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Re: [PATCH] misc/tst-clone3: Fix waiting for exited thread.
On 02/11/2019 01:11 PM, Adhemerval Zanella wrote:
On 11/02/2019 08:59, Florian Weimer wrote:
* Stefan Liebler:
I've first tried to include atomic.h, but it failed building on
x86_64. Thus I'm using the c11 atomic load in the updated patch.
Okay to commit?
As information, I've observed those gcc errors on x86_64:
In file included from ../sysdeps/unix/sysv/linux/x86_64/sysdep.h:30,
from ../sysdeps/x86_64/nptl/tls.h:28,
from ../sysdeps/x86/atomic-machine.h:23,
from ../include/atomic.h:50,
from ../sysdeps/unix/sysv/linux/tst-clone3.c:29:
../sysdeps/unix/sysv/linux/dl-sysdep.h: In function
‘_dl_discover_osversion’:
../sysdeps/unix/sysv/linux/dl-sysdep.h:31:42: error: expected
declaration specifiers before ‘attribute_hidden’
extern int _dl_discover_osversion (void) attribute_hidden;
^~~~~~~~~~~~~~~~
That's because the test isn't in tests-internal.
+ while ((__tid = __atomic_load_n (ctid_ptr, __ATOMIC_ACQUIRE)) != 0) \
Actually, that's not a C11 atomic construct, but I think it's okay to
use that here. (The C11 stuff lives in <stdatomic.h> and should be
functionally equivalent.)
Sorry, this is a pet peeve of mine. We have three different atomic
access facilities that people refer to as C11 atomics: Our own
<atomic.h>, the GCC __atomic builtins, and <stdatomic.h>.
I still think this contributes to cognitive load, and we should
eliminate all but one (leaving us with new-style atomics and the old
macros in <atomic.h>). The GCC __atomic builtins have the best freely
available documentation, so they are a natural candidate IMHO.
It really annoying that C11 standard is not freely available from ISO,
although the working draft is still open [1]. I don't have a strong
opinion, but since there is support in the language itself and they
are fully supported by the compiler I also don't see why not prefer it.
[1] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
Then I have a further update of the patch which uses stdatomic.h and
atomic_load_explicit.
commit 4fe43564677deb86f4b404d20bc48a2128f918a9
Author: Stefan Liebler <stli@linux.ibm.com>
Date: Fri Feb 8 12:42:52 2019 +0100
misc/tst-clone3: Fix waiting for exited thread.
From time to time the test misc/tst-clone3 fails with a timeout.
Then futex_wait is blocking. Usually ctid should be set to zero
due to CLONE_CHILD_CLEARTID and the futex should be waken up.
But the fail occures if the thread has already exited before
ctid is set to the return value of clone(). Then futex_wait() will
block as there will be nobody who wakes the futex up again.
This patch initializes ctid to a known value before calling clone
and the kernel is the only one who updates the value to zero after clone.
If futex_wait is called then it is either waked up due to the exited thread
or the futex syscall fails as *ctid_ptr is already zero instead of the
specified value 1.
ChangeLog:
* sysdeps/unix/sysv/linux/tst-clone3.c (do_test):
Initialize ctid with a known value and remove update of ctid
after clone.
(wait_tid): Adjust arguments and call futex_wait with ctid_val
as assumed current value of ctid_ptr.
diff --git a/sysdeps/unix/sysv/linux/tst-clone3.c b/sysdeps/unix/sysv/linux/tst-clone3.c
index aa8e718afe..e78755fcd3 100644
--- a/sysdeps/unix/sysv/linux/tst-clone3.c
+++ b/sysdeps/unix/sysv/linux/tst-clone3.c
@@ -27,6 +27,7 @@
#include <stackinfo.h> /* For _STACK_GROWS_{UP,DOWN}. */
#include <support/check.h>
+#include <stdatomic.h>
/* Test if clone call with CLONE_THREAD does not call exit_group. The 'f'
function returns '1', which will be used by clone thread to call the
@@ -42,11 +43,14 @@ f (void *a)
/* Futex wait for TID argument, similar to pthread_join internal
implementation. */
-#define wait_tid(tid) \
- do { \
- __typeof (tid) __tid; \
- while ((__tid = (tid)) != 0) \
- futex_wait (&(tid), __tid); \
+#define wait_tid(ctid_ptr, ctid_val) \
+ do { \
+ __typeof (*(ctid_ptr)) __tid; \
+ /* We need acquire MO here so that we synchronize with the \
+ kernel's store to 0 when the clone terminates. */ \
+ while ((__tid = atomic_load_explicit (ctid_ptr, \
+ memory_order_acquire)) != 0) \
+ futex_wait (ctid_ptr, ctid_val); \
} while (0)
static inline int
@@ -64,7 +68,11 @@ do_test (void)
clone_flags |= CLONE_VM | CLONE_SIGHAND;
/* We will used ctid to call on futex to wait for thread exit. */
clone_flags |= CLONE_CHILD_CLEARTID;
- pid_t ctid, tid;
+ /* Initialize with a known value. ctid is set to zero by the kernel after the
+ cloned thread has exited. */
+#define CTID_INIT_VAL 1
+ pid_t ctid = CTID_INIT_VAL;
+ pid_t tid;
#ifdef __ia64__
extern int __clone2 (int (*__fn) (void *__arg), void *__child_stack_base,
@@ -86,8 +94,7 @@ do_test (void)
if (tid == -1)
FAIL_EXIT1 ("clone failed: %m");
- ctid = tid;
- wait_tid (ctid);
+ wait_tid (&ctid, CTID_INIT_VAL);
return 2;
}