On Thu, Feb 05 2026 at 10:08, Yuwen Chen wrote: > There are timing issues in the use of threads in some unit test cases > for futex.
This is handwaving and contains zero technical information. https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#changelog > A potentially good solution is to check whether the thread Either it's a solution or not. > is in the sleep state after creating it. > This patch adds a file named futex_thread.h, in which several thread # git grep 'This patch' Documentation/process/ > related functions are implemented to facilitate the solution of this > problem. > > Signed-off-by: Yuwen Chen <[email protected]> https://www.kernel.org/doc/html/latest/process/submitting-patches.html#using-reported-by-tested-by-reviewed-by-suggested-by-and-fixes > +#ifndef _FUTEX_THREAD_H > +#define _FUTEX_THREAD_H > +#include <stdio.h> > +#include <string.h> > +#include <errno.h> > + > +#define USEC_PER_SEC 1000000L #include <linux/time64.h> > +#define WAIT_THREAD_RETRIES 100 > +#define WAIT_THREAD_CREATE_TIMEOUT_USECS (USEC_PER_SEC) > + > +struct futex_thread { > + pthread_t thread; > + pthread_barrier_t barrier; > + pid_t tid; > + int (*threadfn)(void *arg); > + void *arg; > + int retval; > +}; > + > +static int __wait_for_thread(FILE *fp) > +{ > + char buf[80] = ""; > + > + for (int i = 0; i < WAIT_THREAD_RETRIES; i++) { > + if (!fgets(buf, sizeof(buf), fp)) > + return -EIO; > + if (!strncmp(buf, "futex", 5)) > + return 0; > + usleep(WAIT_THREAD_CREATE_TIMEOUT_USECS / WAIT_THREAD_RETRIES); What's wrong with #define WAIT_THREAD_DELAY (USEC_PER_SEC / WAIT_THREAD_RETRIES) ... usleep(WAIT_THREAD_DELAY); > + rewind(fp); > + } > + return -ETIMEDOUT; > +} > + > +static void *__futex_thread_fn(void *arg) > +{ > + struct futex_thread *t = arg; > + > + t->tid = gettid(); > + pthread_barrier_wait(&t->barrier); > + t->retval = t->threadfn(t->arg); > + return NULL; > +} > + > +/** > + * futex_wait_for_thread - Wait for the child thread to sleep in the futex > context > + * @t: Thread handle. > + */ > +static int futex_wait_for_thread(struct futex_thread *t) > +{ > + char fname[80]; > + FILE *fp; > + int res; > + > + snprintf(fname, sizeof(fname), "/proc/%d/wchan", t->tid); > + fp = fopen(fname, "r"); > + if (!fp) { > + usleep(USEC_PER_SEC / 10); What is this usleep for? > + return -EIO; > + } > + > + res = __wait_for_thread(fp); > + if (res == -EIO) > + usleep(USEC_PER_SEC / 10); Again. What's this usleep for? If the file cannot be opened or fgets() fails then this fails hard. > + fclose(fp); > + return res; > +} > + > +/** > + * futex_thread_create - Create a new thread for testing. > + * @t: The handle of the newly created thread. > + * @threadfn: The new thread starts execution by invoking threadfn > + * @arg: The parameters passed to threadfn. > + */ > +static int futex_thread_create(struct futex_thread *t, int (*threadfn)(void > *), void *arg) > +{ > + int ret; > + > + pthread_barrier_init(&t->barrier, NULL, 2); > + t->tid = 0; > + t->threadfn = threadfn; > + t->arg = arg; > + > + ret = pthread_create(&t->thread, NULL, __futex_thread_fn, t); > + if (ret) > + return ret; > + > + pthread_barrier_wait(&t->barrier); > + return 0; Why aren't you waiting for the thread right here? There is absolutely no point in doing that at the call site of futex_thread_create(). Thanks, tglx
