path: root/debian/patches/0020-libdiskfs-lock-less-reference-counting-of-nodes.patch

From 2bbb125dfe14d575401a924f4d5ac74c507cfc4b Mon Sep 17 00:00:00 2001
From: Justus Winter <4winter@informatik.uni-hamburg.de>
Date: Wed, 14 May 2014 11:19:35 +0200
Subject: [PATCH 20/27] libdiskfs: lock-less reference counting of nodes

* libdiskfs/diskfs.h (struct node): Use refcounts_t for reference counting.
(diskfs_node_refcnt_lock): Remove.
(diskfs_node_norefs,diskfs_drop_node): Change comments accordingly.
* libdiskfs/init-init.c: Likewise.
* libdiskfs/node-drop.c: Likewise.
* libdiskfs/node-make.c: Likewise.
* libdiskfs/node-nput.c: Likewise.
* libdiskfs/node-nputl.c: Likewise.
* libdiskfs/node-nref.c: Likewise.
* libdiskfs/node-nrefl.c: Likewise.
* libdiskfs/node-nrele.c: Likewise.
* libdiskfs/node-nrelel.c: Likewise.
* ext2fs/inode.c: Likewise.
* fatfs/inode.c: Likewise.
* isofs/inode.c: Likewise.
* tmpfs/node.c: Likewise.
* doc/hurd.texi: Likewise.
---
 doc/hurd.texi           | 11 ++--------
 ext2fs/inode.c          |  9 +++------
 fatfs/inode.c           | 21 ++++++-------------
 isofs/inode.c           |  9 +++------
 libdiskfs/diskfs.h      | 12 ++++-------
 libdiskfs/init-init.c   |  2 --
 libdiskfs/node-drop.c   |  9 +++------
 libdiskfs/node-make.c   |  3 +--
 libdiskfs/node-nput.c   | 54 +++++++++++++++++++------------------------------
 libdiskfs/node-nputl.c  | 12 ++++-------
 libdiskfs/node-nref.c   | 10 ++++-----
 libdiskfs/node-nrefl.c  |  6 +++---
 libdiskfs/node-nrele.c  | 48 +++++++++++++++++++++----------------------
 libdiskfs/node-nrelel.c |  9 +++------
 tmpfs/node.c            |  9 +++------
 15 files changed, 83 insertions(+), 141 deletions(-)

diff --git a/doc/hurd.texi b/doc/hurd.texi
index 07ddfb4..6cafdb9 100644
--- a/doc/hurd.texi
+++ b/doc/hurd.texi
@@ -3780,10 +3780,6 @@ new thread and (eventually) get rid of the old one; the old thread won't
 do any more syncs, regardless.
 @end deftypefun
 
-@deftypevar spin_lock_t diskfs_node_refcnt_lock
-Pager reference count lock.
-@end deftypevar
-
 @deftypevar int diskfs_readonly
 Set to zero if the filesystem is currently writable.
 @end deftypevar
@@ -3818,9 +3814,7 @@ Every file or directory is a diskfs @dfn{node}.  The following functions
 help your diskfs callbacks manage nodes and their references:
 
 @deftypefun void diskfs_drop_node (@w{struct node *@var{np}})
-Node @var{np} now has no more references; clean all state.  The
-@var{diskfs_node_refcnt_lock} must be held, and will be released upon
-return.  @var{np} must be locked.
+Node @var{np} now has no more references; clean all state.
 @end deftypefun
 
 @deftypefun void diskfs_node_update (@w{struct node *@var{np}}, @w{int @var{wait}})
@@ -4236,14 +4230,13 @@ without real users.
 @deftypefun void diskfs_try_dropping_softrefs (@w{struct node *@var{np}})
 Node @var{np} has some light references, but has just lost its last hard
 references.  Take steps so that if any light references can be freed,
-they are.  Both @var{diskfs_node_refcnt_lock} and @var{np} are locked.
+they are.  @var{np} is locked.
 This function will be called after @code{diskfs_lost_hardrefs}.
 @end deftypefun
 
 @deftypefun void diskfs_node_norefs (@w{struct node *@var{np}})
 Node @var{np} has no more references; free local state, including
 @code{*@var{np}} if it shouldn't be retained.
-@var{diskfs_node_refcnt_lock} is held.
 @end deftypefun
 
 @deftypefun error_t diskfs_set_hypermetadata (@w{int @var{wait}}, @w{int @var{clean}})
diff --git a/ext2fs/inode.c b/ext2fs/inode.c
index aa070a2..7ec343f 100644
--- a/ext2fs/inode.c
+++ b/ext2fs/inode.c
@@ -190,12 +190,9 @@ diskfs_try_dropping_softrefs (struct node *np)
     {
       /* 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)
+      struct references result;
+      refcounts_references (&np->refcounts, &result);
+      if (result.hard > 0 || result.weak > 1)
         {
           /* A reference was reacquired through a hash table lookup.
              It's fine, we didn't touch anything yet. */
diff --git a/fatfs/inode.c b/fatfs/inode.c
index 8b3385b..d5750b4 100644
--- a/fatfs/inode.c
+++ b/fatfs/inode.c
@@ -237,14 +237,8 @@ diskfs_node_norefs (struct node *np)
   if (np->dn->translator)
     free (np->dn->translator);
 
-  /* It is safe to unlock diskfs_node_refcnt_lock here for a while because
-     all references to the node have been deleted.  */
   if (np->dn->dirnode)
-    {
-      pthread_spin_unlock (&diskfs_node_refcnt_lock);
-      diskfs_nrele (np->dn->dirnode);
-      pthread_spin_lock (&diskfs_node_refcnt_lock);
-    }
+    diskfs_nrele (np->dn->dirnode);
 
   assert (!np->dn->pager);
 
@@ -262,12 +256,9 @@ diskfs_try_dropping_softrefs (struct node *np)
     {
       /* 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)
+      struct references result;
+      refcounts_references (&np->refcounts, &result);
+      if (result.hard > 0 || result.weak > 1)
         {
           /* A reference was reacquired through a hash table lookup.
              It's fine, we didn't touch anything yet. */
@@ -372,7 +363,7 @@ read_node (struct node *np, vm_address_t buf)
   /* Files in fatfs depend on the directory that hold the file.  */
   np->dn->dirnode = dp;
   if (dp)
-    dp->references++;
+    refcounts_ref (&dp->refcounts, NULL);
 
   pthread_rwlock_rdlock (&np->dn->dirent_lock);
 
@@ -814,7 +805,7 @@ diskfs_alloc_node (struct node *dir, mode_t mode, struct node **node)
 
   /* FIXME: We know that readnode couldn't put this in.  */
   np->dn->dirnode = dir;
-  dir->references++;
+  refcounts_ref (&dir->refcounts, NULL);
 
   *node = np;
   return 0;
diff --git a/isofs/inode.c b/isofs/inode.c
index 4f22086..b36a289 100644
--- a/isofs/inode.c
+++ b/isofs/inode.c
@@ -536,12 +536,9 @@ diskfs_try_dropping_softrefs (struct node *np)
 
       /* Check if someone reacquired a reference through the
          node_cache.  */
-      unsigned int references;
-      pthread_spin_lock (&diskfs_node_refcnt_lock);
-      references = np->references;
-      pthread_spin_unlock (&diskfs_node_refcnt_lock);
-
-      if (references > 0)
+      struct references result;
+      refcounts_references (&np->refcounts, &result);
+      if (result.hard > 0 || result.weak > 1)
         {
           /* A reference was reacquired through a hash table lookup.
              It's fine, we didn't touch anything yet. */
diff --git a/libdiskfs/diskfs.h b/libdiskfs/diskfs.h
index acd1910..e10e75e 100644
--- a/libdiskfs/diskfs.h
+++ b/libdiskfs/diskfs.h
@@ -96,8 +96,7 @@ struct node
 
   pthread_mutex_t lock;
 
-  int references;		/* hard references */
-  int light_references;		/* light references */
+  refcounts_t refcounts;
 
   mach_port_t sockaddr;		/* address for S_IFSOCK shortcut */
 
@@ -201,8 +200,6 @@ extern volatile struct mapped_time_value *diskfs_mtime;
    be done by format independent code. */
 extern int diskfs_synchronous;
 
-extern pthread_spinlock_t diskfs_node_refcnt_lock;
-
 extern int pager_port_type;
 
 /* Whether the filesystem is currently writable or not. */
@@ -451,7 +448,7 @@ error_t diskfs_alloc_node (struct node *dp, mode_t mode, struct node **np);
 void diskfs_free_node (struct node *np, mode_t mode);
 
 /* Node NP has no more references; free local state, including *NP
-   if it isn't to be retained.  diskfs_node_refcnt_lock is held. */
+   if it isn't to be retained.  */
 void diskfs_node_norefs (struct node *np);
 
 /* The user must define this function.  Node NP has some light
@@ -614,9 +611,8 @@ void diskfs_spawn_first_thread (ports_demuxer_type demuxer);
    diskfs_init_completed once it has a valid proc and auth port. */
 void diskfs_start_bootstrap ();
 
-/* Node NP now has no more references; clean all state.  The
-   _diskfs_node_refcnt_lock must be held, and will be released
-   upon return.  NP must be locked.  */
+/* Node NP now has no more references; clean all state.  NP must be
+   locked.  */
 void diskfs_drop_node (struct node *np);
 
 /* Set on disk fields from NP->dn_stat; update ctime, atime, and mtime
diff --git a/libdiskfs/init-init.c b/libdiskfs/init-init.c
index 35be7ed..976b4e8 100644
--- a/libdiskfs/init-init.c
+++ b/libdiskfs/init-init.c
@@ -37,8 +37,6 @@ int _diskfs_noatime;
 
 struct hurd_port _diskfs_exec_portcell;
 
-pthread_spinlock_t diskfs_node_refcnt_lock = PTHREAD_SPINLOCK_INITIALIZER;
-
 pthread_spinlock_t _diskfs_control_lock = PTHREAD_SPINLOCK_INITIALIZER;
 int _diskfs_ncontrol_ports;
 
diff --git a/libdiskfs/node-drop.c b/libdiskfs/node-drop.c
index 83eb590..fab3cfa 100644
--- a/libdiskfs/node-drop.c
+++ b/libdiskfs/node-drop.c
@@ -31,9 +31,8 @@ free_modreqs (struct modreq *mr)
 }
 
 
-/* Node NP now has no more references; clean all state.  The
-   diskfs_node_refcnt_lock must be held, and will be released
-   upon return.  NP must be locked.  */
+/* Node NP now has no more references; clean all state.  NP must be
+   locked.  */
 void
 diskfs_drop_node (struct node *np)
 {
@@ -60,8 +59,7 @@ diskfs_drop_node (struct node *np)
 	     and an nput.  The next time through, this routine
 	     will notice that the size is zero, and not have to
 	     do anything. */
-	  np->references++;
-	  pthread_spin_unlock (&diskfs_node_refcnt_lock);
+	  refcounts_ref (&np->refcounts, NULL);
 	  diskfs_truncate (np, 0);
 	  
 	  /* Force allocsize to zero; if truncate consistently fails this
@@ -94,5 +92,4 @@ diskfs_drop_node (struct node *np)
   assert (!np->sockaddr);
 
   diskfs_node_norefs (np);
-  pthread_spin_unlock (&diskfs_node_refcnt_lock);
 }
diff --git a/libdiskfs/node-make.c b/libdiskfs/node-make.c
index ff0cc0d..c7ca3b0 100644
--- a/libdiskfs/node-make.c
+++ b/libdiskfs/node-make.c
@@ -29,8 +29,7 @@ init_node (struct node *np, struct disknode *dn)
   np->dn_stat_dirty = 0;
 
   pthread_mutex_init (&np->lock, NULL);
-  np->references = 1;
-  np->light_references = 0;
+  refcounts_init (&np->refcounts, 1, 0);
   np->owner = 0;
   np->sockaddr = MACH_PORT_NULL;
 
diff --git a/libdiskfs/node-nput.c b/libdiskfs/node-nput.c
index 5043ad1..2935ae2 100644
--- a/libdiskfs/node-nput.c
+++ b/libdiskfs/node-nput.c
@@ -26,56 +26,44 @@
 void
 diskfs_nput (struct node *np)
 {
-  int tried_drop_softrefs = 0;
+  struct references result;
 
- loop:
-  pthread_spin_lock (&diskfs_node_refcnt_lock);
-  assert (np->references);
-  np->references--;
-  if (np->references + np->light_references == 0)
-    diskfs_drop_node (np);
-  else if (np->references == 0 && !tried_drop_softrefs)
-    {
-      pthread_spin_unlock (&diskfs_node_refcnt_lock);
+  /* While we call the diskfs_try_dropping_softrefs, we need to hold
+     one reference.  We use a weak reference for this purpose, which
+     we acquire by demoting our hard reference to a weak one.  */
+  refcounts_demote (&np->refcounts, &result);
 
+  if (result.hard == 0)
+    {
       /* This is our cue that something akin to "last process closes file"
 	 in the POSIX.1 sense happened, so make sure any pending node time
 	 updates now happen in a timely fashion.  */
       diskfs_set_node_times (np);
-
       diskfs_lost_hardrefs (np);
       if (!np->dn_stat.st_nlink)
 	{
-	  /* There are no links.  If there are soft references that
-	     can be dropped, we can't let them postpone deallocation.
-	     So attempt to drop them.  But that's a user-supplied
-	     routine, which might result in further recursive calls to
-	     the ref-counting system.  So we have to reacquire our
-	     reference around the call to forestall disaster. */
-	  pthread_spin_lock (&diskfs_node_refcnt_lock);
-	  np->references++;
-	  pthread_spin_unlock (&diskfs_node_refcnt_lock);
-
 	  if (np->sockaddr != MACH_PORT_NULL)
 	    {
 	      mach_port_deallocate (mach_task_self (), np->sockaddr);
 	      np->sockaddr = MACH_PORT_NULL;
 	    }
 
+	  /* There are no links.  If there are soft references that
+	     can be dropped, we can't let them postpone deallocation.
+	     So attempt to drop them.  But that's a user-supplied
+	     routine, which might result in further recursive calls to
+	     the ref-counting system.  This is not a problem, as we
+	     hold a weak reference ourselves. */
 	  diskfs_try_dropping_softrefs (np);
-
-	  /* But there's no value in looping forever in this
-	     routine; only try to drop soft refs once. */
-	  tried_drop_softrefs = 1;
-
-	  /* Now we can drop the reference back... */
-	  goto loop;
 	}
       pthread_mutex_unlock (&np->lock);
     }
-  else
-    {
-      pthread_spin_unlock (&diskfs_node_refcnt_lock);
-      pthread_mutex_unlock (&np->lock);
-    }
+
+  /* Finally get rid of our reference.  */
+  refcounts_deref_weak (&np->refcounts, &result);
+
+  if (result.hard == 0 && result.weak == 0)
+    diskfs_drop_node (np);
+
+  pthread_mutex_unlock (&np->lock);
 }
diff --git a/libdiskfs/node-nputl.c b/libdiskfs/node-nputl.c
index 1959665..8dac16e 100644
--- a/libdiskfs/node-nputl.c
+++ b/libdiskfs/node-nputl.c
@@ -25,14 +25,10 @@
 void
 diskfs_nput_light (struct node *np)
 {
-  pthread_spin_lock (&diskfs_node_refcnt_lock);
-  assert (np->light_references);
-  np->light_references--;
-  if (np->references + np->light_references == 0)
+  struct references result;
+  refcounts_deref_weak (&np->refcounts, &result);
+  if (result.hard == 0 && result.weak == 0)
     diskfs_drop_node (np);
   else
-    {
-      pthread_spin_unlock (&diskfs_node_refcnt_lock);
-      pthread_mutex_unlock (&np->lock);
-    }
+    pthread_mutex_unlock (&np->lock);
 }
diff --git a/libdiskfs/node-nref.c b/libdiskfs/node-nref.c
index 13cea05..89ffa4f 100644
--- a/libdiskfs/node-nref.c
+++ b/libdiskfs/node-nref.c
@@ -26,12 +26,10 @@
 void
 diskfs_nref (struct node *np)
 {
-  int new_hardref;
-  pthread_spin_lock (&diskfs_node_refcnt_lock);
-  np->references++;
-  new_hardref = (np->references == 1);
-  pthread_spin_unlock (&diskfs_node_refcnt_lock);
-  if (new_hardref)
+  struct references result;
+  refcounts_ref (&np->refcounts, &result);
+  assert (result.hard > 1 || result.weak > 0);
+  if (result.hard == 1)
     {
       pthread_mutex_lock (&np->lock);
       diskfs_new_hardrefs (np);
diff --git a/libdiskfs/node-nrefl.c b/libdiskfs/node-nrefl.c
index 9692247..b7af409 100644
--- a/libdiskfs/node-nrefl.c
+++ b/libdiskfs/node-nrefl.c
@@ -24,7 +24,7 @@
 void
 diskfs_nref_light (struct node *np)
 {
-  pthread_spin_lock (&diskfs_node_refcnt_lock);
-  np->light_references++;
-  pthread_spin_unlock (&diskfs_node_refcnt_lock);
+  struct references result;
+  refcounts_ref_weak (&np->refcounts, &result);
+  assert (result.hard > 0 || result.weak > 1);
 }
diff --git a/libdiskfs/node-nrele.c b/libdiskfs/node-nrele.c
index cc68089..d962846 100644
--- a/libdiskfs/node-nrele.c
+++ b/libdiskfs/node-nrele.c
@@ -28,38 +28,36 @@
 void
 diskfs_nrele (struct node *np)
 {
-  int tried_drop_softrefs = 0;
+  struct references result;
 
- loop:
-  pthread_spin_lock (&diskfs_node_refcnt_lock);
-  assert (np->references);
-  np->references--;
-  if (np->references + np->light_references == 0)
-    {
-      pthread_mutex_lock (&np->lock);
-      diskfs_drop_node (np);
-    }
-  else if (np->references == 0)
+  /* While we call the diskfs_try_dropping_softrefs, we need to hold
+     one reference.  We use a weak reference for this purpose, which
+     we acquire by demoting our hard reference to a weak one.  */
+  refcounts_demote (&np->refcounts, &result);
+
+  if (result.hard == 0)
     {
       pthread_mutex_lock (&np->lock);
-      pthread_spin_unlock (&diskfs_node_refcnt_lock);
       diskfs_lost_hardrefs (np);
-      if (!np->dn_stat.st_nlink && !tried_drop_softrefs)
+      if (!np->dn_stat.st_nlink)
 	{
-	  /* Same issue here as in nput; see that for explanation */
-	  pthread_spin_lock (&diskfs_node_refcnt_lock);
-	  np->references++;
-	  pthread_spin_unlock (&diskfs_node_refcnt_lock);
-
+	  /* There are no links.  If there are soft references that
+	     can be dropped, we can't let them postpone deallocation.
+	     So attempt to drop them.  But that's a user-supplied
+	     routine, which might result in further recursive calls to
+	     the ref-counting system.  This is not a problem, as we
+	     hold a weak reference ourselves. */
 	  diskfs_try_dropping_softrefs (np);
-	  tried_drop_softrefs = 1;
-
-	  /* Now we can drop the reference back... */
-	  pthread_mutex_unlock (&np->lock);
-	  goto loop;
 	}
       pthread_mutex_unlock (&np->lock);
     }
-  else
-    pthread_spin_unlock (&diskfs_node_refcnt_lock);
+
+  /* Finally get rid of our reference.  */
+  refcounts_deref_weak (&np->refcounts, &result);
+
+  if (result.hard == 0 && result.weak == 0)
+    {
+      pthread_mutex_lock (&np->lock);
+      diskfs_drop_node (np);
+    }
 }
diff --git a/libdiskfs/node-nrelel.c b/libdiskfs/node-nrelel.c
index ee53b22..dc4f920 100644
--- a/libdiskfs/node-nrelel.c
+++ b/libdiskfs/node-nrelel.c
@@ -26,14 +26,11 @@
 void
 diskfs_nrele_light (struct node *np)
 {
-  pthread_spin_lock (&diskfs_node_refcnt_lock);
-  assert (np->light_references);
-  np->light_references--;
-  if (np->references + np->light_references == 0)
+  struct references result;
+  refcounts_deref_weak (&np->refcounts, &result);
+  if (result.hard == 0 && result.weak == 0)
     {
       pthread_mutex_lock (&np->lock);
       diskfs_drop_node (np);
     }
-  else
-    pthread_spin_unlock (&diskfs_node_refcnt_lock);
 }
diff --git a/tmpfs/node.c b/tmpfs/node.c
index 74971ec..275bb3c 100644
--- a/tmpfs/node.c
+++ b/tmpfs/node.c
@@ -285,12 +285,9 @@ diskfs_try_dropping_softrefs (struct node *np)
     {
       /* 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)
+      struct references result;
+      refcounts_references (&np->refcounts, &result);
+      if (result.hard > 0 || result.weak > 1)
         {
           /* A reference was reacquired through a hash table lookup.
              It's fine, we didn't touch anything yet. */
-- 
2.0.0.rc2