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