On Tue, Jun 18, 2013 at 02:30:05AM +0400, Glauber Costa wrote:
> On Mon, Jun 17, 2013 at 02:35:08PM -0700, Andrew Morton wrote:
> > On Mon, 17 Jun 2013 19:14:12 +0400 Glauber Costa <glommer@xxxxxxxxx> wrote:
> >
> > > > I managed to trigger:
> > > > [ 1015.776029] kernel BUG at mm/list_lru.c:92!
> > > > [ 1015.776029] invalid opcode: 0000 [#1] SMP
> > > > with Linux next (next-20130607) with https://lkml.org/lkml/2013/6/17/203
> > > > on top.
> > > >
> > > > This is obviously BUG_ON(nlru->nr_items < 0) and
> > > > ffffffff81122d0b: 48 85 c0 test %rax,%rax
> > > > ffffffff81122d0e: 49 89 44 24 18 mov %rax,0x18(%r12)
> > > > ffffffff81122d13: 0f 84 87 00 00 00 je ffffffff81122da0 <list_lru_walk_node+0x110>
> > > > ffffffff81122d19: 49 83 7c 24 18 00 cmpq $0x0,0x18(%r12)
> > > > ffffffff81122d1f: 78 7b js ffffffff81122d9c <list_lru_walk_node+0x10c>
> > > > [...]
> > > > ffffffff81122d9c: 0f 0b ud2
> > > >
> > > > RAX is -1UL.
> > > Yes, fearing those kind of imbalances, we decided to leave the counter as a signed quantity
> > > and BUG, instead of an unsigned quantity.
> > >
> > > >
> > > > I assume that the current backtrace is of no use and it would most
> > > > probably be some shrinker which doesn't behave.
> > > >
> > > There are currently 3 users of list_lru in tree: dentries, inodes and xfs.
> > > Assuming you are not using xfs, we are left with dentries and inodes.
> > >
> > > The first thing to do is to find which one of them is misbehaving. You can try finding
> > > this out by the address of the list_lru, and where it lays in the superblock.
> > >
> > > Once we know each of them is misbehaving, then we'll have to figure out why.
> >
> > The trace says shrink_slab_node->super_cache_scan->prune_icache_sb. So
> > it's inodes?
> >
> Assuming there is no memory corruption of any sort going on , let's check the code.
> nr_item is only manipulated in 3 places:
>
> 1) list_lru_add, where it is increased
> 2) list_lru_del, where it is decreased in case the user have voluntarily removed the
> element from the list
> 3) list_lru_walk_node, where an element is removing during shrink.
>
> All three excerpts seem to be correctly locked, so something like this indicates an imbalance.
> Either the element was never added to the list, or it was added, removed, and we didn't notice
> it. (Again, your backing storage is not XFS, is it? If it is , we have another user to look for)
>
> I will assume that Andrew is correct and this is inode related. list_lru_del reads as follows:
> spin_lock(&nlru->lock);
> if (!list_empty(item)) { ... }
>
> So one possibility is that we are manipulating this list outside this lock somewhere. Going to
> inode.c... We always manipulate the LRU inside the lock, but the element is not always in the LRU,
> if it is in a list. Could it be possible that the element is in the dispose_list, and at the same
> time someone calls list_lru_del at it, creating the imbalance ?
>
> callers:
> iput_final, evict_inodes, invalidate_inodes.
> Both evict_inodes and invalidate_inodes will do the following pattern:
>
> inode->i_state |= I_FREEING;
> inode_lru_list_del(inode);
> spin_unlock(&inode->i_lock);
> list_add(&inode->i_lru, &dispose);
>
> IOW, they will remove the element from the LRU, and add it to the dispose list.
> Both of them will also bail out if they see I_FREEING already set, so they are safe
> against each other - because the flag is manipulated inside the lock.
>
> But how about iput_final? It seems to me that if we are calling iput_final at the
> same time as the other two, this *could* happen (maybe there is some extra protection
> that can be seen from Australia but not from here. Dave?)
>
> Right now this is my best theory.
>
> I am attaching a patch that should make a difference in case I am right.
>
>
>
Which is obviously borked since I did not fix the other callers so to move I_FREEING
after lru del.
Michal, would you mind testing the following patch?
diff --git a/fs/inode.c b/fs/inode.c
index 00b804e..48eafa6 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -419,6 +419,8 @@ void inode_add_lru(struct inode *inode)
static void inode_lru_list_del(struct inode *inode)
{
+ if (inode->i_state & I_FREEING)
+ return;
if (list_lru_del(&inode->i_sb->s_inode_lru, &inode->i_lru))
this_cpu_dec(nr_unused);
@@ -609,8 +611,8 @@ void evict_inodes(struct super_block *sb)
continue;
}
- inode->i_state |= I_FREEING;
inode_lru_list_del(inode);
+ inode->i_state |= I_FREEING;
spin_unlock(&inode->i_lock);
list_add(&inode->i_lru, &dispose);
}
@@ -653,8 +655,8 @@ int invalidate_inodes(struct super_block *sb, bool kill_dirty)
continue;
}
- inode->i_state |= I_FREEING;
inode_lru_list_del(inode);
+ inode->i_state |= I_FREEING;
spin_unlock(&inode->i_lock);
list_add(&inode->i_lru, &dispose);
}
@@ -1381,9 +1383,8 @@ static void iput_final(struct inode *inode)
inode->i_state &= ~I_WILL_FREE;
}
+ inode_lru_list_del(inode);
inode->i_state |= I_FREEING;
- if (!list_empty(&inode->i_lru))
- inode_lru_list_del(inode);
spin_unlock(&inode->i_lock);
evict(inode);