Re: [PATCH-tip v7 08/20] locking/rwsem: Implement lock handoff to prevent lock starvation

[Peter Zijlstra]

On Sun, Apr 28, 2019 at 05:25:45PM -0400, Waiman Long wrote:
> +static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem,
> +                                     enum writer_wait_state wstate)
>  {
>       long new;
>  
> +     lockdep_assert_held(&sem->wait_lock);
> +     do {
> +             bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
>  
> +             if (has_handoff && wstate == WRITER_NOT_FIRST)
> +                     return false;
>  
> +             if (count & RWSEM_LOCK_MASK) {
> +                     if (has_handoff || (wstate != WRITER_HANDOFF))
> +                             return false;
> +                     new = count | RWSEM_FLAG_HANDOFF;
> +             } else {
> +                     new = (count | RWSEM_WRITER_LOCKED) &
> +                             ~RWSEM_FLAG_HANDOFF;
>  
> +                     if (list_is_singular(&sem->wait_list))
> +                             new &= ~RWSEM_FLAG_WAITERS;
> +             }
> +     } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
> +
> +     /*
> +      * We have either acquired the lock with handoff bit cleared or
> +      * set the handoff bit.
> +      */
> +     if (new & RWSEM_FLAG_HANDOFF)
> +             return false;
> +
> +     rwsem_set_owner(sem);
> +     return true;
>  }

I've changed that like so.

--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -406,19 +406,23 @@ static inline bool rwsem_try_write_lock(
        long new;
 
        lockdep_assert_held(&sem->wait_lock);
+
        do {
                bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
 
                if (has_handoff && wstate == WRITER_NOT_FIRST)
                        return false;
 
+               new = count;
+
                if (count & RWSEM_LOCK_MASK) {
                        if (has_handoff || (wstate != WRITER_HANDOFF))
                                return false;
-                       new = count | RWSEM_FLAG_HANDOFF;
+
+                       new |= RWSEM_FLAG_HANDOFF;
                } else {
-                       new = (count | RWSEM_WRITER_LOCKED) &
-                               ~RWSEM_FLAG_HANDOFF;
+                       new |= RWSEM_WRITER_LOCKED;
+                       new &= ~RWSEM_FLAG_HANDOFF;
 
                        if (list_is_singular(&sem->wait_list))
                                new &= ~RWSEM_FLAG_WAITERS;