From 884bddba07e2679e4f0281dda9ac5507bb1f1354 Mon Sep 17 00:00:00 2001 From: Justus Winter <4winter@informatik.uni-hamburg.de> Date: Tue, 13 May 2014 13:09:15 +0200 Subject: [PATCH 3/9] ext2fs: use a seperate lock to protect nodehash Previously, ext2fs used diskfs_node_refcnt_lock to serialize access to the nodehash. Use a separate lock to protect nodehash. Adjust the reference counting accordingly. Every node in the nodehash carries a light reference. When we are asked to give up that light reference, we reacquire our lock momentarily to check whether someone else reacquired a reference through the nodehash. * ext2fs/inode.c (nodecache_lock): New lock. (diskfs_cached_lookup): Use a separate lock to protect nodehash. Adjust the reference counting accordingly. (ifind): Likewise. (diskfs_node_iterate): Likewise. (diskfs_node_norefs): Move the code removing the node from nodehash... (diskfs_try_dropping_softrefs): ... here, where we check whether someone reacquired a reference, and if so hold on to our light reference. --- ext2fs/inode.c | 78 ++++++++++++++++++++++++++++++++++++++++++---------------- 1 file changed, 57 insertions(+), 21 deletions(-) diff --git a/ext2fs/inode.c b/ext2fs/inode.c index ed78265..835c8c7 100644 --- a/ext2fs/inode.c +++ b/ext2fs/inode.c @@ -46,8 +46,18 @@ #define INOHASH(ino) (((unsigned)(ino))%INOHSZ) #endif +/* The nodehash is a cache of nodes. + + Access to nodehash and nodehash_nr_items is protected by + nodecache_lock. + + Every node in the nodehash carries a light reference. When we are + asked to give up that light reference, we reacquire our lock + momentarily to check whether someone else reacquired a reference + through the nodehash. */ static struct node *nodehash[INOHSZ]; static size_t nodehash_nr_items; +static pthread_rwlock_t nodecache_lock = PTHREAD_RWLOCK_INITIALIZER; static error_t read_node (struct node *np); @@ -71,24 +81,22 @@ diskfs_cached_lookup (ino_t inum, struct node **npp) struct node *np; struct disknode *dn; - pthread_spin_lock (&diskfs_node_refcnt_lock); + pthread_rwlock_rdlock (&nodecache_lock); for (np = nodehash[INOHASH(inum)]; np; np = np->dn->hnext) if (np->cache_id == inum) { - np->references++; - pthread_spin_unlock (&diskfs_node_refcnt_lock); + diskfs_nref (np); + pthread_rwlock_unlock (&nodecache_lock); pthread_mutex_lock (&np->lock); *npp = np; return 0; } + pthread_rwlock_unlock (&nodecache_lock); /* Format specific data for the new node. */ dn = malloc (sizeof (struct disknode)); if (! dn) - { - pthread_spin_unlock (&diskfs_node_refcnt_lock); - return ENOMEM; - } + return ENOMEM; dn->dirents = 0; dn->dir_idx = 0; dn->pager = 0; @@ -102,14 +110,15 @@ diskfs_cached_lookup (ino_t inum, struct node **npp) pthread_mutex_lock (&np->lock); /* Put NP in NODEHASH. */ + pthread_rwlock_wrlock (&nodecache_lock); dn->hnext = nodehash[INOHASH(inum)]; if (dn->hnext) dn->hnext->dn->hprevp = &dn->hnext; dn->hprevp = &nodehash[INOHASH(inum)]; nodehash[INOHASH(inum)] = np; + diskfs_nref_light (np); nodehash_nr_items += 1; - - pthread_spin_unlock (&diskfs_node_refcnt_lock); + pthread_rwlock_unlock (&nodecache_lock); /* Get the contents of NP off disk. */ err = read_node (np); @@ -140,14 +149,13 @@ ifind (ino_t inum) { struct node *np; - pthread_spin_lock (&diskfs_node_refcnt_lock); + pthread_rwlock_rdlock (&nodecache_lock); for (np = nodehash[INOHASH(inum)]; np; np = np->dn->hnext) { if (np->cache_id != inum) continue; - assert (np->references); - pthread_spin_unlock (&diskfs_node_refcnt_lock); + pthread_rwlock_unlock (&nodecache_lock); return np; } assert (0); @@ -158,11 +166,6 @@ ifind (ino_t inum) void diskfs_node_norefs (struct node *np) { - *np->dn->hprevp = np->dn->hnext; - if (np->dn->hnext) - np->dn->hnext->dn->hprevp = np->dn->hprevp; - nodehash_nr_items -= 1; - if (np->dn->dirents) free (np->dn->dirents); assert (!np->dn->pager); @@ -180,6 +183,35 @@ diskfs_node_norefs (struct node *np) void diskfs_try_dropping_softrefs (struct node *np) { + pthread_rwlock_wrlock (&nodecache_lock); + if (np->dn->hnext != NULL) + { + /* Check if someone reacquired a reference through the + nodehash. */ + unsigned int references; + pthread_spin_lock (&diskfs_node_refcnt_lock); + references = np->references; + pthread_spin_unlock (&diskfs_node_refcnt_lock); + + /* An additional reference is acquired by libdiskfs across calls + to diskfs_try_dropping_softrefs. */ + if (references > 1) + { + /* A reference was reacquired through a hash table lookup. + It's fine, we didn't touch anything yet. */ + pthread_rwlock_unlock (&nodecache_lock); + return; + } + + *np->dn->hprevp = np->dn->hnext; + if (np->dn->hnext) + np->dn->hnext->dn->hprevp = np->dn->hprevp; + np->dn->hnext = NULL; + nodehash_nr_items -= 1; + diskfs_nrele_light (np); + } + pthread_rwlock_unlock (&nodecache_lock); + drop_pager_softrefs (np); } @@ -556,12 +588,12 @@ diskfs_node_iterate (error_t (*fun)(struct node *)) size_t num_nodes; struct node *node, **node_list, **p; - pthread_spin_lock (&diskfs_node_refcnt_lock); + pthread_rwlock_rdlock (&nodecache_lock); /* We must copy everything from the hash table into another data structure to avoid running into any problems with the hash-table being modified during processing (normally we delegate access to hash-table with - diskfs_node_refcnt_lock, but we can't hold this while locking the + nodecache_lock, but we can't hold this while locking the individual node locks). */ num_nodes = nodehash_nr_items; @@ -570,7 +602,7 @@ diskfs_node_iterate (error_t (*fun)(struct node *)) node_list = malloc (num_nodes * sizeof (struct node *)); if (node_list == NULL) { - pthread_spin_unlock (&diskfs_node_refcnt_lock); + pthread_rwlock_unlock (&nodecache_lock); ext2_debug ("unable to allocate temporary node table"); return ENOMEM; } @@ -580,10 +612,14 @@ diskfs_node_iterate (error_t (*fun)(struct node *)) for (node = nodehash[n]; node; node = node->dn->hnext) { *p++ = node; + + /* We acquire a hard reference for node, but without using + diskfs_nref. We do this so that diskfs_new_hardrefs will not + get called. */ node->references++; } - pthread_spin_unlock (&diskfs_node_refcnt_lock); + pthread_rwlock_unlock (&nodecache_lock); p = node_list; while (num_nodes-- > 0) -- 2.0.0.rc2