diff mbox series

[v2] i386: Use pthread_barrier for synchronization on tst-bz21269

Message ID 20230221192640.3650934-1-adhemerval.zanella@linaro.org
State Superseded
Headers
Series [v2] i386: Use pthread_barrier for synchronization on tst-bz21269 |

Checks

Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent
dj/TryBot-32bit success Build for i686

Commit Message

Adhemerval Zanella Netto Feb. 21, 2023, 7:26 p.m. UTC 
  To improve the false negative in patchwork buildbot.

Checked on i686-linux-gnu.
---
 sysdeps/unix/sysv/linux/i386/tst-bz21269.c | 62 +++++++---------------
 1 file changed, 19 insertions(+), 43 deletions(-)

Comments

DJ Delorie Feb. 23, 2023, 1:08 a.m. UTC | #1 
Adhemerval Zanella <adhemerval.zanella@linaro.org> writes:

> -   - C11 atomics instead of plain access.
> +   - Use pthread_barrier instead of atomic and futexes.

Ok.

> -#include <stdatomic.h>
> -
>  #include <asm/ldt.h>
> -#include <linux/futex.h>

Ok.

> +#include <support/xsignal.h>
> +
> +#define NITER 5

Ok.

> -static int
> -futex (int *uaddr, int futex_op, int val, void *timeout, int *uaddr2,
> -	int val3)
> -{
> -  return syscall (SYS_futex, uaddr, futex_op, val, timeout, uaddr2, val3);
> -}
> -

Ok.

> -  TEST_VERIFY_EXIT (sigaction (sig, &sa, 0) == 0);
> +  xsigaction (sig, &sa, 0);

Ok.

> -/* Possible values of futex:
> -   0: thread is idle.
> -   1: thread armed.
> -   2: thread should clear LDT entry 0.
> -   3: thread should exit.  */
> -static atomic_uint ftx;
> +static pthread_barrier_t barrier;

Ok.

> -  while (1)
> +  for (int i = 0; i < NITER; i++)

Matches the loop in main, ok.

> -      futex ((int *) &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
> -      while (atomic_load (&ftx) != 2)
> -	{
> -	  if (atomic_load (&ftx) >= 3)
> -	    return NULL;
> -	}
> +      xpthread_barrier_wait (&barrier);

First barrier, ok.  Both threads start here

>        /* clear LDT entry 0.  */
>        const struct user_desc desc = { 0 };
>        xmodify_ldt (1, &desc, sizeof (desc));

While we're doing this, main thread is also modifying its ldt?

> -      /* If ftx == 2, set it to zero,  If ftx == 100, quit.  */
> -      if (atomic_fetch_add (&ftx, -2) != 2)
> -	return NULL;
> +      /* Wait for 'ss' set in main thread.  */
> +      xpthread_barrier_wait (&barrier);

Second barrier.  Ok.  After this, the main thread sets SS...

I think the barriers are still in the wrong locations.

the original code did this (wrt main):

* wait for thread to be idle
* xmodify_ldt()
* set thread to "armed"
* set SS to something "funny"
* tell thread to run (=2)
- thread calls xmodify_ltd and goes idle (=0)
* wait for thread idle

Then, it sets SS *before* the thread clears the LDT.

So I think the barriers need to be placed like this:

  -main-  -thread-

for (;;) {
  xmodify_ldt (set it)  -
  set SS       -
  -- barrier --
  -         xmodify_ldt (clear it)
  -- barrier --
  fail        -
}
Adhemerval Zanella Netto Feb. 24, 2023, 3:45 p.m. UTC | #2 
On 22/02/23 22:08, DJ Delorie wrote:
> Adhemerval Zanella <adhemerval.zanella@linaro.org> writes:
> 
>> -   - C11 atomics instead of plain access.
>> +   - Use pthread_barrier instead of atomic and futexes.
> 
> Ok.
> 
>> -#include <stdatomic.h>
>> -
>>  #include <asm/ldt.h>
>> -#include <linux/futex.h>
> 
> Ok.
> 
>> +#include <support/xsignal.h>
>> +
>> +#define NITER 5
> 
> Ok.
> 
>> -static int
>> -futex (int *uaddr, int futex_op, int val, void *timeout, int *uaddr2,
>> -	int val3)
>> -{
>> -  return syscall (SYS_futex, uaddr, futex_op, val, timeout, uaddr2, val3);
>> -}
>> -
> 
> Ok.
> 
>> -  TEST_VERIFY_EXIT (sigaction (sig, &sa, 0) == 0);
>> +  xsigaction (sig, &sa, 0);
> 
> Ok.
> 
>> -/* Possible values of futex:
>> -   0: thread is idle.
>> -   1: thread armed.
>> -   2: thread should clear LDT entry 0.
>> -   3: thread should exit.  */
>> -static atomic_uint ftx;
>> +static pthread_barrier_t barrier;
> 
> Ok.
> 
>> -  while (1)
>> +  for (int i = 0; i < NITER; i++)
> 
> Matches the loop in main, ok.
> 
>> -      futex ((int *) &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
>> -      while (atomic_load (&ftx) != 2)
>> -	{
>> -	  if (atomic_load (&ftx) >= 3)
>> -	    return NULL;
>> -	}
>> +      xpthread_barrier_wait (&barrier);
> 
> First barrier, ok.  Both threads start here
> 
>>        /* clear LDT entry 0.  */
>>        const struct user_desc desc = { 0 };
>>        xmodify_ldt (1, &desc, sizeof (desc));
> 
> While we're doing this, main thread is also modifying its ldt?
> 
>> -      /* If ftx == 2, set it to zero,  If ftx == 100, quit.  */
>> -      if (atomic_fetch_add (&ftx, -2) != 2)
>> -	return NULL;
>> +      /* Wait for 'ss' set in main thread.  */
>> +      xpthread_barrier_wait (&barrier);
> 
> Second barrier.  Ok.  After this, the main thread sets SS...
> 
> I think the barriers are still in the wrong locations.
> 
> the original code did this (wrt main):
> 
> * wait for thread to be idle
> * xmodify_ldt()
> * set thread to "armed"
> * set SS to something "funny"
> * tell thread to run (=2)
> - thread calls xmodify_ltd and goes idle (=0)
> * wait for thread idle
> 
> Then, it sets SS *before* the thread clears the LDT.
> 
> So I think the barriers need to be placed like this:
> 
>   -main-  -thread-
> 
> for (;;) {
>   xmodify_ldt (set it)  -
>   set SS       -
>   -- barrier --
>   -         xmodify_ldt (clear it)
>   -- barrier --
>   fail        -
> }
> 

Ok I thint it works as well.  I will update the patch.
diff mbox series

Patch

diff --git a/sysdeps/unix/sysv/linux/i386/tst-bz21269.c b/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
index 1850240c34..bc2a380a11 100644
--- a/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
+++ b/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
@@ -20,16 +20,13 @@ 
    more specifically in do_multicpu_tests function.  The main changes
    are:
 
-   - C11 atomics instead of plain access.
+   - Use pthread_barrier instead of atomic and futexes.
    - Remove x86_64 support which simplifies the syscall handling
      and fallbacks.
    - Replicate only the test required to trigger the issue for the
      BZ#21269.  */
 
-#include <stdatomic.h>
-
 #include <asm/ldt.h>
-#include <linux/futex.h>
 
 #include <setjmp.h>
 #include <signal.h>
@@ -40,6 +37,9 @@ 
 #include <support/xunistd.h>
 #include <support/check.h>
 #include <support/xthread.h>
+#include <support/xsignal.h>
+
+#define NITER 5
 
 static int
 xset_thread_area (struct user_desc *u_info)
@@ -55,13 +55,6 @@  xmodify_ldt (int func, const void *ptr, unsigned long bytecount)
   TEST_VERIFY_EXIT (syscall (SYS_modify_ldt, 1, ptr, bytecount) == 0);
 }
 
-static int
-futex (int *uaddr, int futex_op, int val, void *timeout, int *uaddr2,
-	int val3)
-{
-  return syscall (SYS_futex, uaddr, futex_op, val, timeout, uaddr2, val3);
-}
-
 static void
 xsethandler (int sig, void (*handler)(int, siginfo_t *, void *), int flags)
 {
@@ -69,7 +62,7 @@  xsethandler (int sig, void (*handler)(int, siginfo_t *, void *), int flags)
   sa.sa_sigaction = handler;
   sa.sa_flags = SA_SIGINFO | flags;
   TEST_VERIFY_EXIT (sigemptyset (&sa.sa_mask) == 0);
-  TEST_VERIFY_EXIT (sigaction (sig, &sa, 0) == 0);
+  xsigaction (sig, &sa, 0);
 }
 
 static jmp_buf jmpbuf;
@@ -123,33 +116,24 @@  setup_low_user_desc (void)
   low_user_desc_clear->seg_not_present = 1;
 }
 
-/* Possible values of futex:
-   0: thread is idle.
-   1: thread armed.
-   2: thread should clear LDT entry 0.
-   3: thread should exit.  */
-static atomic_uint ftx;
+static pthread_barrier_t barrier;
 
 static void *
 threadproc (void *ctx)
 {
-  while (1)
+  for (int i = 0; i < NITER; i++)
     {
-      futex ((int *) &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
-      while (atomic_load (&ftx) != 2)
-	{
-	  if (atomic_load (&ftx) >= 3)
-	    return NULL;
-	}
+      xpthread_barrier_wait (&barrier);
 
       /* clear LDT entry 0.  */
       const struct user_desc desc = { 0 };
       xmodify_ldt (1, &desc, sizeof (desc));
 
-      /* If ftx == 2, set it to zero,  If ftx == 100, quit.  */
-      if (atomic_fetch_add (&ftx, -2) != 2)
-	return NULL;
+      /* Wait for 'ss' set in main thread.  */
+      xpthread_barrier_wait (&barrier);
     }
+
+  return NULL;
 }
 
 
@@ -180,18 +164,19 @@  do_test (void)
   /* Some kernels send SIGBUS instead.  */
   xsethandler (SIGBUS, sigsegv_handler, 0);
 
+  xpthread_barrier_init (&barrier, NULL, 2);
+
   thread = xpthread_create (0, threadproc, 0);
 
   asm volatile ("mov %%ss, %0" : "=rm" (orig_ss));
 
-  for (int i = 0; i < 5; i++)
+  for (int i = 0; i < NITER; i++)
     {
       if (sigsetjmp (jmpbuf, 1) != 0)
 	continue;
 
       /* Make sure the thread is ready after the last test. */
-      while (atomic_load (&ftx) != 0)
-	;
+      xpthread_barrier_wait (&barrier);
 
       struct user_desc desc = {
 	.entry_number       = 0,
@@ -207,28 +192,19 @@  do_test (void)
 
       xmodify_ldt (0x11, &desc, sizeof (desc));
 
-      /* Arm the thread.  */
-      ftx = 1;
-      futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
+      xpthread_barrier_wait (&barrier);
 
       asm volatile ("mov %0, %%ss" : : "r" (0x7));
 
-      /* Fire up thread modify_ldt call.  */
-      atomic_store (&ftx, 2);
-
-      while (atomic_load (&ftx) != 0)
-	;
-
       /* On success, modify_ldt will segfault us synchronously and we will
 	 escape via siglongjmp.  */
       support_record_failure ();
     }
 
-  atomic_store (&ftx, 100);
-  futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
-
   xpthread_join (thread);
 
+  xpthread_barrier_destroy (&barrier);
+
   return 0;
 }