[Devel] [PATCH RH7] cgroup: rework reference acquisition for cgroup_find_inode
Pavel Tikhomirov
ptikhomirov at virtuozzo.com
Mon Oct 26 11:02:19 MSK 2020
On 10/26/20 9:48 AM, Andrey Zhadchenko wrote:
> Use more generic igrab instead of atomic inc. Move cgroup_hash_del to eviction
> stage to avoid deadlock.
>
> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
> ---
> kernel/cgroup.c | 25 ++++++++-----------------
> 1 file changed, 8 insertions(+), 17 deletions(-)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 27d7a5e..3bcbae9 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1522,21 +1522,10 @@ static struct inode *cgroup_find_inode(unsigned long fh[2], char take_ref)
> struct inode *ret = NULL;
>
> spin_lock(&cgroup_inode_table_lock);
> - item = cgroup_find_item_no_lock(fh);
>
> - /*
> - * If we need to increase refcount, we should be aware of possible
> - * deadlock. Another thread may have started deleting this inode:
> - * iput->iput_final->cgroup_delete_inode->cgroup_hash_del
> - * If we just call igrab, it will try to take i_lock and this will
> - * result in deadlock, because deleting thread has already taken it
> - * and waits on cgroup_inode_table_lock to find inode in hashtable.
> - *
> - * If i_count is zero, someone is deleting it -> skip.
> - */
> - if (take_ref && item)
> - if (!atomic_inc_not_zero(&item->inode->i_count))
> - item = NULL;
> + item = cgroup_find_item_no_lock(fh);
> + if (item && take_ref && !igrab(item->inode))
> + item = NULL;
>
> spin_unlock(&cgroup_inode_table_lock);
>
> @@ -1634,15 +1623,17 @@ static const struct export_operations cgroup_export_ops = {
> .fh_to_dentry = cgroup_fh_to_dentry,
> };
>
> -static int cgroup_delete_inode(struct inode *inode)
> +static void cgroup_evict_inode(struct inode *inode)
> {
> cgroup_hash_del(inode);
> - return generic_delete_inode(inode);
> + clear_inode(inode);
> + truncate_inode_pages_final(&inode->i_data);
Why clear_inode/truncate_inode_pages_final order is different from the
order in evict()? And probably also we need to call cgroup_hash_del
after clear_inode to be consistent with mqueue_evict_inode.
I don't like different order of calls in different places, though it is
probably ok now, this can hurt in future.
> }
>
> static const struct super_operations cgroup_ops = {
> .statfs = simple_statfs,
> - .drop_inode = cgroup_delete_inode,
> + .drop_inode = generic_delete_inode,
> + .evict_inode = cgroup_evict_inode,
> .show_options = cgroup_show_options,
> #ifdef CONFIG_VE
> .show_path = cgroup_show_path,
>
--
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.
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