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From 76d845df17dbd06d394ee4b56b918750b5ef8dce 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 15/20] 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 (nodehash_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 | 70 ++++++++++++++++++++++++++++++++++++++++++----------------
1 file changed, 51 insertions(+), 19 deletions(-)
diff --git a/ext2fs/inode.c b/ext2fs/inode.c
index ed78265..bb6581e 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
+ nodehash_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_mutex_t nodehash_lock = PTHREAD_MUTEX_INITIALIZER;
static error_t read_node (struct node *np);
@@ -71,12 +81,12 @@ diskfs_cached_lookup (ino_t inum, struct node **npp)
struct node *np;
struct disknode *dn;
- pthread_spin_lock (&diskfs_node_refcnt_lock);
+ pthread_mutex_lock (&nodehash_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_mutex_unlock (&nodehash_lock);
pthread_mutex_lock (&np->lock);
*npp = np;
return 0;
@@ -86,7 +96,7 @@ diskfs_cached_lookup (ino_t inum, struct node **npp)
dn = malloc (sizeof (struct disknode));
if (! dn)
{
- pthread_spin_unlock (&diskfs_node_refcnt_lock);
+ pthread_mutex_unlock (&nodehash_lock);
return ENOMEM;
}
dn->dirents = 0;
@@ -107,9 +117,10 @@ diskfs_cached_lookup (ino_t inum, struct node **npp)
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_mutex_unlock (&nodehash_lock);
/* Get the contents of NP off disk. */
err = read_node (np);
@@ -140,14 +151,13 @@ ifind (ino_t inum)
{
struct node *np;
- pthread_spin_lock (&diskfs_node_refcnt_lock);
+ pthread_mutex_lock (&nodehash_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_mutex_unlock (&nodehash_lock);
return np;
}
assert (0);
@@ -158,11 +168,6 @@ 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 +185,33 @@ diskfs_node_norefs (struct node *np)
void
diskfs_try_dropping_softrefs (struct node *np)
{
+ pthread_mutex_lock (&nodehash_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);
+
+ if (references > 0)
+ {
+ /* A reference was reacquired through a hash table lookup.
+ It's fine, we didn't touch anything yet. */
+ pthread_mutex_unlock (&nodehash_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_mutex_unlock (&nodehash_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_mutex_lock (&nodehash_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
+ nodehash_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_mutex_unlock (&nodehash_lock);
ext2_debug ("unable to allocate temporary node table");
return ENOMEM;
}
@@ -580,10 +612,10 @@ diskfs_node_iterate (error_t (*fun)(struct node *))
for (node = nodehash[n]; node; node = node->dn->hnext)
{
*p++ = node;
- node->references++;
+ diskfs_nref_light (node);
}
- pthread_spin_unlock (&diskfs_node_refcnt_lock);
+ pthread_mutex_unlock (&nodehash_lock);
p = node_list;
while (num_nodes-- > 0)
@@ -595,7 +627,7 @@ diskfs_node_iterate (error_t (*fun)(struct node *))
err = (*fun)(node);
pthread_mutex_unlock (&node->lock);
}
- diskfs_nrele (node);
+ diskfs_nrele_light (node);
}
free (node_list);
--
2.0.0.rc2
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