[Cluster-devel] [PATCH v2] GFS2: Use rht_for_each_entry_rcu in glock_hash_walk

[Bob Peterson <rpeterso@redhat.com>]

----- Original Message -----
> This lockdep splat was being triggered on umount:
> 
> [55715.973122] ===============================
> [55715.980169] [ INFO: suspicious RCU usage. ]
> [55715.981021] 4.3.0-11553-g8d3de01-dirty #15 Tainted: G        W
> [55715.982353] -------------------------------
> [55715.983301] fs/gfs2/glock.c:1427 suspicious rcu_dereference_protected()
> usage!
> 
> The code it refers to is the rht_for_each_entry_safe usage in
> glock_hash_walk. The condition that triggers the warning is
> lockdep_rht_bucket_is_held(tbl, hash) which is checked in the
> __rcu_dereference_protected macro.
> 
> The rhashtable buckets are not changed in glock_hash_walk so it's safe
> to rely on the rcu protection. Replace the rht_for_each_entry_safe()
> usage with rht_for_each_entry_rcu(), which doesn't care whether the
> bucket lock is held if the rcu read lock is held.
> 
> Signed-off-by: Andrew Price <anprice@redhat.com>
> ---
>  fs/gfs2/glock.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
> index 32e7471..430326e 100644
> --- a/fs/gfs2/glock.c
> +++ b/fs/gfs2/glock.c
> @@ -1417,14 +1417,14 @@ static struct shrinker glock_shrinker = {
>  static void glock_hash_walk(glock_examiner examiner, const struct gfs2_sbd
>  *sdp)
>  {
>  	struct gfs2_glock *gl;
> -	struct rhash_head *pos, *next;
> +	struct rhash_head *pos;
>  	const struct bucket_table *tbl;
>  	int i;
>  
>  	rcu_read_lock();
>  	tbl = rht_dereference_rcu(gl_hash_table.tbl, &gl_hash_table);
>  	for (i = 0; i < tbl->size; i++) {
> -		rht_for_each_entry_safe(gl, pos, next, tbl, i, gl_node) {
> +		rht_for_each_entry_rcu(gl, pos, tbl, i, gl_node) {
>  			if ((gl->gl_name.ln_sbd == sdp) &&
>  			    lockref_get_not_dead(&gl->gl_lockref))
>  				examiner(gl);
> --
> 2.4.3
> 
> 
Hi,

Thanks. This is now applied to the for-next branch of the linux-gfs2 tree:
https://git.kernel.org/cgit/linux/kernel/git/gfs2/linux-gfs2.git/commit/fs/gfs2?h=for-next&id=3dd1dd8c696bdb7c8dcc9456cb23558ad1b336b8

Regards,

Bob Peterson
Red Hat File Systems