On Wed, Apr 07, 2021 at 02:46:11PM -0700, Daniel Xu wrote:
> +static void fini_seq_pagecache(void *priv_data)
> +{
> + struct bpf_iter_seq_pagecache_info *info = priv_data;
> + struct radix_tree_iter iter;
> + struct super_block *sb;
> + void **slot;
> +
> + radix_tree_for_each_slot(slot, &info->superblocks, &iter, 0) {
> + sb = (struct super_block *)iter.index;
> + atomic_dec(&sb->s_active);
> + radix_tree_delete(&info->superblocks, iter.index);
> + }
... and if in the meanwhile all other contributors to ->s_active have
gone away, that will result in...?
IOW, NAK. The objects you are playing with have non-trivial lifecycle
and poking into the guts of data structures without bothering to
understand it is not a good idea.
Rule of the thumb: if your code ends up using fields that are otherwise
handled by a small part of codebase, the odds are that you need to be
bloody careful. In particular, ->ns_lock has 3 users - all in
fs/namespace.c. ->list/->mnt_list: all users in fs/namespace.c and
fs/pnode.c. ->s_active: majority in fs/super.c, with several outliers
in filesystems and safety of those is not trivial.
Any time you see that kind of pattern, you are risking to reprise
a scene from The Modern Times - the one with Charlie taking a trip
through the guts of machinery.
