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authorAl Viro <viro@zeniv.linux.org.uk>2008-07-15 01:44:23 -0400
committerAl Viro <viro@zeniv.linux.org.uk>2008-07-26 20:53:09 -0400
commitf7e6ced4061da509f737541ca4dbd44d83a6e82f (patch)
tree065a6907a085854da1eb176da1314c472e69a364 /kernel/sysctl.c
parent734550921e9b7ab924a43aa3d0bd4239dac4fbf1 (diff)
[PATCH] allow delayed freeing of ctl_table_header
Refcount the sucker; instead of freeing it by the end of unregistration just drop the refcount and free only when it hits zero. Make sure that we _always_ make ->unregistering non-NULL in start_unregistering(). That allows anybody to get a reference to such puppy, preventing its freeing and reuse. It does *not* block unregistration. Anybody who holds such a reference can * try to grab a "use" reference (ctl_head_grab()); that will succeeds if and only if it hadn't entered unregistration yet. If it succeeds, we can use it in all normal ways until we release the "use" reference (with ctl_head_finish()). Note that this relies on having ->unregistering become non-NULL in all cases when one starts to unregister the sucker. * keep pointers to ctl_table entries; they *can* be freed if the entire thing is unregistered. However, if ctl_head_grab() succeeds, we know that unregistration had not happened (and will not happen until ctl_head_finish()) and such pointers can be used safely. IOW, now we can have inodes under /proc/sys keep references to ctl_table entries, protecting them with references to ctl_table_header and grabbing the latter for the duration of operations that require access to ctl_table. That won't cause deadlocks, since unregistration will not be stopped by mere keeping a reference to ctl_table_header. Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Diffstat (limited to 'kernel/sysctl.c')
-rw-r--r--kernel/sysctl.c37
1 files changed, 36 insertions, 1 deletions
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 8ee4a0619fb..60d9357e717 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1387,6 +1387,9 @@ static void start_unregistering(struct ctl_table_header *p)
spin_unlock(&sysctl_lock);
wait_for_completion(&wait);
spin_lock(&sysctl_lock);
+ } else {
+ /* anything non-NULL; we'll never dereference it */
+ p->unregistering = ERR_PTR(-EINVAL);
}
/*
* do not remove from the list until nobody holds it; walking the
@@ -1395,6 +1398,32 @@ static void start_unregistering(struct ctl_table_header *p)
list_del_init(&p->ctl_entry);
}
+void sysctl_head_get(struct ctl_table_header *head)
+{
+ spin_lock(&sysctl_lock);
+ head->count++;
+ spin_unlock(&sysctl_lock);
+}
+
+void sysctl_head_put(struct ctl_table_header *head)
+{
+ spin_lock(&sysctl_lock);
+ if (!--head->count)
+ kfree(head);
+ spin_unlock(&sysctl_lock);
+}
+
+struct ctl_table_header *sysctl_head_grab(struct ctl_table_header *head)
+{
+ if (!head)
+ BUG();
+ spin_lock(&sysctl_lock);
+ if (!use_table(head))
+ head = ERR_PTR(-ENOENT);
+ spin_unlock(&sysctl_lock);
+ return head;
+}
+
void sysctl_head_finish(struct ctl_table_header *head)
{
if (!head)
@@ -1771,6 +1800,7 @@ struct ctl_table_header *__register_sysctl_paths(
header->unregistering = NULL;
header->root = root;
sysctl_set_parent(NULL, header->ctl_table);
+ header->count = 1;
#ifdef CONFIG_SYSCTL_SYSCALL_CHECK
if (sysctl_check_table(namespaces, header->ctl_table)) {
kfree(header);
@@ -1834,8 +1864,9 @@ void unregister_sysctl_table(struct ctl_table_header * header)
spin_lock(&sysctl_lock);
start_unregistering(header);
+ if (!--header->count)
+ kfree(header);
spin_unlock(&sysctl_lock);
- kfree(header);
}
void setup_sysctl_set(struct ctl_table_set *p,
@@ -1869,6 +1900,10 @@ void setup_sysctl_set(struct ctl_table_set *p,
{
}
+void sysctl_head_put(struct ctl_table_header *head)
+{
+}
+
#endif /* CONFIG_SYSCTL */
/*