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

["H.J. Lu" <hjl.tools@gmail.com>]

On Tue, Nov 16, 2021 at 6:24 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> 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?

Adding LLL_MUTEX_READ_LOCK just avoids the more expensive
LLL_MUTEX_TRYLOCK.  It doesn't change the flow.

> 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
> >



-- 
H.J.