[v6,4/4] Avoid extra load with CAS in __pthread_mutex_clocklock_common [BZ #28537]
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Build for i686
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Commit Message
Replace boolean CAS with value CAS to avoid the extra load.
---
nptl/pthread_mutex_timedlock.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
Comments
The 11/11/2021 08:24, H.J. Lu via Libc-alpha wrote:
> Replace boolean CAS with value CAS to avoid the extra load.
this looks good in general.
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
> ---
> nptl/pthread_mutex_timedlock.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/nptl/pthread_mutex_timedlock.c b/nptl/pthread_mutex_timedlock.c
> index 57f3f28869..f763cfc7fa 100644
> --- a/nptl/pthread_mutex_timedlock.c
> +++ b/nptl/pthread_mutex_timedlock.c
> @@ -233,12 +233,12 @@ __pthread_mutex_clocklock_common (pthread_mutex_t *mutex,
> meantime. */
> if ((oldval & FUTEX_WAITERS) == 0)
> {
> - if (atomic_compare_and_exchange_bool_acq (&mutex->__data.__lock,
> - oldval | FUTEX_WAITERS,
> - oldval)
> - != 0)
> + int val;
> + if ((val = atomic_compare_and_exchange_val_acq
> + (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
> + oldval)) != oldval)
> {
> - oldval = mutex->__data.__lock;
> + oldval = val;
> continue;
> }
> oldval |= FUTEX_WAITERS;
> --
> 2.33.1
>
On Nov 11 2021, H.J. Lu wrote:
> diff --git a/nptl/pthread_mutex_timedlock.c b/nptl/pthread_mutex_timedlock.c
> index 57f3f28869..f763cfc7fa 100644
> --- a/nptl/pthread_mutex_timedlock.c
> +++ b/nptl/pthread_mutex_timedlock.c
> @@ -233,12 +233,12 @@ __pthread_mutex_clocklock_common (pthread_mutex_t *mutex,
> meantime. */
> if ((oldval & FUTEX_WAITERS) == 0)
> {
> - if (atomic_compare_and_exchange_bool_acq (&mutex->__data.__lock,
> - oldval | FUTEX_WAITERS,
> - oldval)
> - != 0)
> + int val;
> + if ((val = atomic_compare_and_exchange_val_acq
> + (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
> + oldval)) != oldval)
Please move the assignment out of the condition.
Andreas.
On Fri, Nov 12, 2021 at 10:53 AM Andreas Schwab <schwab@linux-m68k.org> wrote:
>
> On Nov 11 2021, H.J. Lu wrote:
>
> > diff --git a/nptl/pthread_mutex_timedlock.c b/nptl/pthread_mutex_timedlock.c
> > index 57f3f28869..f763cfc7fa 100644
> > --- a/nptl/pthread_mutex_timedlock.c
> > +++ b/nptl/pthread_mutex_timedlock.c
> > @@ -233,12 +233,12 @@ __pthread_mutex_clocklock_common (pthread_mutex_t *mutex,
> > meantime. */
> > if ((oldval & FUTEX_WAITERS) == 0)
> > {
> > - if (atomic_compare_and_exchange_bool_acq (&mutex->__data.__lock,
> > - oldval | FUTEX_WAITERS,
> > - oldval)
> > - != 0)
> > + int val;
> > + if ((val = atomic_compare_and_exchange_val_acq
> > + (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
> > + oldval)) != oldval)
>
> Please move the assignment out of the condition.
>
> Andreas.
>
> --
> Andreas Schwab, schwab@linux-m68k.org
> GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510 2552 DF73 E780 A9DA AEC1
> "And now for something completely different."
I would like to backport this patch to release branch 2.33 and 2.34
Any comments/suggestions or objections on this.
commit 0b82747dc48d5bf0871bdc6da8cb6eec1256355f
Author: H.J. Lu <hjl.tools@gmail.com>
Date: Thu Nov 11 06:31:51 2021 -0800
Avoid extra load with CAS in __pthread_mutex_lock_full [BZ #28537]
Replace boolean CAS with value CAS to avoid the extra load.
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
On 9/11/2022 1:12 PM, Sunil Pandey wrote:
> On Fri, Nov 12, 2021 at 10:53 AM Andreas Schwab <schwab@linux-m68k.org> wrote:
>>
>> On Nov 11 2021, H.J. Lu wrote:
>>
>>> diff --git a/nptl/pthread_mutex_timedlock.c b/nptl/pthread_mutex_timedlock.c
>>> index 57f3f28869..f763cfc7fa 100644
>>> --- a/nptl/pthread_mutex_timedlock.c
>>> +++ b/nptl/pthread_mutex_timedlock.c
>>> @@ -233,12 +233,12 @@ __pthread_mutex_clocklock_common (pthread_mutex_t *mutex,
>>> meantime. */
>>> if ((oldval & FUTEX_WAITERS) == 0)
>>> {
>>> - if (atomic_compare_and_exchange_bool_acq (&mutex->__data.__lock,
>>> - oldval | FUTEX_WAITERS,
>>> - oldval)
>>> - != 0)
>>> + int val;
>>> + if ((val = atomic_compare_and_exchange_val_acq
>>> + (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
>>> + oldval)) != oldval)
>>
>> Please move the assignment out of the condition.
>>
>> Andreas.
>>
>> --
>> Andreas Schwab, schwab@linux-m68k.org
>> GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510 2552 DF73 E780 A9DA AEC1
>> "And now for something completely different."
>
> I would like to backport this patch to release branch 2.33 and 2.34
>
what exactly is the stable branch policy that would suggest to backport performance improvements like this ?
(most projects are sticking to "strict bugfixes and other gross oversights" as much as possible)
* Arjan van de Ven via Libc-alpha:
> what exactly is the stable branch policy that would suggest to
> backport performance improvements like this ? (most projects are
> sticking to "strict bugfixes and other gross oversights" as much as
> possible)
We occasionally backport safe (and not-so-safe) performance
optimizations. We draw the line at ABI changes, those aren't possible.
It's also very desirable that all future releases have a superset of the
backports.
But anything else depends on who is willing to do the work.
Thanks,
Florian
On Sun, Sep 11, 2022 at 4:13 PM Sunil Pandey via Libc-stable
<libc-stable@sourceware.org> wrote:
>
> On Fri, Nov 12, 2021 at 10:53 AM Andreas Schwab <schwab@linux-m68k.org> wrote:
> >
> > On Nov 11 2021, H.J. Lu wrote:
> >
> > > diff --git a/nptl/pthread_mutex_timedlock.c b/nptl/pthread_mutex_timedlock.c
> > > index 57f3f28869..f763cfc7fa 100644
> > > --- a/nptl/pthread_mutex_timedlock.c
> > > +++ b/nptl/pthread_mutex_timedlock.c
> > > @@ -233,12 +233,12 @@ __pthread_mutex_clocklock_common (pthread_mutex_t *mutex,
> > > meantime. */
> > > if ((oldval & FUTEX_WAITERS) == 0)
> > > {
> > > - if (atomic_compare_and_exchange_bool_acq (&mutex->__data.__lock,
> > > - oldval | FUTEX_WAITERS,
> > > - oldval)
> > > - != 0)
> > > + int val;
> > > + if ((val = atomic_compare_and_exchange_val_acq
> > > + (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
> > > + oldval)) != oldval)
> >
> > Please move the assignment out of the condition.
> >
> > Andreas.
> >
> > --
> > Andreas Schwab, schwab@linux-m68k.org
> > GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510 2552 DF73 E780 A9DA AEC1
> > "And now for something completely different."
>
> I would like to backport this patch to release branch 2.33 and 2.34
>
> Any comments/suggestions or objections on this.
>
> commit 0b82747dc48d5bf0871bdc6da8cb6eec1256355f
> Author: H.J. Lu <hjl.tools@gmail.com>
> Date: Thu Nov 11 06:31:51 2021 -0800
>
> Avoid extra load with CAS in __pthread_mutex_lock_full [BZ #28537]
>
> Replace boolean CAS with value CAS to avoid the extra load.
>
> Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Fine by me.
@@ -233,12 +233,12 @@ __pthread_mutex_clocklock_common (pthread_mutex_t *mutex,
meantime. */
if ((oldval & FUTEX_WAITERS) == 0)
{
- if (atomic_compare_and_exchange_bool_acq (&mutex->__data.__lock,
- oldval | FUTEX_WAITERS,
- oldval)
- != 0)
+ int val;
+ if ((val = atomic_compare_and_exchange_val_acq
+ (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
+ oldval)) != oldval)
{
- oldval = mutex->__data.__lock;
+ oldval = val;
continue;
}
oldval |= FUTEX_WAITERS;