Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list is not NULL.

[Peter Zijlstra]

On Fri, Jan 22, 2016 at 11:56:52AM +0100, Peter Zijlstra wrote:
> On Fri, Jan 22, 2016 at 11:53:12AM +0100, Peter Zijlstra wrote:
> 
> > There might be other details, but this is the one that stood out.
> 
> I think this also does the wrong thing for use_ww_ctx.

Something like so? 

---
 kernel/locking/mutex.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 0551c219c40e..070a0ac34aa7 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -512,6 +512,7 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned int subclass,
        struct task_struct *task = current;
        struct mutex_waiter waiter;
        unsigned long flags;
+       bool acquired;
        int ret;
 
        preempt_disable();
@@ -543,6 +544,7 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned int subclass,
        lock_contended(&lock->dep_map, ip);
 
        for (;;) {
+               acquired = false;
                /*
                 * Lets try to take the lock again - this is needed even if
                 * we get here for the first time (shortly after failing to
@@ -577,7 +579,16 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned int subclass,
                /* didn't get the lock, go to sleep: */
                spin_unlock_mutex(&lock->wait_lock, flags);
                schedule_preempt_disabled();
+
+               if (mutex_is_locked(lock))
+                       acquired = mutex_optimistic_spin(lock, ww_ctx, 
use_ww_ctx);
+
                spin_lock_mutex(&lock->wait_lock, flags);
+
+               if (acquired) {
+                       atomic_set(&lock->count, -1);
+                       break;
+               }
        }
        __set_task_state(task, TASK_RUNNING);
 
@@ -587,6 +598,9 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned int subclass,
                atomic_set(&lock->count, 0);
        debug_mutex_free_waiter(&waiter);
 
+       if (acquired)
+               goto unlock;
+
 skip_wait:
        /* got the lock - cleanup and rejoice! */
        lock_acquired(&lock->dep_map, ip);
@@ -597,6 +611,7 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned int subclass,
                ww_mutex_set_context_slowpath(ww, ww_ctx);
        }
 
+unlock:
        spin_unlock_mutex(&lock->wait_lock, flags);
        preempt_enable();
        return 0;