On Thu, Aug 17, 2023 at 04:37:36PM +0200, Jan Kara wrote:
> On Thu 17-08-23 12:47:44, Christian Brauner wrote:
> > Recent rework moved block device closing out of sb->put_super() and into
> > sb->kill_sb() to avoid deadlocks as s_umount is held in put_super() and
> > blkdev_put() can end up taking s_umount again.
> >
> > That means we need to move the removal of the superblock from @fs_supers
> > out of generic_shutdown_super() and into deactivate_locked_super() to
> > ensure that concurrent mounters don't fail to open block devices that
> > are still in use because blkdev_put() in sb->kill_sb() hasn't been
> > called yet.
> >
> > We can now do this as we can make iterators through @fs_super and
> > @super_blocks wait without holding s_umount. Concurrent mounts will wait
> > until a dying superblock is fully dead so until sb->kill_sb() has been
> > called and SB_DEAD been set. Concurrent iterators can already discard
> > any SB_DYING superblock.
> >
> > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
> > ---
> > fs/super.c | 71 +++++++++++++++++++++++++++++++++++++++++++++++++-----
> > include/linux/fs.h | 1 +
> > 2 files changed, 66 insertions(+), 6 deletions(-)
>
> <snip>
>
> > @@ -456,6 +497,25 @@ void deactivate_locked_super(struct super_block *s)
> > list_lru_destroy(&s->s_dentry_lru);
> > list_lru_destroy(&s->s_inode_lru);
> >
> > + /*
> > + * Remove it from @fs_supers so it isn't found by new
> > + * sget{_fc}() walkers anymore. Any concurrent mounter still
> > + * managing to grab a temporary reference is guaranteed to
> > + * already see SB_DYING and will wait until we notify them about
> > + * SB_DEAD.
> > + */
> > + spin_lock(&sb_lock);
> > + hlist_del_init(&s->s_instances);
> > + spin_unlock(&sb_lock);
> > +
> > + /*
> > + * Let concurrent mounts know that this thing is really dead.
> > + * We don't need @sb->s_umount here as every concurrent caller
> > + * will see SB_DYING and either discard the superblock or wait
> > + * for SB_DEAD.
> > + */
> > + super_wake(s, SB_DEAD);
> > +
> > put_filesystem(fs);
> > put_super(s);
> > } else {
> > @@ -638,15 +698,14 @@ void generic_shutdown_super(struct super_block *sb)
> > spin_unlock(&sb->s_inode_list_lock);
> > }
> > }
> > - spin_lock(&sb_lock);
> > - /* should be initialized for __put_super_and_need_restart() */
> > - hlist_del_init(&sb->s_instances);
> > - spin_unlock(&sb_lock);
>
> OK, but we have several checks of hlist_unhashed(&sb->s_instances) in the
> code whose meaning is now subtly changed. We have:
If by changed meaning you mean they can be dropped, then yes.
That's what I understand you as saying given the following list.
> trylock_super() - needs SB_DYING check instead of s_instances check
> __iterate_supers() - probably we should add SB_DYING check to not block
> emergency operations on s_umount unnecessarily and drop s_instances
> check
> iterate_supers() - we can drop s_instances check
> get_super() - we can drop s_instances check
> get_active_super() - we can drop s_instances check
> user_get_super() - we can drop s_instances check
But does this otherwise look reasonable?
(Btw, just because I noticed it, do you prefer suse.cz or suse.com?)
