by: Trond Myklebust --- fs/nfs/dir.c | 43 +++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 41 insertions(+), 2 deletions(-) (limited to 'fs/nfs') diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c index 2bfbde0f7176..1b5f38f48dab 100644 --- a/fs/nfs/dir.c +++ b/fs/nfs/dir.c @@ -2079,7 +2079,7 @@ MODULE_PARM_DESC(nfs_access_max_cachesize, "NFS access maximum total cache lengt static void nfs_access_free_entry(struct nfs_access_entry *entry) { put_rpccred(entry->cred); - kfree(entry); + kfree_rcu(entry, rcu_head); smp_mb__before_atomic(); atomic_long_dec(&nfs_access_nr_entries); smp_mb__after_atomic(); @@ -2257,6 +2257,38 @@ out_zap: return -ENOENT; } +static int nfs_access_get_cached_rcu(struct inode *inode, struct rpc_cred *cred, struct nfs_access_entry *res) +{ + /* Only check the most recently returned cache entry, + * but do it without locking. + */ + struct nfs_inode *nfsi = NFS_I(inode); + struct nfs_access_entry *cache; + int err = -ECHILD; + struct list_head *lh; + + rcu_read_lock(); + if (nfsi->cache_validity & NFS_INO_INVALID_ACCESS) + goto out; + lh = rcu_dereference(nfsi->access_cache_entry_lru.prev); + cache = list_entry(lh, struct nfs_access_entry, lru); + if (lh == &nfsi->access_cache_entry_lru || + cred != cache->cred) + cache = NULL; + if (cache == NULL) + goto out; + if (!nfs_have_delegated_attributes(inode) && + !time_in_range_open(jiffies, cache->jiffies, cache->jiffies + nfsi->attrtimeo)) + goto out; + res->jiffies = cache->jiffies; + res->cred = cache->cred; + res->mask = cache->mask; + err = 0; +out: + rcu_read_unlock(); + return err; +} + static void nfs_access_add_rbtree(struct inode *inode, struct nfs_access_entry *set) { struct nfs_inode *nfsi = NFS_I(inode); @@ -2300,6 +2332,11 @@ void nfs_access_add_cache(struct inode *inode, struct nfs_access_entry *set) cache->cred = get_rpccred(set->cred); cache->mask = set->mask; + /* The above field assignments must be visible + * before this item appears on the lru. We cannot easily + * use rcu_assign_pointer, so just force the memory barrier. + */ + smp_wmb(); nfs_access_add_rbtree(inode, cache); /* Update accounting */ @@ -2339,7 +2376,9 @@ static int nfs_do_access(struct inode *inode, struct rpc_cred *cred, int mask) trace_nfs_access_enter(inode); - status = nfs_access_get_cached(inode, cred, &cache); + status = nfs_access_get_cached_rcu(inode, cred, &cache); + if (status != 0) + status = nfs_access_get_cached(inode, cred, &cache); if (status == 0) goto out_cached; -- cgit v1.2.3