locking/mutex: Fix HANDOFF condition

Yanfei reported that setting HANDOFF should not depend on recomputing
@first, only on @first state. Which would then give:

  if (ww_ctx || !first)
    first = __mutex_waiter_is_first(lock, &waiter);
  if (first)
    __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);

But because 'ww_ctx || !first' is basically 'always' and the test for
first is relatively cheap, omit that first branch entirely.

Reported-by: Yanfei Xu <yanfei.xu@windriver.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Waiman Long <longman@redhat.com>
Reviewed-by: Yanfei Xu <yanfei.xu@windriver.com>
Link: https://lore.kernel.org/r/20210630154114.896786297@infradead.org

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index cab7163f9731f9edf104ed018c66f1bfd661e426..8c3d4993b1e0ff40a6c0a0e262a2718a9012cfc8 100644 (file)
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -909,7 +909,6 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
                    struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
 {
        struct mutex_waiter waiter;
-       bool first = false;
        struct ww_mutex *ww;
        int ret;
 
@@ -988,6 +987,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 
        set_current_state(state);
        for (;;) {
+               bool first;
+
                /*
                 * Once we hold wait_lock, we're serialized against
                 * mutex_unlock() handing the lock off to us, do a trylock
@@ -1016,15 +1017,9 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
                spin_unlock(&lock->wait_lock);
                schedule_preempt_disabled();
 
-               /*
-                * ww_mutex needs to always recheck its position since its waiter
-                * list is not FIFO ordered.
-                */
-               if (ww_ctx || !first) {
-                       first = __mutex_waiter_is_first(lock, &waiter);
-                       if (first)
-                               __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
-               }
+               first = __mutex_waiter_is_first(lock, &waiter);
+               if (first)
+                       __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
 
                set_current_state(state);
                /*