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

[David Laight <David.Laight@ACULAB.COM>]

From: Peter Zijlstra
> Sent: 03 May 2019 14:11
> To: Waiman Long
...
> 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;

The compiler will probably convert it back :-)

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)