[Cluster-devel] [PATCH] GFS2: Hold gl_hash_table bucket locks 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.
> 
> lockdep_rht_bucket_is_held returns false when rht_bucket_lock(tbl, hash)
> is not held, which suggests that glock_hash_walk should be holding the
> lock for each rhashtable bucket it looks at. Holding those locks clears
> up the warning.
> 
> Signed-off-by: Andrew Price <anprice@redhat.com>
> ---
>  fs/gfs2/glock.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
> index 32e7471..7384cbb 100644
> --- a/fs/gfs2/glock.c
> +++ b/fs/gfs2/glock.c
> @@ -1424,11 +1424,13 @@ static void glock_hash_walk(glock_examiner examiner,
> const struct gfs2_sbd *sdp)
>  	rcu_read_lock();
>  	tbl = rht_dereference_rcu(gl_hash_table.tbl, &gl_hash_table);
>  	for (i = 0; i < tbl->size; i++) {
> +		spin_lock(rht_bucket_lock(tbl, i));
>  		rht_for_each_entry_safe(gl, pos, next, tbl, i, gl_node) {
>  			if ((gl->gl_name.ln_sbd == sdp) &&
>  			    lockref_get_not_dead(&gl->gl_lockref))
>  				examiner(gl);
>  		}
> +		spin_unlock(rht_bucket_lock(tbl, i));
>  	}
>  	rcu_read_unlock();
>  	cond_resched();
> --
> 2.4.3
> 
> 
Hi Andy,

Thanks. I'll hold onto this until after the merge window is closed.

Regards,

Bob Peterson
Red Hat File Systems