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Message-ID: <20170219084202.GJ29622@ZenIV.linux.org.uk>
Date: Sun, 19 Feb 2017 08:42:12 +0000
From: Al Viro <viro@...IV.linux.org.uk>
To: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc: linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] proc/sysctl: prune stale dentries during unregistering
On Sat, Feb 18, 2017 at 09:55:28PM +0300, Konstantin Khlebnikov wrote:
> This patch has locking problem. I've got lockdep splat under LTP.
>
> d_lock nests inside i_lock
> sysctl_lock nests inside d_lock in d_compare
>
> This patch adds i_lock nesting inside sysctl_lock.
Once ->unregistering is set, you can drop sysctl_lock just fine. So I'd
try something like this - use rcu_read_lock() in proc_sys_prune_dcache(),
drop sysctl_lock() before it and regain after. Make sure that no inodes
are added to the list ones ->unregistering has been set and use RCU list
primitives for modifying the inode list, with sysctl_lock still used to
serialize its modifications.
Freeing struct inode is RCU-delayed (see proc_destroy_inode()), so doing
igrab() is safe there. Since we don't drop inode reference until after we'd
passed beyond it in the list, list_for_each_entry_rcu() should be fine,
AFAICS. Below is a completely untested modification of your patch along
those lines:
diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 842a5ff5b85c..7ad9ed7958af 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -43,10 +43,11 @@ static void proc_evict_inode(struct inode *inode)
de = PDE(inode);
if (de)
pde_put(de);
+
head = PROC_I(inode)->sysctl;
if (head) {
RCU_INIT_POINTER(PROC_I(inode)->sysctl, NULL);
- sysctl_head_put(head);
+ proc_sys_evict_inode(inode, head);
}
}
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index 2de5194ba378..ed1d762160e6 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -65,6 +65,7 @@ struct proc_inode {
struct proc_dir_entry *pde;
struct ctl_table_header *sysctl;
struct ctl_table *sysctl_entry;
+ struct list_head sysctl_inodes;
const struct proc_ns_operations *ns_ops;
struct inode vfs_inode;
};
@@ -249,10 +250,12 @@ extern void proc_thread_self_init(void);
*/
#ifdef CONFIG_PROC_SYSCTL
extern int proc_sys_init(void);
-extern void sysctl_head_put(struct ctl_table_header *);
+extern void proc_sys_evict_inode(struct inode *inode,
+ struct ctl_table_header *head);
#else
static inline void proc_sys_init(void) { }
-static inline void sysctl_head_put(struct ctl_table_header *head) { }
+static inline void proc_sys_evict_inode(struct inode *inode,
+ struct ctl_table_header *head) { }
#endif
/*
diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 55313d994895..6477c4a2dc6c 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -190,6 +190,7 @@ static void init_header(struct ctl_table_header *head,
head->set = set;
head->parent = NULL;
head->node = node;
+ INIT_LIST_HEAD(&head->inodes);
if (node) {
struct ctl_table *entry;
for (entry = table; entry->procname; entry++, node++)
@@ -259,6 +260,26 @@ static void unuse_table(struct ctl_table_header *p)
complete(p->unregistering);
}
+static void proc_sys_prune_dcache(struct ctl_table_header *head)
+{
+ struct inode *inode, *prev = NULL;
+ struct proc_inode *ei;
+
+ rcu_read_lock();
+ list_for_each_entry_rcu(ei, &head->inodes, sysctl_inodes) {
+ inode = igrab(&ei->vfs_inode);
+ if (inode) {
+ rcu_read_unlock();
+ iput(prev);
+ prev = inode;
+ d_prune_aliases(inode);
+ rcu_read_lock();
+ }
+ }
+ rcu_read_unlock();
+ iput(prev);
+}
+
/* called under sysctl_lock, will reacquire if has to wait */
static void start_unregistering(struct ctl_table_header *p)
{
@@ -272,31 +293,22 @@ static void start_unregistering(struct ctl_table_header *p)
p->unregistering = &wait;
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);
+ spin_unlock(&sysctl_lock);
}
/*
+ * Prune dentries for unregistered sysctls: namespaced sysctls
+ * can have duplicate names and contaminate dcache very badly.
+ */
+ proc_sys_prune_dcache(p);
+ /*
* do not remove from the list until nobody holds it; walking the
* list in do_sysctl() relies on that.
*/
- erase_header(p);
-}
-
-static 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_rcu(head, rcu);
- spin_unlock(&sysctl_lock);
+ erase_header(p);
}
static struct ctl_table_header *sysctl_head_grab(struct ctl_table_header *head)
@@ -440,10 +452,20 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
inode->i_ino = get_next_ino();
- sysctl_head_get(head);
ei = PROC_I(inode);
+
+ spin_lock(&sysctl_lock);
+ if (unlikely(head->unregistering)) {
+ spin_unlock(&sysctl_lock);
+ iput(inode);
+ inode = NULL;
+ goto out;
+ }
ei->sysctl = head;
ei->sysctl_entry = table;
+ list_add_rcu(&ei->sysctl_inodes, &head->inodes);
+ head->count++;
+ spin_unlock(&sysctl_lock);
inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
inode->i_mode = table->mode;
@@ -466,6 +488,15 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
return inode;
}
+void proc_sys_evict_inode(struct inode *inode, struct ctl_table_header *head)
+{
+ spin_lock(&sysctl_lock);
+ list_del_rcu(&PROC_I(inode)->sysctl_inodes);
+ if (!--head->count)
+ kfree_rcu(head, rcu);
+ spin_unlock(&sysctl_lock);
+}
+
static struct ctl_table_header *grab_header(struct inode *inode)
{
struct ctl_table_header *head = PROC_I(inode)->sysctl;
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index adf4e51cf597..b7e82049fec7 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -143,6 +143,7 @@ struct ctl_table_header
struct ctl_table_set *set;
struct ctl_dir *parent;
struct ctl_node *node;
+ struct list_head inodes; /* head for proc_inode->sysctl_inodes */
};
struct ctl_dir {
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