Re: [PATCH 2/2 v2] rseq/selftests: test MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU

[Peter Oskolkov]

On Thu, Aug 6, 2020 at 5:27 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
>
> On Thu, Aug 06, 2020 at 10:05:44AM -0700, Peter Oskolkov wrote:
> > Based on Google-internal RSEQ work done by
> > Paul Turner and Andrew Hunter.
> >
> > This patch adds a selftest for MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU.
> > The test quite often fails without the previous patch in this patchset,
> > but consistently passes with it.
> >
> > Signed-off-by: Peter Oskolkov <posk@xxxxxxxxxx>
> > ---
> >  .../selftests/rseq/basic_percpu_ops_test.c    | 181 ++++++++++++++++++
> >  1 file changed, 181 insertions(+)
> >
> > diff --git a/tools/testing/selftests/rseq/basic_percpu_ops_test.c b/tools/testing/selftests/rseq/basic_percpu_ops_test.c
> > index eb3f6db36d36..147c80deac19 100644
> > --- a/tools/testing/selftests/rseq/basic_percpu_ops_test.c
> > +++ b/tools/testing/selftests/rseq/basic_percpu_ops_test.c
> > @@ -3,16 +3,21 @@
> >  #include <assert.h>
> >  #include <pthread.h>
> >  #include <sched.h>
> > +#include <stdatomic.h>
> >  #include <stdint.h>
> >  #include <stdio.h>
> >  #include <stdlib.h>
> >  #include <string.h>
> >  #include <stddef.h>
> > +#include <syscall.h>
> > +#include <unistd.h>
> >
> >  #include "rseq.h"
> >
> >  #define ARRAY_SIZE(arr)      (sizeof(arr) / sizeof((arr)[0]))
> >
> > +#define MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU   (1<<7)
> > +
> >  struct percpu_lock_entry {
> >       intptr_t v;
> >  } __attribute__((aligned(128)));
> > @@ -289,6 +294,180 @@ void test_percpu_list(void)
> >       assert(sum == expected_sum);
> >  }
> >
> > +struct test_membarrier_thread_args {
> > +     int stop;
> > +     intptr_t percpu_list_ptr;
> > +};
> > +
> > +/* Worker threads modify data in their "active" percpu lists. */
> > +void *test_membarrier_worker_thread(void *arg)
> > +{
> > +     struct test_membarrier_thread_args *args =
> > +             (struct test_membarrier_thread_args *)arg;
> > +     const int iters = 10 * 1000 * 1000;
> > +     int i;
> > +
> > +     if (rseq_register_current_thread()) {
> > +             fprintf(stderr, "Error: rseq_register_current_thread(...) failed(%d): %s\n",
> > +                     errno, strerror(errno));
> > +             abort();
> > +     }
> > +
> > +     for (i = 0; i < iters; ++i) {
> > +             while (true) {
> > +                     int cpu, ret;
> > +                     struct percpu_list *list_ptr = (struct percpu_list *)
> > +                             atomic_load(&args->percpu_list_ptr);
> > +
>
> What if the manager thread update ->percpu_list_ptr and call
> membarrier() here? I.e.
>
>         CPU0                    CPU1
>                                 list_ptr = atomic_load(&args->percpu_list_ptr); // read list_b
>
>         atomic_store(&args->percpu_list_ptr, list_a);
>         sys_membarrier(MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU, 1); // send ipi to restart rseq.cs on CPU1
>
>                                 <got IPI, but not in a rseq.cs, so nothing to do>
>                                 cpu = rseq_cpu_start(); // start a rseq.cs and accessing list_b!
>
> The thing is, atomic_load() is an reference to ->percpu_list_ptr, which
> is outside the rseq.cs, simply restarting rseq doesn't kill this
> reference.
>
> Am I missing something subtle?

rseq_cmpeqv_cmpeqv_store is used below to make sure the reference is
the one that should be used; if it is no longer "active", the
iteration is restarted.

>
> Regards,
> Boqun
>
> > +                     if (!list_ptr)
> > +                             continue;  /* Not yet initialized. */
> > +
> > +                     cpu = rseq_cpu_start();
> > +                     struct percpu_list_node *node = list_ptr->c[cpu].head;
> > +                     const intptr_t prev = node->data;
> > +
> > +                     ret = rseq_cmpeqv_cmpeqv_storev(&node->data, prev,
> > +                                     &args->percpu_list_ptr,
> > +                                     (intptr_t)list_ptr, prev + 1, cpu);
> > +                     if (!ret)
> > +                             break;  /* Success. */
> > +             }
> > +     }
> > +
> > +     if (rseq_unregister_current_thread()) {
> > +             fprintf(stderr, "Error: rseq_unregister_current_thread(...) failed(%d): %s\n",
> > +                     errno, strerror(errno));
> > +             abort();
> > +     }
> > +     return NULL;
> > +}
> > +
> > +void test_membarrier_init_percpu_list(struct percpu_list *list)
> > +{
> > +     int i;
> > +
> > +     memset(list, 0, sizeof(*list));
> > +     for (i = 0; i < CPU_SETSIZE; i++) {
> > +             struct percpu_list_node *node;
> > +
> > +             node = malloc(sizeof(*node));
> > +             assert(node);
> > +             node->data = 0;
> > +             node->next = NULL;
> > +             list->c[i].head = node;
> > +     }
> > +}
> > +
> > +void test_membarrier_free_percpu_list(struct percpu_list *list)
> > +{
> > +     int i;
> > +
> > +     for (i = 0; i < CPU_SETSIZE; i++)
> > +             free(list->c[i].head);
> > +}
> > +
> > +static int sys_membarrier(int cmd, int flags)
> > +{
> > +     return syscall(__NR_membarrier, cmd, flags);
> > +}
> > +
> > +/*
> > + * The manager thread swaps per-cpu lists that worker threads see,
> > + * and validates that there are no unexpected modifications.
> > + */
> > +void *test_membarrier_manager_thread(void *arg)
> > +{
> > +     struct test_membarrier_thread_args *args =
> > +             (struct test_membarrier_thread_args *)arg;
> > +     struct percpu_list list_a, list_b;
> > +     intptr_t expect_a = 0, expect_b = 0;
> > +     int cpu_a = 0, cpu_b = 0;
> > +
> > +     if (rseq_register_current_thread()) {
> > +             fprintf(stderr, "Error: rseq_register_current_thread(...) failed(%d): %s\n",
> > +                     errno, strerror(errno));
> > +             abort();
> > +     }
> > +
> > +     /* Init lists. */
> > +     test_membarrier_init_percpu_list(&list_a);
> > +     test_membarrier_init_percpu_list(&list_b);
> > +
> > +     atomic_store(&args->percpu_list_ptr, (intptr_t)&list_a);
> > +
> > +     while (!atomic_load(&args->stop)) {
> > +             /* list_a is "active". */
> > +             cpu_a = rand() % CPU_SETSIZE;
> > +             /*
> > +              * As list_b is "inactive", we should never see changes
> > +              * to list_b.
> > +              */
> > +             if (expect_b != atomic_load(&list_b.c[cpu_b].head->data)) {
> > +                     fprintf(stderr, "Membarrier test failed\n");
> > +                     abort();
> > +             }
> > +
> > +             /* Make list_b "active". */
> > +             atomic_store(&args->percpu_list_ptr, (intptr_t)&list_b);
> > +             sys_membarrier(MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU, cpu_a);
> > +             /*
> > +              * Cpu A should now only modify list_b, so we values
> > +              * in list_a should be stable.
> > +              */
> > +             expect_a = atomic_load(&list_a.c[cpu_a].head->data);
> > +
> > +             cpu_b = rand() % CPU_SETSIZE;
> > +             /*
> > +              * As list_a is "inactive", we should never see changes
> > +              * to list_a.
> > +              */
> > +             if (expect_a != atomic_load(&list_a.c[cpu_a].head->data)) {
> > +                     fprintf(stderr, "Membarrier test failed\n");
> > +                     abort();
> > +             }
> > +
> > +             /* Make list_a "active". */
> > +             atomic_store(&args->percpu_list_ptr, (intptr_t)&list_a);
> > +             sys_membarrier(MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU, cpu_b);
> > +             /* Remember a value from list_b. */
> > +             expect_b = atomic_load(&list_b.c[cpu_b].head->data);
> > +     }
> > +
> > +     test_membarrier_free_percpu_list(&list_a);
> > +     test_membarrier_free_percpu_list(&list_b);
> > +
> > +     if (rseq_unregister_current_thread()) {
> > +             fprintf(stderr, "Error: rseq_unregister_current_thread(...) failed(%d): %s\n",
> > +                     errno, strerror(errno));
> > +             abort();
> > +     }
> > +     return NULL;
> > +}
> > +
> > +/* Test MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU membarrier command. */
> > +void test_membarrier(void)
> > +{
> > +     struct test_membarrier_thread_args thread_args;
> > +     pthread_t worker_threads[CPU_SETSIZE];
> > +     pthread_t manager_thread;
> > +     int i;
> > +
> > +     thread_args.stop = 0;
> > +     thread_args.percpu_list_ptr = 0;
> > +     pthread_create(&manager_thread, NULL,
> > +                    test_membarrier_manager_thread, &thread_args);
> > +
> > +     for (i = 0; i < CPU_SETSIZE; i++)
> > +             pthread_create(&worker_threads[i], NULL,
> > +                    test_membarrier_worker_thread, &thread_args);
> > +
> > +     for (i = 0; i < CPU_SETSIZE; i++)
> > +             pthread_join(worker_threads[i], NULL);
> > +
> > +     atomic_store(&thread_args.stop, 1);
> > +     pthread_join(manager_thread, NULL);
> > +}
> > +
> >  int main(int argc, char **argv)
> >  {
> >       if (rseq_register_current_thread()) {
> > @@ -300,6 +479,8 @@ int main(int argc, char **argv)
> >       test_percpu_spinlock();
> >       printf("percpu_list\n");
> >       test_percpu_list();
> > +     printf("membarrier\n");
> > +     test_membarrier();
> >       if (rseq_unregister_current_thread()) {
> >               fprintf(stderr, "Error: rseq_unregister_current_thread(...) failed(%d): %s\n",
> >                       errno, strerror(errno));
> > --
> > 2.28.0.163.g6104cc2f0b6-goog
> >