Re: [CRIU] BUG: Dentry ffff9f795a08fe60{i=af565f, n=lo} still in use (1) [unmount of proc proc]

[Eric W. Biederman]

Andrei Vagin <ava...@gmail.com> writes:

> I did a few experiments and found that the bug is reproduced for 6-12
> hours on the our test server. Then I reverted two patches and the server
> is working normally for more than 24 hours already, so the bug is
> probably in one of these patches.
>
> commit e3d0065ab8535cbeee69a4c46a59f4d7360803ae
> Author: Andrei Vagin <ava...@openvz.org>
> Date:   Sun Jul 2 07:41:25 2017 +0200
>
>     Revert "proc/sysctl: prune stale dentries during unregistering"
>     
>     This reverts commit d6cffbbe9a7e51eb705182965a189457c17ba8a3.
>
> commit 2d3c50dac81011c1da4d2f7a63b84bd75287e320
> Author: Andrei Vagin <ava...@openvz.org>
> Date:   Sun Jul 2 07:40:08 2017 +0200
>
>     Revert "proc/sysctl: Don't grab i_lock under sysctl_lock."
>     
>     This reverts commit ace0c791e6c3cf5ef37cad2df69f0d90ccc40ffb.
>
>
> FYI: This bug has been reproduced on 4.11.7

Instead of the revert could you test the patch below?

This should fix the issue by grabbing a s_active reference
to the proc super block for every inode we flush.

diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index c5ae09b6c726..18694598bebf 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -67,7 +67,7 @@ struct proc_inode {
        struct proc_dir_entry *pde;
        struct ctl_table_header *sysctl;
        struct ctl_table *sysctl_entry;
-       struct list_head sysctl_inodes;
+       struct hlist_node sysctl_inodes;
        const struct proc_ns_operations *ns_ops;
        struct inode vfs_inode;
 };
diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 67985a7233c2..9bf06e2b1284 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -191,7 +191,7 @@ static void init_header(struct ctl_table_header *head,
        head->set = set;
        head->parent = NULL;
        head->node = node;
-       INIT_LIST_HEAD(&head->inodes);
+       INIT_HLIST_HEAD(&head->inodes);
        if (node) {
                struct ctl_table *entry;
                for (entry = table; entry->procname; entry++, node++)
@@ -261,25 +261,42 @@ static void unuse_table(struct ctl_table_header *p)
                        complete(p->unregistering);
 }
 
-/* called under sysctl_lock */
 static void proc_sys_prune_dcache(struct ctl_table_header *head)
 {
-       struct inode *inode, *prev = NULL;
+       struct inode *inode;
        struct proc_inode *ei;
+       struct hlist_node *node;
+       struct super_block *sb;
 
        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);
+       for (;;) {
+               node = hlist_first_rcu(&head->inodes);
+               if (!node)
+                       break;
+               ei = hlist_entry(node, struct proc_inode, sysctl_inodes);
+               spin_lock(&sysctl_lock);
+               hlist_del_init_rcu(&ei->sysctl_inodes);
+               spin_unlock(&sysctl_lock);
+
+               inode = &ei->vfs_inode;
+               sb = inode->i_sb;
+               if (!atomic_inc_not_zero(&sb->s_active))
+                       continue;
+               inode = igrab(inode);
+               rcu_read_unlock();
+               if (unlikely(!inode)) {
+                       deactivate_super(sb);
                        rcu_read_lock();
+                       continue;
                }
+
+               d_prune_aliases(inode);
+               iput(inode);
+               deactivate_super(sb);
+
+               rcu_read_lock();
        }
        rcu_read_unlock();
-       iput(prev);
 }
 
 /* called under sysctl_lock, will reacquire if has to wait */
@@ -461,7 +478,7 @@ static struct inode *proc_sys_make_inode(struct super_block 
*sb,
        }
        ei->sysctl = head;
        ei->sysctl_entry = table;
-       list_add_rcu(&ei->sysctl_inodes, &head->inodes);
+       hlist_add_head_rcu(&ei->sysctl_inodes, &head->inodes);
        head->count++;
        spin_unlock(&sysctl_lock);
 
@@ -489,7 +506,7 @@ static struct inode *proc_sys_make_inode(struct super_block 
*sb,
 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);
+       hlist_del_init_rcu(&PROC_I(inode)->sysctl_inodes);
        if (!--head->count)
                kfree_rcu(head, rcu);
        spin_unlock(&sysctl_lock);
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 80d07816def0..1c04a26bfd2f 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -143,7 +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 hlist_head inodes; /* head for proc_inode->sysctl_inodes */
 };
 
 struct ctl_dir {