From ace0c791e6c3cf5ef37cad2df69f0d90ccc40ffb Mon Sep 17 00:00:00 2001 From: "Eric W. Biederman" Date: Mon, 20 Feb 2017 18:17:03 +1300 Subject: [PATCH] proc/sysctl: Don't grab i_lock under sysctl_lock. Konstantin Khlebnikov writes: > This patch has locking problem. I've got lockdep splat under LTP. > > [ 6633.115456] ====================================================== > [ 6633.115502] [ INFO: possible circular locking dependency detected ] > [ 6633.115553] 4.9.10-debug+ #9 Tainted: G L > [ 6633.115584] ------------------------------------------------------- > [ 6633.115627] ksm02/284980 is trying to acquire lock: > [ 6633.115659] (&sb->s_type->i_lock_key#4){+.+...}, at: [] igrab+0x1e/0x80 > [ 6633.115834] but task is already holding lock: > [ 6633.115882] (sysctl_lock){+.+...}, at: [] unregister_sysctl_table+0x6b/0x110 > [ 6633.116026] which lock already depends on the new lock. > [ 6633.116026] > [ 6633.116080] > [ 6633.116080] the existing dependency chain (in reverse order) is: > [ 6633.116117] > -> #2 (sysctl_lock){+.+...}: > -> #1 (&(&dentry->d_lockref.lock)->rlock){+.+...}: > -> #0 (&sb->s_type->i_lock_key#4){+.+...}: > > d_lock nests inside i_lock > sysctl_lock nests inside d_lock in d_compare > > This patch adds i_lock nesting inside sysctl_lock. Al Viro replied: > 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. I agree with Al Viro's analsysis of the situtation. Fixes: d6cffbbe9a7e ("proc/sysctl: prune stale dentries during unregistering") Reported-by: Konstantin Khlebnikov Tested-by: Konstantin Khlebnikov Suggested-by: Al Viro Signed-off-by: "Eric W. Biederman" --- fs/proc/proc_sysctl.c | 31 ++++++++++++++++++------------- 1 file changed, 18 insertions(+), 13 deletions(-) diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c index 8efb1e10b025..3e64c6502dc8 100644 --- a/fs/proc/proc_sysctl.c +++ b/fs/proc/proc_sysctl.c @@ -266,21 +266,19 @@ static void proc_sys_prune_dcache(struct ctl_table_header *head) struct inode *inode, *prev = NULL; struct proc_inode *ei; - list_for_each_entry(ei, &head->inodes, sysctl_inodes) { + rcu_read_lock(); + list_for_each_entry_rcu(ei, &head->inodes, sysctl_inodes) { inode = igrab(&ei->vfs_inode); if (inode) { - spin_unlock(&sysctl_lock); + rcu_read_unlock(); iput(prev); prev = inode; d_prune_aliases(inode); - spin_lock(&sysctl_lock); + rcu_read_lock(); } } - if (prev) { - spin_unlock(&sysctl_lock); - iput(prev); - spin_lock(&sysctl_lock); - } + rcu_read_unlock(); + iput(prev); } /* called under sysctl_lock, will reacquire if has to wait */ @@ -296,10 +294,10 @@ 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 @@ -310,6 +308,7 @@ static void start_unregistering(struct ctl_table_header *p) * do not remove from the list until nobody holds it; walking the * list in do_sysctl() relies on that. */ + spin_lock(&sysctl_lock); erase_header(p); } @@ -455,11 +454,17 @@ static struct inode *proc_sys_make_inode(struct super_block *sb, inode->i_ino = get_next_ino(); ei = PROC_I(inode); - ei->sysctl = head; - ei->sysctl_entry = table; spin_lock(&sysctl_lock); - list_add(&ei->sysctl_inodes, &head->inodes); + 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); @@ -487,7 +492,7 @@ out: void proc_sys_evict_inode(struct inode *inode, struct ctl_table_header *head) { spin_lock(&sysctl_lock); - list_del(&PROC_I(inode)->sysctl_inodes); + list_del_rcu(&PROC_I(inode)->sysctl_inodes); if (!--head->count) kfree_rcu(head, rcu); spin_unlock(&sysctl_lock); -- 2.20.1