lkml.org 
[lkml]   [2018]   [Mar]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH] gfs2: Stop using rhashtable_walk_peek
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 <agruenba@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
[unhandled content-type:application/pgp-signature]
\
 
 \ /
  Last update: 2018-03-28 23:54    [W:0.045 / U:5.924 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site