On Sat, Dec 16, 2017 at 08:42:51AM +0200, Nikolay Borisov wrote: > > > On 15.12.2017 21:58, Chris Mason wrote: > > refcounts have a generic implementation and an asm optimized one. The > > generic version has extra debugging to make sure that once a refcount > > goes to zero, refcount_inc won't increase it. > ^^^^^^^^ > > I guess you meant to say refcount_add
refcount_inc may also just throw a warning without bumping the refcnt. Thanks, -liubo > > > > The btrfs delayed inode code wasn't expecting this, and we're tripping > > over the warnings when the generic refcounts are used. We ended up with > > this race: > > > > Process A Process B > > btrfs_get_delayed_node() > > spin_lock(root->inode_lock) > > radix_tree_lookup() > > __btrfs_release_delayed_node() > > refcount_dec_and_test(&delayed_node->refs) > > our refcount is now zero > > refcount_add(2) <--- > > warning here, refcount > > unchanged > > > > spin_lock(root->inode_lock) > > radix_tree_delete() > > > > With the generic refcounts, we actually warn again when process B above > > tries to release his refcount because refcount_add() turned into a > > no-op. > > > > We saw this in production on older kernels without the asm optimized > > refcounts. > > > > The fix used here is to use refcount_inc_not_zero() to detect when the > > object is in the middle of being freed and return NULL. This is almost > > always the right answer anyway, since we usually end up pitching the > > delayed_node if it didn't have fresh data in it. > > > > This also changes __btrfs_release_delayed_node() to remove the extra > > check for zero refcounts before radix tree deletion. > > btrfs_get_delayed_node() was the only path that was allowing refcounts > > to go from zero to one. > > > > Signed-off-by: Chris Mason <c...@fb.com> > > Fixes: 6de5f18e7b0da > > cc: <sta...@vger.kernel.org> #4.12+ > > --- > > fs/btrfs/delayed-inode.c | 45 ++++++++++++++++++++++++++++++++++----------- > > 1 file changed, 34 insertions(+), 11 deletions(-) > > > > diff --git a/fs/btrfs/delayed-inode.c b/fs/btrfs/delayed-inode.c > > index 5d73f79..84c54af 100644 > > --- a/fs/btrfs/delayed-inode.c > > +++ b/fs/btrfs/delayed-inode.c > > @@ -87,6 +87,7 @@ static struct btrfs_delayed_node *btrfs_get_delayed_node( > > > > spin_lock(&root->inode_lock); > > node = radix_tree_lookup(&root->delayed_nodes_tree, ino); > > + > > if (node) { > > if (btrfs_inode->delayed_node) { > > refcount_inc(&node->refs); /* can be accessed */ > > @@ -94,9 +95,30 @@ static struct btrfs_delayed_node *btrfs_get_delayed_node( > > spin_unlock(&root->inode_lock); > > return node; > > } > > - btrfs_inode->delayed_node = node; > > - /* can be accessed and cached in the inode */ > > - refcount_add(2, &node->refs); > > + > > + /* it's possible that we're racing into the middle of > > + * removing this node from the radix tree. In this case, > > + * the refcount was zero and it should never go back > > + * to one. Just return NULL like it was never in the radix > > + * at all; our release function is in the process of removing > > + * it. > > + * > > + * Some implementations of refcount_inc refuse to > > + * bump the refcount once it has hit zero. If we don't do > > + * this dance here, refcount_inc() may decide to > > + * just WARN_ONCE() instead of actually bumping the refcount. > > + * > > + * If this node is properly in the radix, we want to > > + * bump the refcount twice, once for the inode > > + * and once for this get operation. > > + */ > > + if (refcount_inc_not_zero(&node->refs)) { > > + refcount_inc(&node->refs); > > + btrfs_inode->delayed_node = node; > > + } else { > > + node = NULL; > > + } > > + > > spin_unlock(&root->inode_lock); > > return node; > > } > > @@ -254,17 +276,18 @@ static void __btrfs_release_delayed_node( > > mutex_unlock(&delayed_node->mutex); > > > > if (refcount_dec_and_test(&delayed_node->refs)) { > > - bool free = false; > > struct btrfs_root *root = delayed_node->root; > > + > > spin_lock(&root->inode_lock); > > - if (refcount_read(&delayed_node->refs) == 0) { > > - radix_tree_delete(&root->delayed_nodes_tree, > > - delayed_node->inode_id); > > - free = true; > > - } > > + /* > > + * once our refcount goes to zero, nobody is allowed to > > + * bump it back up. We can delete it now > > + */ > > + ASSERT(refcount_read(&delayed_node->refs) == 0); > > + radix_tree_delete(&root->delayed_nodes_tree, > > + delayed_node->inode_id); > > spin_unlock(&root->inode_lock); > > - if (free) > > - kmem_cache_free(delayed_node_cache, delayed_node); > > + kmem_cache_free(delayed_node_cache, delayed_node); > > } > > } > > > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html