Re: [PATCH v6 1/4] Add LLL_MUTEX_READ_LOCK [BZ #28537]

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Thu, Nov 11, 2021 at 10:24 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> CAS instruction is expensive.  From the x86 CPU's point of view, getting
> a cache line for writing is more expensive than reading.  See Appendix
> A.2 Spinlock in:
>
> https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/xeon-lock-scaling-analysis-paper.pdf
>
> The full compare and swap will grab the cache line exclusive and cause
> excessive cache line bouncing.
>
> Add LLL_MUTEX_READ_LOCK to do an atomic load and skip CAS in spinlock
> loop if compare may fail to reduce cache line bouncing on contended locks.
> ---
>  nptl/pthread_mutex_lock.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
> index 2bd41767e0..72058c719c 100644
> --- a/nptl/pthread_mutex_lock.c
> +++ b/nptl/pthread_mutex_lock.c
> @@ -64,6 +64,11 @@ lll_mutex_lock_optimized (pthread_mutex_t *mutex)
>  # define PTHREAD_MUTEX_VERSIONS 1
>  #endif
>
> +#ifndef LLL_MUTEX_READ_LOCK
> +# define LLL_MUTEX_READ_LOCK(mutex) \
> +  atomic_load_relaxed (&(mutex)->__data.__lock)
> +#endif
> +
>  static int __pthread_mutex_lock_full (pthread_mutex_t *mutex)
>       __attribute_noinline__;
>
> @@ -141,6 +146,8 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
>                   break;
>                 }
>               atomic_spin_nop ();
> +             if (LLL_MUTEX_READ_LOCK (mutex) != 0)
> +               continue;

Now that the lock spins on a simple read should `max_cnt` be adjusted?
https://sourceware.org/git/?p=glibc.git;a=blob;f=nptl/pthread_mutex_lock.c;h=762059b230ba97140d6ca16c7273b489592dd3bc;hb=d672a98a1af106bd68deb15576710cd61363f7a6#l143
>             }
>           while (LLL_MUTEX_TRYLOCK (mutex) != 0);
>
> --
> 2.33.1
>