Patchwork misc/tst-clone3: Fix waiting for exited thread.

login
register
mail settings
Submitter Stefan Liebler
Date Feb. 11, 2019, 10:23 a.m.
Message ID <ccccdc4c-f67c-2ab4-46ae-63e201db1753@linux.ibm.com>
Download mbox | patch
Permalink /patch/31395/
State New
Headers show

Comments

Stefan Liebler - Feb. 11, 2019, 10:23 a.m.
Attached the updated patch.
See also notes below.

On 02/08/2019 07:37 PM, Adhemerval Zanella wrote:
> 
> 
> On 08/02/2019 13:12, Stefan Liebler wrote:
>> Hi,
>>
>> from time to time the test misc/tst-clone3 fails with an 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.
>>
>> Okay to commit?
>>
>> Bye
>> Stefan
>>
>> 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.
>>
>> 20190208_misc_tst-clone3.patch
>>
>> commit 5a8f80973dbbf06a0eebc2064d2a14521c6a6131
>> 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 an 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.
> 
> Thanks for catching it.
> 
>>
>> diff --git a/sysdeps/unix/sysv/linux/tst-clone3.c b/sysdeps/unix/sysv/linux/tst-clone3.c
>> index aa8e718afe..ffa2056eb6 100644
>> --- a/sysdeps/unix/sysv/linux/tst-clone3.c
>> +++ b/sysdeps/unix/sysv/linux/tst-clone3.c
>> @@ -42,11 +42,11 @@ f (void *a)
>>   
>>   /* Futex wait for TID argument, similar to pthread_join internal
>>      implementation.  */
>> -#define wait_tid(tid) \
>> +#define wait_tid(ctid_ptr, ctid_val)	\
>>     do {					\
>> -    __typeof (tid) __tid;		\
>> -    while ((__tid = (tid)) != 0)	\
>> -      futex_wait (&(tid), __tid);	\
>> +    __typeof (*(ctid_ptr)) __tid;	\
>> +    while ((__tid = *(ctid_ptr)) != 0)	\
>> +      futex_wait (ctid_ptr, ctid_val);	\
>>     } while (0)
> 
> lll_wait_tid uses an atomic load with acquire semantic, I think we should
> use it as well. We can either include the atomic header or use c11 atomic.
> 
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;
                                           ^~~~~~~~~~~~~~~~
In file included from ../sysdeps/x86_64/nptl/tls.h:31,
                  from ../sysdeps/x86/atomic-machine.h:23,
                  from ../include/atomic.h:50,
                  from ../sysdeps/unix/sysv/linux/tst-clone3.c:29:
../sysdeps/generic/dl-dtv.h:22:1: error: empty declaration [-Werror]
  struct dtv_pointer
  ^~~~~~
../sysdeps/generic/dl-dtv.h:33:3: error: storage class specified for 
parameter ‘dtv_t’
  } dtv_t;
    ^~~~~
...
many many more errors
...

I've also tried to add tst-clone3 to tests_internal instead of tests in 
the Makefile, but then I've got:
In file included from ../sysdeps/x86_64/nptl/tls.h:130,
                  from ../sysdeps/x86/atomic-machine.h:23,
                  from ../include/atomic.h:50,
                  from ../sysdeps/unix/sysv/linux/tst-clone3.c:29:
../nptl/descr.h:104:8: error: redefinition of ‘struct robust_list_head’
  struct robust_list_head
         ^~~~~~~~~~~~~~~~
In file included from ../sysdeps/unix/sysv/linux/tst-clone3.c:26:
/usr/include/linux/futex.h:70:8: note: originally defined here
  struct robust_list_head {
         ^~~~~~~~~~~~~~~~

>>   
>>   static inline int
>> @@ -64,7 +64,11 @@ 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 +90,7 @@ do_test (void)
>>     if (tid == -1)
>>       FAIL_EXIT1 ("clone failed: %m");
>>   
>> -  ctid = tid;
>> -  wait_tid (ctid);
>> +  wait_tid (&ctid, CTID_INIT_VAL);
>>   
>>     return 2;
>>   }
>>
>
Florian Weimer - Feb. 11, 2019, 10:59 a.m.
* 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.

Thanks,
Florian
Adhemerval Zanella Netto - Feb. 11, 2019, 12:11 p.m.
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

Patch

commit 3672d05bd3c5c51831ef8b91ee2b0ff491fd2986
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..b345d04b4d 100644
--- a/sysdeps/unix/sysv/linux/tst-clone3.c
+++ b/sysdeps/unix/sysv/linux/tst-clone3.c
@@ -42,11 +42,13 @@  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_n (ctid_ptr, __ATOMIC_ACQUIRE)) != 0)	\
+      futex_wait (ctid_ptr, ctid_val);					\
   } while (0)
 
 static inline int
@@ -64,7 +66,11 @@  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 +92,7 @@  do_test (void)
   if (tid == -1)
     FAIL_EXIT1 ("clone failed: %m");
 
-  ctid = tid;
-  wait_tid (ctid);
+  wait_tid (&ctid, CTID_INIT_VAL);
 
   return 2;
 }