Re: [PATCH] gfs2: Stop using rhashtable_walk_peek

[NeilBrown]

On Wed, Mar 28 2018, Andreas Gruenbacher wrote:

> Function rhashtable_walk_peek is problematic because there is no
> guarantee that the glock previously returned still exists; when that key
> is deleted, rhashtable_walk_peek can end up returning a different key,
> which would cause an inconsistent glock dump.  So instead of using
> rhashtable_walk_peek, keep track of the current glock in the seq file
> iterator functions.
>
> Signed-off-by: Andreas Gruenbacher <agrue...@redhat.com>
> ---
>  fs/gfs2/glock.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
>
> diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
> index 82fb5583445c..f1fc353875d3 100644
> --- a/fs/gfs2/glock.c
> +++ b/fs/gfs2/glock.c
> @@ -55,6 +55,7 @@ struct gfs2_glock_iter {
>       struct gfs2_sbd *sdp;           /* incore superblock           */
>       struct rhashtable_iter hti;     /* rhashtable iterator         */
>       struct gfs2_glock *gl;          /* current glock struct        */
> +     bool gl_held;
>       loff_t last_pos;                /* last position               */
>  };
>  
> @@ -1923,9 +1924,11 @@ void gfs2_glock_exit(void)
>  
>  static void gfs2_glock_iter_next(struct gfs2_glock_iter *gi, loff_t n)
>  {
> -     if (n == 0)
> -             gi->gl = rhashtable_walk_peek(&gi->hti);
> -     else {
> +     if (n != 0 || !gi->gl) {
> +             if (gi->gl_held) {
> +                     gfs2_glock_queue_put(gi->gl);
> +                     gi->gl_held = false;
> +             }
>               gi->gl = rhashtable_walk_next(&gi->hti);
>               n--;
>       }

Thank for this patch!
The above looks a bit fragile to me.
gfs2_glock_iter_next() (And hence gfs2_glock_seq_start()) will sometimes
exit with gl_held true, and sometimes with it false.
gfs2_glock_seq_stop() assumes that it is false.
Normally gfs2_glock_seq_next() will normally be called between these
two and will clear gl_held, but I don't think there is a hard guarantee
of that.
Maybe we should always 'put' gi->gl in iter_next if gl_held??

Thanks,
NeilBrown



> @@ -1988,7 +1991,10 @@ static void gfs2_glock_seq_stop(struct seq_file *seq, 
> void *iter_ptr)
>  {
>       struct gfs2_glock_iter *gi = seq->private;
>  
> -     gi->gl = NULL;
> +     if (gi->gl) {
> +             lockref_get(&gi->gl->gl_lockref);
> +             gi->gl_held = true;
> +     }
>       rhashtable_walk_stop(&gi->hti);
>  }
>  
> @@ -2061,6 +2067,7 @@ static int __gfs2_glocks_open(struct inode *inode, 
> struct file *file,
>                */
>               gi->last_pos = -1;
>               gi->gl = NULL;
> +             gi->gl_held = false;
>               rhashtable_walk_enter(&gl_hash_table, &gi->hti);
>       }
>       return ret;
> @@ -2076,7 +2083,8 @@ static int gfs2_glocks_release(struct inode *inode, 
> struct file *file)
>       struct seq_file *seq = file->private_data;
>       struct gfs2_glock_iter *gi = seq->private;
>  
> -     gi->gl = NULL;
> +     if (gi->gl_held)
> +             gfs2_glock_put(gi->gl);
>       rhashtable_walk_exit(&gi->hti);
>       return seq_release_private(inode, file);
>  }
> -- 
> 2.14.3

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