* [PATCH] Use C11 atomics instead atomic_decrement_and_test
@ 2022-09-08 14:06 Wilco Dijkstra
2022-09-21 17:27 ` Adhemerval Zanella Netto
0 siblings, 1 reply; 4+ messages in thread
From: Wilco Dijkstra @ 2022-09-08 14:06 UTC (permalink / raw)
To: 'GNU C Library'; +Cc: Adhemerval Zanella
Replace atomic_decrement_and_test with atomic_fetch_add_relaxed.
Passes regress on AArch64.
---
diff --git a/htl/pt-dealloc.c b/htl/pt-dealloc.c
index c776e3471dc376977ca7058f0d143f0a842d5799..86bbb3091fbd3758b06294ca705dc774cf8bf0eb 100644
--- a/htl/pt-dealloc.c
+++ b/htl/pt-dealloc.c
@@ -33,7 +33,7 @@ extern pthread_mutex_t __pthread_free_threads_lock;
void
__pthread_dealloc (struct __pthread *pthread)
{
- if (!atomic_decrement_and_test (&pthread->nr_refs))
+ if (atomic_fetch_add_relaxed (&pthread->nr_refs, -1) != 1)
return;
/* Withdraw this thread from the thread ID lookup table. */
diff --git a/htl/pt-exit.c b/htl/pt-exit.c
index f0759c8738e6da4c8e0ba47f6d15720203f6954e..3c0a8c52f34ed64b94ce71a0764512cb76351a3a 100644
--- a/htl/pt-exit.c
+++ b/htl/pt-exit.c
@@ -50,7 +50,7 @@ __pthread_exit (void *status)
/* Decrease the number of threads. We use an atomic operation to
make sure that only the last thread calls `exit'. */
- if (atomic_decrement_and_test (&__pthread_total))
+ if (atomic_fetch_add_relaxed (&__pthread_total, -1) == 1)
/* We are the last thread. */
exit (0);
diff --git a/manual/llio.texi b/manual/llio.texi
index 4e6e3fb672bb8ecbb1bb52faf3d70d9b7b33973f..eba9a0e6593f17116b41ff302cfca05846727155 100644
--- a/manual/llio.texi
+++ b/manual/llio.texi
@@ -2614,7 +2614,7 @@ aiocb64}, since the LFS transparently replaces the old interface.
@c free @ascuheap @acsmem
@c libc_thread_freeres
@c libc_thread_subfreeres ok
-@c atomic_decrement_and_test ok
+@c atomic_fetch_add_relaxed ok
@c td_eventword ok
@c td_eventmask ok
@c atomic_compare_exchange_bool_acq ok
diff --git a/nptl/cond-perf.c b/nptl/cond-perf.c
index 9c9488e2743f17eb4beeaadf45ef83a38a9f9bf9..baf69e6d34fdb3ffef97b32fb9db422077a0359b 100644
--- a/nptl/cond-perf.c
+++ b/nptl/cond-perf.c
@@ -24,7 +24,7 @@ cons (void *arg)
do
{
- if (atomic_decrement_and_test (&ntogo))
+ if (atomic_fetch_add_relaxed (&ntogo, -1) == 1)
{
pthread_mutex_lock (&mut2);
alldone = true;
diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
index 2602dba54e872f36bd54955af7587378e5dc3812..ada3fdd4d440c3fdcf4567734a452c8ba4d0fce1 100644
--- a/nptl/pthread_create.c
+++ b/nptl/pthread_create.c
@@ -489,7 +489,7 @@ start_thread (void *arg)
the breakpoint reports TD_THR_RUN state rather than TD_THR_ZOMBIE. */
atomic_fetch_or_relaxed (&pd->cancelhandling, EXITING_BITMASK);
- if (__glibc_unlikely (atomic_decrement_and_test (&__nptl_nthreads)))
+ if (__glibc_unlikely (atomic_fetch_add_relaxed (&__nptl_nthreads, -1) == 1))
/* This was the last thread. */
exit (0);
diff --git a/sysdeps/nptl/libc_start_call_main.h b/sysdeps/nptl/libc_start_call_main.h
index a9e85f2b098dfc6790d719bba9662e428ab237e6..c10a16b2c4a61e74f6bf18396935b87208df4f7e 100644
--- a/sysdeps/nptl/libc_start_call_main.h
+++ b/sysdeps/nptl/libc_start_call_main.h
@@ -65,7 +65,7 @@ __libc_start_call_main (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
/* One less thread. Decrement the counter. If it is zero we
terminate the entire process. */
result = 0;
- if (! atomic_decrement_and_test (&__nptl_nthreads))
+ if (atomic_fetch_add_relaxed (&__nptl_nthreads, -1) != 1)
/* Not much left to do but to exit the thread, not the process. */
while (1)
INTERNAL_SYSCALL_CALL (exit, 0);
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] Use C11 atomics instead atomic_decrement_and_test
2022-09-08 14:06 [PATCH] Use C11 atomics instead atomic_decrement_and_test Wilco Dijkstra
@ 2022-09-21 17:27 ` Adhemerval Zanella Netto
2022-09-22 13:55 ` Wilco Dijkstra
0 siblings, 1 reply; 4+ messages in thread
From: Adhemerval Zanella Netto @ 2022-09-21 17:27 UTC (permalink / raw)
To: Wilco Dijkstra, 'GNU C Library'
On 08/09/22 11:06, Wilco Dijkstra wrote:
> Replace atomic_decrement_and_test with atomic_fetch_add_relaxed.
>
> Passes regress on AArch64.
Some questions below.
>
> ---
>
> diff --git a/htl/pt-dealloc.c b/htl/pt-dealloc.c
> index c776e3471dc376977ca7058f0d143f0a842d5799..86bbb3091fbd3758b06294ca705dc774cf8bf0eb 100644
> --- a/htl/pt-dealloc.c
> +++ b/htl/pt-dealloc.c
> @@ -33,7 +33,7 @@ extern pthread_mutex_t __pthread_free_threads_lock;
> void
> __pthread_dealloc (struct __pthread *pthread)
> {
> - if (!atomic_decrement_and_test (&pthread->nr_refs))
> + if (atomic_fetch_add_relaxed (&pthread->nr_refs, -1) != 1)
> return;
>
> /* Withdraw this thread from the thread ID lookup table. */
Ok (and I am not sure why __pthread_create_internal usage does not use atomic at all).
> diff --git a/htl/pt-exit.c b/htl/pt-exit.c
> index f0759c8738e6da4c8e0ba47f6d15720203f6954e..3c0a8c52f34ed64b94ce71a0764512cb76351a3a 100644
> --- a/htl/pt-exit.c
> +++ b/htl/pt-exit.c
> @@ -50,7 +50,7 @@ __pthread_exit (void *status)
>
> /* Decrease the number of threads. We use an atomic operation to
> make sure that only the last thread calls `exit'. */
> - if (atomic_decrement_and_test (&__pthread_total))
> + if (atomic_fetch_add_relaxed (&__pthread_total, -1) == 1)
> /* We are the last thread. */
> exit (0);
>
> diff --git a/manual/llio.texi b/manual/llio.texi
I am not sure if MO is suffice here, shouldn't it synchronize with the update
from __pthread_create_internal?
> index 4e6e3fb672bb8ecbb1bb52faf3d70d9b7b33973f..eba9a0e6593f17116b41ff302cfca05846727155 100644
> --- a/manual/llio.texi
> +++ b/manual/llio.texi
> @@ -2614,7 +2614,7 @@ aiocb64}, since the LFS transparently replaces the old interface.
> @c free @ascuheap @acsmem
> @c libc_thread_freeres
> @c libc_thread_subfreeres ok
> -@c atomic_decrement_and_test ok
> +@c atomic_fetch_add_relaxed ok
> @c td_eventword ok
> @c td_eventmask ok
> @c atomic_compare_exchange_bool_acq ok
Ok.
> diff --git a/nptl/cond-perf.c b/nptl/cond-perf.c
> index 9c9488e2743f17eb4beeaadf45ef83a38a9f9bf9..baf69e6d34fdb3ffef97b32fb9db422077a0359b 100644
> --- a/nptl/cond-perf.c
> +++ b/nptl/cond-perf.c
> @@ -24,7 +24,7 @@ cons (void *arg)
>
> do
> {
> - if (atomic_decrement_and_test (&ntogo))
> + if (atomic_fetch_add_relaxed (&ntogo, -1) == 1)
> {
> pthread_mutex_lock (&mut2);
> alldone = true;
Ok, although this code is not used anywhere (neither for testing). Maybe it would be better
to just remove it.
> diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
> index 2602dba54e872f36bd54955af7587378e5dc3812..ada3fdd4d440c3fdcf4567734a452c8ba4d0fce1 100644
> --- a/nptl/pthread_create.c
> +++ b/nptl/pthread_create.c
> @@ -489,7 +489,7 @@ start_thread (void *arg)
> the breakpoint reports TD_THR_RUN state rather than TD_THR_ZOMBIE. */
> atomic_fetch_or_relaxed (&pd->cancelhandling, EXITING_BITMASK);
>
> - if (__glibc_unlikely (atomic_decrement_and_test (&__nptl_nthreads)))
> + if (__glibc_unlikely (atomic_fetch_add_relaxed (&__nptl_nthreads, -1) == 1))
> /* This was the last thread. */
> exit (0);
>
Same question from __pthread_total, is relaxed MO suffice here?
> diff --git a/sysdeps/nptl/libc_start_call_main.h b/sysdeps/nptl/libc_start_call_main.h
> index a9e85f2b098dfc6790d719bba9662e428ab237e6..c10a16b2c4a61e74f6bf18396935b87208df4f7e 100644
> --- a/sysdeps/nptl/libc_start_call_main.h
> +++ b/sysdeps/nptl/libc_start_call_main.h
> @@ -65,7 +65,7 @@ __libc_start_call_main (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
> /* One less thread. Decrement the counter. If it is zero we
> terminate the entire process. */
> result = 0;
> - if (! atomic_decrement_and_test (&__nptl_nthreads))
> + if (atomic_fetch_add_relaxed (&__nptl_nthreads, -1) != 1)
> /* Not much left to do but to exit the thread, not the process. */
> while (1)
> INTERNAL_SYSCALL_CALL (exit, 0);
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] Use C11 atomics instead atomic_decrement_and_test
2022-09-21 17:27 ` Adhemerval Zanella Netto
@ 2022-09-22 13:55 ` Wilco Dijkstra
2022-09-23 13:28 ` Adhemerval Zanella Netto
0 siblings, 1 reply; 4+ messages in thread
From: Wilco Dijkstra @ 2022-09-22 13:55 UTC (permalink / raw)
To: Adhemerval Zanella Netto, 'GNU C Library'
Hi Adhemerval,
> + if (atomic_fetch_add_relaxed (&pthread->nr_refs, -1) != 1)
> return;
>
> /* Withdraw this thread from the thread ID lookup table. */
> Ok (and I am not sure why __pthread_create_internal usage does not use atomic at all).
Yes the non-atomic increment is a bug, an in/decrement could be lost if there is
a data race. Is it still used or could we default to nptl and remove the htl stuff?
> I am not sure if MO is suffice here, shouldn't it synchronize with the update
> from __pthread_create_internal?
As discussed in my previous mail, __pthread_total and __nptl_nthreads are simple
counters that decide when to call exit for the last thread.
> Ok, although this code is not used anywhere (neither for testing). Maybe it would be better
> to just remove it.
Yes, it looks like it was added 20 years ago as some kind of internal benchmark - I'll remove it
in the commit.
Cheers,
Wilco
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] Use C11 atomics instead atomic_decrement_and_test
2022-09-22 13:55 ` Wilco Dijkstra
@ 2022-09-23 13:28 ` Adhemerval Zanella Netto
0 siblings, 0 replies; 4+ messages in thread
From: Adhemerval Zanella Netto @ 2022-09-23 13:28 UTC (permalink / raw)
To: Wilco Dijkstra, 'GNU C Library'
On 22/09/22 10:55, Wilco Dijkstra wrote:
> Hi Adhemerval,
>
>> + if (atomic_fetch_add_relaxed (&pthread->nr_refs, -1) != 1)
>> return;
>>
>> /* Withdraw this thread from the thread ID lookup table. */
>
>> Ok (and I am not sure why __pthread_create_internal usage does not use atomic at all).
>
> Yes the non-atomic increment is a bug, an in/decrement could be lost if there is
> a data race. Is it still used or could we default to nptl and remove the htl stuff?
The htl is used the pthread implementation on Hurd. I am not sure if this support
multiprocessor environments, so I will let Hurd developers figure out if they need
to fix it.
>
>> I am not sure if MO is suffice here, shouldn't it synchronize with the update
>> from __pthread_create_internal?
>
> As discussed in my previous mail, __pthread_total and __nptl_nthreads are simple
> counters that decide when to call exit for the last thread.
Ack.
>
>> Ok, although this code is not used anywhere (neither for testing). Maybe it would be better
>> to just remove it.
>
> Yes, it looks like it was added 20 years ago as some kind of internal benchmark - I'll remove it
> in the commit.
Ok.
>
> Cheers,
> Wilco
Patch looks good me then.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
^ permalink raw reply [flat|nested] 4+ messages in thread
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2022-09-21 17:27 ` Adhemerval Zanella Netto
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2022-09-23 13:28 ` Adhemerval Zanella Netto
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