rcuref: Plug slowpath race in rcuref_put()

Kernel test robot reported an "imbalanced put" in the rcuref_put() slow
path, which turned out to be a false positive. Consider the following race:

            ref  = 0 (via rcuref_init(ref, 1))
 T1                                      T2
 rcuref_put(ref)
 -> atomic_add_negative_release(-1, ref)                                         # ref -> 0xffffffff
 -> rcuref_put_slowpath(ref)
                                         rcuref_get(ref)
                                         -> atomic_add_negative_relaxed(1, &ref->refcnt)
                                           -> return true;                       # ref -> 0

                                         rcuref_put(ref)
                                         -> atomic_add_negative_release(-1, ref) # ref -> 0xffffffff
                                         -> rcuref_put_slowpath()

    -> cnt = atomic_read(&ref->refcnt);                                          # cnt -> 0xffffffff / RCUREF_NOREF
    -> atomic_try_cmpxchg_release(&ref->refcnt, &cnt, RCUREF_DEAD))              # ref -> 0xe0000000 / RCUREF_DEAD
       -> return true
                                           -> cnt = atomic_read(&ref->refcnt);   # cnt -> 0xe0000000 / RCUREF_DEAD
                                           -> if (cnt > RCUREF_RELEASED)         # 0xe0000000 > 0xc0000000
                                             -> WARN_ONCE(cnt >= RCUREF_RELEASED, "rcuref - imbalanced put()")

The problem is the additional read in the slow path (after it
decremented to RCUREF_NOREF) which can happen after the counter has been
marked RCUREF_DEAD.

Prevent this by reusing the return value of the decrement. Now every "final"
put uses RCUREF_NOREF in the slow path and attempts the final cmpxchg() to
RCUREF_DEAD.

[ bigeasy: Add changelog ]

Fixes: ee1ee6d ("atomics: Provide rcuref - scalable reference counting")
Reported-by: kernel test robot <[email protected]>
Debugged-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Sebastian Andrzej Siewior <[email protected]>
Cc: [email protected]
Closes: https://lore.kernel.org/oe-lkp/[email protected]

include/linux/rcuref.h

@@ -71,27 +71,30 @@ static inline __must_check bool rcuref_get(rcuref_t *ref)
 	return rcuref_get_slowpath(ref);
 }
 
-extern __must_check bool rcuref_put_slowpath(rcuref_t *ref);
+extern __must_check bool rcuref_put_slowpath(rcuref_t *ref, unsigned int cnt);
 
 /*
  * Internal helper. Do not invoke directly.
  */
 static __always_inline __must_check bool __rcuref_put(rcuref_t *ref)
 {
+	int cnt;
+
 	RCU_LOCKDEP_WARN(!rcu_read_lock_held() && preemptible(),
 			 "suspicious rcuref_put_rcusafe() usage");
 	/*
 	 * Unconditionally decrease the reference count. The saturation and
 	 * dead zones provide enough tolerance for this.
 	 */
-	if (likely(!atomic_add_negative_release(-1, &ref->refcnt)))
+	cnt = atomic_sub_return_release(1, &ref->refcnt);
+	if (likely(cnt >= 0))
 		return false;
 
 	/*
 	 * Handle the last reference drop and cases inside the saturation
 	 * and dead zones.
 	 */
-	return rcuref_put_slowpath(ref);
+	return rcuref_put_slowpath(ref, cnt);
 }
 
 /**

lib/rcuref.c

@@ -220,6 +220,7 @@ EXPORT_SYMBOL_GPL(rcuref_get_slowpath);
 /**
  * rcuref_put_slowpath - Slowpath of __rcuref_put()
  * @ref:	Pointer to the reference count
+ * @cnt:	The resulting value of the fastpath decrement
  *
  * Invoked when the reference count is outside of the valid zone.
  *
@@ -233,10 +234,8 @@ EXPORT_SYMBOL_GPL(rcuref_get_slowpath);
  *	with a concurrent get()/put() pair. Caller is not allowed to
  *	deconstruct the protected object.
  */
-bool rcuref_put_slowpath(rcuref_t *ref)
+bool rcuref_put_slowpath(rcuref_t *ref, unsigned int cnt)
 {
-	unsigned int cnt = atomic_read(&ref->refcnt);
-
 	/* Did this drop the last reference? */
 	if (likely(cnt == RCUREF_NOREF)) {
 		/*