lightnvm: pblk: add comments wrt locking in recovery path

pblk's recovery path is single threaded and therefore a number of
assumptions regarding concurrency can be made. To avoid confusion, make
this explicit with a couple of comments in the code.
Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/lightnvm/pblk-core.c

@@ -1276,6 +1276,7 @@ static int pblk_line_prepare(struct pblk *pblk, struct pblk_line *line)
 	return 0;
 }
 
+/* Line allocations in the recovery path are always single threaded */
 int pblk_line_recov_alloc(struct pblk *pblk, struct pblk_line *line)
 {
 	struct pblk_line_mgmt *l_mg = &pblk->l_mg;

drivers/lightnvm/pblk-recovery.c

@@ -13,6 +13,9 @@
  * General Public License for more details.
  *
  * pblk-recovery.c - pblk's recovery path
+ *
+ * The L2P recovery path is single threaded as the L2P table is updated in order
+ * following the line sequence ID.
  */
 
 #include "pblk.h"