1 files changed, 10 insertions, 0 deletions

diff --git a/lib/idr.c b/lib/idr.c
index 1a30272066c6..73f4d53c02f3 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -569,6 +569,7 @@ void idr_remove(struct idr *idp, int id)
         struct idr_layer *p;
         struct idr_layer *to_free;
 
+        /* see comment in idr_find_slowpath() */
         if (WARN_ON_ONCE(id < 0))
                 return;
 
@@ -666,6 +667,14 @@ void *idr_find_slowpath(struct idr *idp, int id)
         int n;
         struct idr_layer *p;
 
+        /*
+         * If @id is negative, idr_find() used to ignore the sign bit and
+         * performed lookup with the rest of bits, which is weird and can
+         * lead to very obscure bugs.  We're now returning NULL for all
+         * negative IDs but just in case somebody was depending on the sign
+         * bit being ignored, let's trigger WARN_ON_ONCE() so that they can
+         * be detected and fixed.  WARN_ON_ONCE() can later be removed.
+         */
         if (WARN_ON_ONCE(id < 0))
                 return NULL;
 
@@ -815,6 +824,7 @@ void *idr_replace(struct idr *idp, void *ptr, int id)
         int n;
         struct idr_layer *p, *old_p;
 
+        /* see comment in idr_find_slowpath() */
         if (WARN_ON_ONCE(id < 0))
                 return ERR_PTR(-EINVAL);