Linux futex syscall spurious wakes with return value 0?

Question

I had a problem with the sysall Linux futex( FUTEX_WAIT) operation , sometimes returning early, seemingly for no reason. The documentation indicates certain conditions that can lead to an early (without FUTEX_WAKE) return , but all of them include non-zero return values: EAGAINif the value at the futex address does not match, it ETIMEDOUTexpects the timer to timeout EINTRwhen interrupted by a signal (without restarting), etc. d. But I see a return value of 0. What other than FUTEX_WAKEor aborting a thread whose pointer set_tid_addresspoints to futex can lead to a return FUTEX_WAITwith a return value of 0?

In case this is useful, the specific futex I was expecting is the address of the tid stream (set by syscall clonec CLONE_CHILD_CLEARTID), and the stream has not . My (apparently wrong) assumption is that an operation FUTEX_WAITreturning 0 can only happen when the termination of a thread leads to serious errors in program logic, which have since been fixed by cyclizing and retrying, even if it returns 0 but now I'm curious why this happened.

Here is a minimal test case:

#define _GNU_SOURCE
#include <sched.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <linux/futex.h>
#include <signal.h>

static char stack[32768];
static int tid;

static int foo(void *p)
{
        syscall(SYS_getpid);
        syscall(SYS_getpid);
        syscall(SYS_exit, 0);
}

int main()
{
        int pid = getpid();
        for (;;) {
                int x = clone(foo, stack+sizeof stack,
                        CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND
                        |CLONE_THREAD|CLONE_SYSVSEM //|CLONE_SETTLS
                        |CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID
                        |CLONE_DETACHED,
                        0, &tid, 0, &tid);
                syscall(SYS_futex, &tid, FUTEX_WAIT, x, 0);
                /* Should fail... */
                syscall(SYS_tgkill, pid, tid, SIGKILL);
        }
}

Let it start for a while, it should end with Killed( SIGKILL), which is possible only if the stream still exists when it returns FUTEX_WAIT.

, - , , futex, ( ), , FUTEX_WAIT.

Answer 1

, ? , , foo() clone(), , . - , . , futex , , , ?

, CLONE_VFORK , . , , , .