[v4] nptl: Effectively skip CAS in spinlock loop
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Commit Message
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(-)
Comments
On Thu, Jan 20, 2022 at 5:08 AM H.J. Lu via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> 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
>
I would like to backport this patch to release branch 2.33 and 2.34
Any comments/suggestions or objections on this.
commit 6b8dbbd03ac88f169b65b5c7d7278576a11d2e44
Author: Jangwoong Kim <6812skiii@gmail.com>
Date: Tue Dec 14 21:30:51 2021 +0900
nptl: Effectively skip CAS in spinlock loop
The commit:
"Add LLL_MUTEX_READ_LOCK [BZ #28537]"
SHA1: d672a98a1af106bd68deb15576710cd61363f7a6
On Sun, Sep 11, 2022 at 4:24 PM Sunil Pandey via Libc-stable
<libc-stable@sourceware.org> wrote:
>
> On Thu, Jan 20, 2022 at 5:08 AM H.J. Lu via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> >
> > 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
> >
>
> I would like to backport this patch to release branch 2.33 and 2.34
Fine by me.
>
> Any comments/suggestions or objections on this.
>
> commit 6b8dbbd03ac88f169b65b5c7d7278576a11d2e44
> Author: Jangwoong Kim <6812skiii@gmail.com>
> Date: Tue Dec 14 21:30:51 2021 +0900
>
> nptl: Effectively skip CAS in spinlock loop
>
> The commit:
> "Add LLL_MUTEX_READ_LOCK [BZ #28537]"
> SHA1: d672a98a1af106bd68deb15576710cd61363f7a6
@@ -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;
}