[v2] nptl: Effectively skip CAS in spinlock loop
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| dj/TryBot-apply_patch |
success
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Patch applied to master at the time it was sent
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| dj/TryBot-32bit |
success
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Build for i686
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Commit Message
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 | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)
Comments
On Dez 14 2021, Jangwoong Kim via Libc-alpha wrote:
> diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
> index 47b88a6b5b..3ad199a5e1 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 (1)
> {
> if (cnt++ >= max_cnt)
> {
> @@ -146,10 +146,9 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
> break;
> }
> atomic_spin_nop ();
> - if (LLL_MUTEX_READ_LOCK (mutex) != 0)
> - continue;
> + if (LLL_MUTEX_READ_LOCK (mutex) == 0 && LLL_MUTEX_TRYLOCK (mutex) == 0)
> + break;
> }
> - while (LLL_MUTEX_TRYLOCK (mutex) != 0);
You can keep the condition in the loop trailer.
@@ -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 (1)
{
if (cnt++ >= max_cnt)
{
@@ -146,10 +146,9 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
break;
}
atomic_spin_nop ();
- if (LLL_MUTEX_READ_LOCK (mutex) != 0)
- continue;
+ if (LLL_MUTEX_READ_LOCK (mutex) == 0 && LLL_MUTEX_TRYLOCK (mutex) == 0)
+ break;
}
- while (LLL_MUTEX_TRYLOCK (mutex) != 0);
mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8;
}