[PATCH v3 chao/erofs-dev 08/14] staging: erofs: locked before registering for all new workgroups

[Gao Xiang]

Let's make sure that the one registering a workgroup will also
take the primary work lock at first for two reasons:
  1) There's no need to introduce such a race window (and consequently
     overhead) between registering and locking, other tasks could break
     in by chance, and the race seems unnecessary (no benefit at all);

  2) It's better to take the primary work when a workgroup
     is registered to apply the cache managed policy, for example,
     if some other tasks break in, it could turn into the in-place
     decompression rather than use as the cached decompression.

Reviewed-by: Chao Yu <yuchao0 at huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25 at huawei.com>
---
 drivers/staging/erofs/unzip_vle.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/erofs/unzip_vle.c b/drivers/staging/erofs/unzip_vle.c
index 4e5843e8ee35..a1376f3c6065 100644
--- a/drivers/staging/erofs/unzip_vle.c
+++ b/drivers/staging/erofs/unzip_vle.c
@@ -420,18 +420,22 @@ z_erofs_vle_work_register(const struct z_erofs_vle_work_finder *f,
 	work = z_erofs_vle_grab_primary_work(grp);
 	work->pageofs = f->pageofs;
 
+	/* lock all primary followed works before visible to others */
+	if (unlikely(!mutex_trylock(&work->lock)))
+		/* for a new workgroup, try_lock *never* fails */
+		DBG_BUGON(1);
+
 	if (gnew) {
 		int err = erofs_register_workgroup(f->sb, &grp->obj, 0);
 
 		if (err) {
+			mutex_unlock(&work->lock);
 			kmem_cache_free(z_erofs_workgroup_cachep, grp);
 			return ERR_PTR(-EAGAIN);
 		}
 	}
 
 	*f->owned_head = *f->grp_ret = grp;
-
-	mutex_lock(&work->lock);
 	return work;
 }
 
-- 
2.14.4