[bug report] staging: erofs: tidy up decompression frontend

[Matthew Wilcox]

On Thu, Nov 14, 2019 at 10:10:03PM +0300, Dan Carpenter wrote:
> 	fs/erofs/zdata.c:443 z_erofs_register_collection()
> 	error: double unlocked 'cl->lock' (orig line 439)
> 
> fs/erofs/zdata.c
>    432          cl = z_erofs_primarycollection(pcl);
>    433          cl->pageofs = map->m_la & ~PAGE_MASK;
>    434  
>    435          /*
>    436           * lock all primary followed works before visible to others
>    437           * and mutex_trylock *never* fails for a new pcluster.
>    438           */
>    439          mutex_trylock(&cl->lock);
>                 ^^^^^^^^^^^^^^^^^^^^^^^^
>    440  
>    441          err = erofs_register_workgroup(inode->i_sb, &pcl->obj, 0);
>    442          if (err) {
>    443                  mutex_unlock(&cl->lock);
>                         ^^^^^^^^^^^^^^^^^^^^^^^
> How can we unlock if we don't know that the trylock succeeded?

The comment says it'll always succeed.  That said, this is an uncommon
pattern -- usually we just mutex_lock().  If there's a good reason to use
mutex_trylock() instead, then I'd prefer it to be guarded with a BUG_ON.