i386: Use pthread_barrier for synchronization on tst-bz21269
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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
To improve the false negative in patchwork buildbot.
Checked on i686-linux-gnu.
---
sysdeps/unix/sysv/linux/i386/tst-bz21269.c | 58 +++++++---------------
1 file changed, 17 insertions(+), 41 deletions(-)
Comments
The resulting test has two calls to pthread_barrier_wait() in the
subthread but three calls in main(). These will quickly get out of
sync.
Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org> writes:
> - - C11 atomics instead of plain access.
> + - Use pthread_barrier instead of atomic and futexes.
Is this true relative to the original testcase? Still, merely a
comment, so OK.
> -#include <stdatomic.h>
> -
> #include <asm/ldt.h>
> -#include <linux/futex.h>
Ok.
> +#include <support/xsignal.h>
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);
> -}
We remove all calls to futex, so no longer need this. 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;
Ok.
> +static pthread_barrier_t barrier;
New, ok.
> static void *
> threadproc (void *ctx)
> {
> - while (1)
> + for (int i = 0; i < 5; i++)
This matches the loop in main. Ok. In the future, a #define loop limit
would be appropriate, to prevent these getting out of sync. Or a
comment that it has to match main().
> {
> - 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.
> /* clear LDT entry 0. */
> const struct user_desc desc = { 0 };
> xmodify_ldt (1, &desc, sizeof (desc));
Leave the stuff the thread is actually testing ;-)
> - /* 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.
> }
> +
> + return NULL;
Ok, moved from above.
> }
>
>
> @@ -180,6 +162,8 @@ do_test (void)
> /* Some kernels send SIGBUS instead. */
> xsethandler (SIGBUS, sigsegv_handler, 0);
>
> + xpthread_barrier_init (&barrier, NULL, 2);
Initialize; must have two callers. Ok - main and thread both call.
> thread = xpthread_create (0, threadproc, 0);
>
> asm volatile ("mov %%ss, %0" : "=rm" (orig_ss));
> @@ -190,8 +174,7 @@ do_test (void)
> continue;
>
> /* Make sure the thread is ready after the last test. */
> - while (atomic_load (&ftx) != 0)
> - ;
> + xpthread_barrier_wait (&barrier);
First barrier, ok.
> struct user_desc desc = {
> .entry_number = 0,
> @@ -207,28 +190,21 @@ 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);
Second barrier, ok.
> asm volatile ("mov %0, %%ss" : : "r" (0x7));
>
> - /* Fire up thread modify_ldt call. */
> - atomic_store (&ftx, 2);
> -
> - while (atomic_load (&ftx) != 0)
> - ;
> + xpthread_barrier_wait (&barrier);
Third barrier? This puts main and the thread out of sync.
> /* 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);
> -
Ok.
> xpthread_join (thread);
>
> + xpthread_barrier_destroy (&barrier);
> +
Ok.
On 20/02/23 19:55, DJ Delorie wrote:
>
> The resulting test has two calls to pthread_barrier_wait() in the
> subthread but three calls in main(). These will quickly get out of
> sync.
>
>
> Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org> writes:
>> - - C11 atomics instead of plain access.
>> + - Use pthread_barrier instead of atomic and futexes.
>
> Is this true relative to the original testcase? Still, merely a
> comment, so OK.
The original refers to the kernel tests in here.
>
>> -#include <stdatomic.h>
>> -
>> #include <asm/ldt.h>
>> -#include <linux/futex.h>
>
> Ok.
>
>> +#include <support/xsignal.h>
>
> 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);
>> -}
>
> We remove all calls to futex, so no longer need this. 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;
>
> Ok.
>
>> +static pthread_barrier_t barrier;
>
> New, ok.
>
>> static void *
>> threadproc (void *ctx)
>> {
>> - while (1)
>> + for (int i = 0; i < 5; i++)
>
> This matches the loop in main. Ok. In the future, a #define loop limit
> would be appropriate, to prevent these getting out of sync. Or a
> comment that it has to match main().
Ack.
>
>> {
>> - 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.
>
>> /* clear LDT entry 0. */
>> const struct user_desc desc = { 0 };
>> xmodify_ldt (1, &desc, sizeof (desc));
>
> Leave the stuff the thread is actually testing ;-)
>
>> - /* 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.
>
>> }
>> +
>> + return NULL;
>
> Ok, moved from above.
>
>> }
>>
>>
>> @@ -180,6 +162,8 @@ do_test (void)
>> /* Some kernels send SIGBUS instead. */
>> xsethandler (SIGBUS, sigsegv_handler, 0);
>>
>> + xpthread_barrier_init (&barrier, NULL, 2);
>
> Initialize; must have two callers. Ok - main and thread both call.
>
>> thread = xpthread_create (0, threadproc, 0);
>>
>> asm volatile ("mov %%ss, %0" : "=rm" (orig_ss));
>> @@ -190,8 +174,7 @@ do_test (void)
>> continue;
>>
>> /* Make sure the thread is ready after the last test. */
>> - while (atomic_load (&ftx) != 0)
>> - ;
>> + xpthread_barrier_wait (&barrier);
>
> First barrier, ok.
>
>> struct user_desc desc = {
>> .entry_number = 0,
>> @@ -207,28 +190,21 @@ 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);
>
> Second barrier, ok.
>
>> asm volatile ("mov %0, %%ss" : : "r" (0x7));
>>
>> - /* Fire up thread modify_ldt call. */
>> - atomic_store (&ftx, 2);
>> -
>> - while (atomic_load (&ftx) != 0)
>> - ;
>> + xpthread_barrier_wait (&barrier);
>
> Third barrier? This puts main and the thread out of sync.
Indeed, this is wrong. I will remove it.
>
>> /* 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);
>> -
>
> Ok.
>
>> xpthread_join (thread);
>>
>> + xpthread_barrier_destroy (&barrier);
>> +
>
> Ok.
>
@@ -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 +37,7 @@
#include <support/xunistd.h>
#include <support/check.h>
#include <support/xthread.h>
+#include <support/xsignal.h>
static int
xset_thread_area (struct user_desc *u_info)
@@ -55,13 +53,6 @@ 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 +60,7 @@ 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 +114,24 @@ 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 < 5; 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;
}
@@ -180,6 +162,8 @@ 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));
@@ -190,8 +174,7 @@ do_test (void)
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 +190,21 @@ 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)
- ;
+ xpthread_barrier_wait (&barrier);
/* 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;
}