path: root/debian/patches/0018-tmpfs-use-a-seperate-lock-to-protect-all_nodes.patch

From 169ef4426b3237510025287ff5dcfb83cc143982 Mon Sep 17 00:00:00 2001
From: Justus Winter <4winter@informatik.uni-hamburg.de>
Date: Tue, 13 May 2014 15:35:42 +0200
Subject: [PATCH 18/20] tmpfs: use a seperate lock to protect all_nodes

Previously, tmpfs used diskfs_node_refcnt_lock to serialize access to
the all_nodes and some other related global state related to memory
consumption.

Use a separate lock to protect all_nodes, and to the state related to
memory consumption as this is updated during insertion and removal
operations. Adjust the reference counting accordingly.  Every node in
the all_nodes 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 all_nodes.

* tmpfs/node.c (all_nodes_lock): New lock.
(diskfs_alloc_node): Use a separate lock to protect all_nodes.
Adjust the reference counting accordingly.
(diskfs_free_node): Likewise.
(diskfs_cached_lookup):Likewise.
(diskfs_node_iterate): Likewise.
(diskfs_node_norefs): Move the code removing the node from all_nodes...
(diskfs_try_dropping_softrefs): ... here, where we check whether
someone reacquired a reference, and if so hold on to our light
reference.
* tmpfs/tmpfs.c (diskfs_set_statfs): Use all_nodes_lock.
* tmpfs/tmpfs.h (all_nodes_lock): New declaration.
(adjust_used): Use all_nodes_lock.
---
 tmpfs/node.c  | 78 +++++++++++++++++++++++++++++++++++++++++++----------------
 tmpfs/tmpfs.c |  4 +--
 tmpfs/tmpfs.h |  6 +++--
 3 files changed, 63 insertions(+), 25 deletions(-)

diff --git a/tmpfs/node.c b/tmpfs/node.c
index acc029a..689777f 100644
--- a/tmpfs/node.c
+++ b/tmpfs/node.c
@@ -29,8 +29,18 @@ the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.  */
 unsigned int num_files;
 static unsigned int gen;
 
+/* all_nodes is a cache of nodes.
+
+   Access to all_nodes and all_nodes_nr_items is protected by
+   all_nodes_lock.
+
+   Every node in the all_nodes 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 all_nodes.  */
 struct node *all_nodes;
 static size_t all_nodes_nr_items;
+pthread_mutex_t all_nodes_lock = PTHREAD_MUTEX_INITIALIZER;
 
 error_t
 diskfs_alloc_node (struct node *dp, mode_t mode, struct node **npp)
@@ -40,18 +50,18 @@ diskfs_alloc_node (struct node *dp, mode_t mode, struct node **npp)
   dn = calloc (1, sizeof *dn);
   if (dn == 0)
     return ENOSPC;
-  pthread_spin_lock (&diskfs_node_refcnt_lock);
+  pthread_mutex_lock (&all_nodes_lock);
   if (round_page (tmpfs_space_used + sizeof *dn) / vm_page_size
       > tmpfs_page_limit)
     {
-      pthread_spin_unlock (&diskfs_node_refcnt_lock);
+      pthread_mutex_unlock (&all_nodes_lock);
       free (dn);
       return ENOSPC;
     }
   dn->gen = gen++;
   ++num_files;
   tmpfs_space_used += sizeof *dn;
-  pthread_spin_unlock (&diskfs_node_refcnt_lock);
+  pthread_mutex_unlock (&all_nodes_lock);
 
   dn->type = IFTODT (mode & S_IFMT);
   return diskfs_cached_lookup ((ino_t) (uintptr_t) dn, npp);
@@ -75,15 +85,18 @@ diskfs_free_node (struct node *np, mode_t mode)
       free (np->dn->u.lnk);
       break;
     }
+
+  pthread_mutex_lock (&all_nodes_lock);
   *np->dn->hprevp = np->dn->hnext;
   if (np->dn->hnext != 0)
     np->dn->hnext->dn->hprevp = np->dn->hprevp;
   all_nodes_nr_items -= 1;
-  free (np->dn);
-  np->dn = 0;
-
   --num_files;
   tmpfs_space_used -= sizeof *np->dn;
+  pthread_mutex_unlock (&all_nodes_lock);
+
+  free (np->dn);
+  np->dn = 0;
 }
 
 void
@@ -117,14 +130,6 @@ diskfs_node_norefs (struct node *np)
 	  np->dn->u.chr = np->dn_stat.st_rdev;
 	  break;
 	}
-
-      /* Remove this node from the cache list rooted at `all_nodes'.  */
-      *np->dn->hprevp = np->dn->hnext;
-      if (np->dn->hnext != 0)
-	np->dn->hnext->dn->hprevp = np->dn->hprevp;
-      all_nodes_nr_items -= 1;
-      np->dn->hnext = 0;
-      np->dn->hprevp = 0;
     }
 
   free (np);
@@ -167,11 +172,14 @@ diskfs_cached_lookup (ino_t inum, struct node **npp)
 
   assert (npp);
 
+  pthread_mutex_lock (&all_nodes_lock);
+
   if (dn->hprevp != 0)		/* There is already a node.  */
     {
       np = *dn->hprevp;
       assert (np->dn == dn);
       assert (*dn->hprevp == np);
+      pthread_mutex_unlock (&all_nodes_lock);
 
       diskfs_nref (np);
     }
@@ -183,14 +191,14 @@ diskfs_cached_lookup (ino_t inum, struct node **npp)
       np = diskfs_make_node (dn);
       np->cache_id = (ino_t) (uintptr_t) dn;
 
-      pthread_spin_lock (&diskfs_node_refcnt_lock);
       dn->hnext = all_nodes;
       if (dn->hnext)
 	dn->hnext->dn->hprevp = &dn->hnext;
       dn->hprevp = &all_nodes;
       all_nodes = np;
       all_nodes_nr_items += 1;
-      pthread_spin_unlock (&diskfs_node_refcnt_lock);
+      diskfs_nref_light (np);
+      pthread_mutex_unlock (&all_nodes_lock);
 
       st = &np->dn_stat;
       memset (st, 0, sizeof *st);
@@ -229,12 +237,12 @@ 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 (&all_nodes_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
+     all_nodes_lock, but we can't hold this while locking the
      individual node locks).  */
 
   num_nodes = all_nodes_nr_items;
@@ -243,10 +251,10 @@ diskfs_node_iterate (error_t (*fun) (struct node *))
   for (node = all_nodes; node != 0; node = node->dn->hnext)
     {
       *p++ = node;
-      node->references++;
+      diskfs_nref_light (node);
     }
 
-  pthread_spin_unlock (&diskfs_node_refcnt_lock);
+  pthread_mutex_unlock (&all_nodes_lock);
 
   p = node_list;
   while (num_nodes-- > 0)
@@ -258,7 +266,7 @@ diskfs_node_iterate (error_t (*fun) (struct node *))
 	  err = (*fun) (node);
 	  pthread_mutex_unlock (&node->lock);
 	}
-      diskfs_nrele (node);
+      diskfs_nrele_light (node);
     }
 
   return err;
@@ -272,6 +280,34 @@ diskfs_node_iterate (error_t (*fun) (struct node *))
 void
 diskfs_try_dropping_softrefs (struct node *np)
 {
+  pthread_mutex_lock (&all_nodes_lock);
+  if (np->dn->hnext != NULL)
+    {
+      /* Check if someone reacquired a reference through the
+         all_nodes.  */
+      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 (&all_nodes_lock);
+          return;
+        }
+
+      /* Remove this node from the cache list rooted at `all_nodes'.  */
+      *np->dn->hprevp = np->dn->hnext;
+      if (np->dn->hnext != 0)
+	np->dn->hnext->dn->hprevp = np->dn->hprevp;
+      all_nodes_nr_items -= 1;
+      np->dn->hnext = NULL;
+      np->dn->hprevp = NULL;
+      diskfs_nrele_light (np);
+    }
+  pthread_mutex_unlock (&all_nodes_lock);
 }
 
 /* The user must define this funcction.  Node NP has some light
diff --git a/tmpfs/tmpfs.c b/tmpfs/tmpfs.c
index a45d343..024b818 100644
--- a/tmpfs/tmpfs.c
+++ b/tmpfs/tmpfs.c
@@ -67,10 +67,10 @@ diskfs_set_statfs (struct statfs *st)
   st->f_bsize = vm_page_size;
   st->f_blocks = tmpfs_page_limit;
 
-  pthread_spin_lock (&diskfs_node_refcnt_lock);
+  pthread_mutex_lock (&all_nodes_lock);
   st->f_files = num_files;
   pages = round_page (tmpfs_space_used) / vm_page_size;
-  pthread_spin_unlock (&diskfs_node_refcnt_lock);
+  pthread_mutex_unlock (&all_nodes_lock);
 
   st->f_bfree = pages < tmpfs_page_limit ? tmpfs_page_limit - pages : 0;
   st->f_bavail = st->f_bfree;
diff --git a/tmpfs/tmpfs.h b/tmpfs/tmpfs.h
index b3c636d..187249e 100644
--- a/tmpfs/tmpfs.h
+++ b/tmpfs/tmpfs.h
@@ -24,6 +24,7 @@ the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.  */
 #include <sys/types.h>
 #include <dirent.h>
 #include <stdint.h>
+#include <pthread.h>
 
 struct disknode
 {
@@ -71,15 +72,16 @@ struct tmpfs_dirent
 
 extern unsigned int num_files;
 extern off_t tmpfs_page_limit, tmpfs_space_used;
+extern pthread_mutex_t all_nodes_lock;
 
 extern mach_port_t default_pager;
 
 static inline void
 adjust_used (off_t change)
 {
-  pthread_spin_lock (&diskfs_node_refcnt_lock);
+  pthread_mutex_lock (&all_nodes_lock);
   tmpfs_space_used += change;
-  pthread_spin_unlock (&diskfs_node_refcnt_lock);
+  pthread_mutex_unlock (&all_nodes_lock);
 }
 
 #endif
-- 
2.0.0.rc2