Re: [PATCH] locking/rtmutex: remove unused cmpxchg_relaxed

[Davidlohr Bueso]

On Tue, 04 Feb 2020, Alex Shi wrote:

> Thanks Thomas and David!
> Is this following patch ok?

So if anything, this really wants to be two patches.

> ---
> From 4cf9e38a73c67c6894f3addb2ddca26bb51b1a28 Mon Sep 17 00:00:00 2001
> From: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
> Date: Tue, 21 Jan 2020 15:03:33 +0800
> Subject: [PATCH v2] locking/rtmutex: optimize rt_mutex_cmpxchg_xxx series func
>
> rt_mutex_cmpxchg_relexed isn't interested by anyone, so remove it.
> And Davidlohr Bueso suggests check l->owner before cmpxchg to reduce
> lock contention.
>
> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
> kernel/locking/rtmutex.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index 851bbb10819d..eb26f4e57ce4 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -141,9 +141,10 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
>  * set up.
>  */
> #ifndef CONFIG_DEBUG_RT_MUTEXES
> -# define rt_mutex_cmpxchg_relaxed(l,c,n) (cmpxchg_relaxed(&l->owner, c, n) == c)
> -# define rt_mutex_cmpxchg_acquire(l,c,n) (cmpxchg_acquire(&l->owner, c, n) == c)
> -# define rt_mutex_cmpxchg_release(l,c,n) (cmpxchg_release(&l->owner, c, n) == c)
> +# define rt_mutex_cmpxchg_acquire(l,c,n)	\
> +		(l->owner == c && cmpxchg_acquire(&l->owner, c, n) == c)
> +# define rt_mutex_cmpxchg_release(l,c,n)	\
> +		(l->owner == c && cmpxchg_release(&l->owner, c, n) == c)

Thomas, should I resend the top-waiter spin series? Otherwise yeah,
I see little point to the CCAS fastpath thing.

Thanks,
Davidlohr