[PATCH v4] nptl: Effectively skip CAS in spinlock loop

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

On Thu, Jan 20, 2022 at 04:53:27AM -0800, H.J. Lu wrote:
> On Tue, Dec 14, 2021 at 4:31 AM Jangwoong Kim via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> >
> > The commit:
> > "Add LLL_MUTEX_READ_LOCK [BZ #28537]"
> > SHA1: d672a98a1af106bd68deb15576710cd61363f7a6
> >
> > introduced LLL_MUTEX_READ_LOCK, to skip CAS in spinlock loop
> > if atomic load fails. But, "continue" inside of do-while loop
> > does not skip the evaluation of escape expression, thus CAS
> > is not skipped.
> >
> > Replace do-while with while and skip LLL_MUTEX_TRYLOCK if
> > LLL_MUTEX_READ_LOCK fails.
> > ---
> >  nptl/pthread_mutex_lock.c | 5 +----
> >  1 file changed, 1 insertion(+), 4 deletions(-)
> >
> > diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
> > index 47b88a6b5b..06274c68e4 100644
> > --- a/nptl/pthread_mutex_lock.c
> > +++ b/nptl/pthread_mutex_lock.c
> > @@ -138,7 +138,7 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
> >           int cnt = 0;
> >           int max_cnt = MIN (max_adaptive_count (),
> >                              mutex->__data.__spins * 2 + 10);
> > -         do
> > +         while (LLL_MUTEX_READ_LOCK (mutex) != 0 || LLL_MUTEX_TRYLOCK (mutex) != 0)
> >             {
> >               if (cnt++ >= max_cnt)
> >                 {
> > @@ -146,10 +146,7 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
> >                   break;
> >                 }
> >               atomic_spin_nop ();
> > -             if (LLL_MUTEX_READ_LOCK (mutex) != 0)
> > -               continue;
> >             }
> > -         while (LLL_MUTEX_TRYLOCK (mutex) != 0);
> >
> >           mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8;
> >         }
> > --
> > 2.25.1
> >
> 
> LGTM.
> 
> Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
> 
> Thanks.
> 

This is the v4 patch I am checkinng in.

H.J.
---
From: Jangwoong Kim <6812skiii@gmail.com>
Date: Tue, 14 Dec 2021 21:30:51 +0900

The commit:
"Add LLL_MUTEX_READ_LOCK [BZ #28537]"
SHA1: d672a98a1af106bd68deb15576710cd61363f7a6

introduced LLL_MUTEX_READ_LOCK, to skip CAS in spinlock loop
if atomic load fails. But, "continue" inside of do-while loop
does not skip the evaluation of escape expression, thus CAS
is not skipped.

Replace do-while with while and skip LLL_MUTEX_TRYLOCK if
LLL_MUTEX_READ_LOCK fails.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
---
 nptl/pthread_mutex_lock.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
index 31dfc7f1d7..d2e652d151 100644
--- a/nptl/pthread_mutex_lock.c
+++ b/nptl/pthread_mutex_lock.c
@@ -146,10 +146,9 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
 		  break;
 		}
 	      atomic_spin_nop ();
-	      if (LLL_MUTEX_READ_LOCK (mutex) != 0)
-		continue;
 	    }
-	  while (LLL_MUTEX_TRYLOCK (mutex) != 0);
+	  while (LLL_MUTEX_READ_LOCK (mutex) != 0
+		 || LLL_MUTEX_TRYLOCK (mutex) != 0);
 
 	  mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8;
 	}
-- 
2.34.1