Add a "fork" of musl as //fusl.

fusl/src/thread/synccall.c

+#include "pthread_impl.h"
+#include <semaphore.h>
+#include <unistd.h>
+#include <dirent.h>
+#include <string.h>
+#include <ctype.h>
+#include "futex.h"
+#include "atomic.h"
+#include "../dirent/__dirent.h"
+
+static struct chain {
+        struct chain *next;
+        int tid;
+        sem_t target_sem, caller_sem;
+} *volatile head;
+
+static volatile int synccall_lock[2];
+static volatile int target_tid;
+static void (*callback)(void *), *context;
+static volatile int dummy = 0;
+weak_alias(dummy, __block_new_threads);
+
+static void handler(int sig)
+{
+        struct chain ch;
+        int old_errno = errno;
+
+        sem_init(&ch.target_sem, 0, 0);
+        sem_init(&ch.caller_sem, 0, 0);
+
+        ch.tid = __syscall(SYS_gettid);
+
+        do ch.next = head;
+        while (a_cas_p(&head, ch.next, &ch) != ch.next);
+
+        if (a_cas(&target_tid, ch.tid, 0) == (ch.tid | 0x80000000))
+                __syscall(SYS_futex, &target_tid, FUTEX_UNLOCK_PI|FUTEX_PRIVATE);
+
+        sem_wait(&ch.target_sem);
+        callback(context);
+        sem_post(&ch.caller_sem);
+        sem_wait(&ch.target_sem);
+
+        errno = old_errno;
+}
+
+void __synccall(void (*func)(void *), void *ctx)
+{
+        sigset_t oldmask;
+        int cs, i, r, pid, self;;
+        DIR dir = {0};
+        struct dirent *de;
+        struct sigaction sa = { .sa_flags = 0, .sa_handler = handler };
+        struct chain *cp, *next;
+        struct timespec ts;
+
+        /* Blocking signals in two steps, first only app-level signals
+         * before taking the lock, then all signals after taking the lock,
+         * is necessary to achieve AS-safety. Blocking them all first would
+         * deadlock if multiple threads called __synccall. Waiting to block
+         * any until after the lock would allow re-entry in the same thread
+         * with the lock already held. */
+        __block_app_sigs(&oldmask);
+        LOCK(synccall_lock);
+        __block_all_sigs(0);
+        pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
+
+        head = 0;
+
+        if (!libc.threaded) goto single_threaded;
+
+        callback = func;
+        context = ctx;
+
+        /* This atomic store ensures that any signaled threads will see the
+         * above stores, and prevents more than a bounded number of threads,
+         * those already in pthread_create, from creating new threads until
+         * the value is cleared to zero again. */
+        a_store(&__block_new_threads, 1);
+
+        /* Block even implementation-internal signals, so that nothing
+         * interrupts the SIGSYNCCALL handlers. The main possible source
+         * of trouble is asynchronous cancellation. */
+        memset(&sa.sa_mask, -1, sizeof sa.sa_mask);
+        __libc_sigaction(SIGSYNCCALL, &sa, 0);
+
+        pid = __syscall(SYS_getpid);
+        self = __syscall(SYS_gettid);
+
+        /* Since opendir is not AS-safe, the DIR needs to be setup manually
+         * in automatic storage. Thankfully this is easy. */
+        dir.fd = open("/proc/self/task", O_RDONLY|O_DIRECTORY|O_CLOEXEC);
+        if (dir.fd < 0) goto out;
+
+        /* Initially send one signal per counted thread. But since we can't
+         * synchronize with thread creation/exit here, there could be too
+         * few signals. This initial signaling is just an optimization, not
+         * part of the logic. */
+        for (i=libc.threads_minus_1; i; i--)
+                __syscall(SYS_kill, pid, SIGSYNCCALL);
+
+        /* Loop scanning the kernel-provided thread list until it shows no
+         * threads that have not already replied to the signal. */
+        for (;;) {
+                int miss_cnt = 0;
+                while ((de = readdir(&dir))) {
+                        if (!isdigit(de->d_name[0])) continue;
+                        int tid = atoi(de->d_name);
+                        if (tid == self || !tid) continue;
+
+                        /* Set the target thread as the PI futex owner before
+                         * checking if it's in the list of caught threads. If it
+                         * adds itself to the list after we check for it, then
+                         * it will see its own tid in the PI futex and perform
+                         * the unlock operation. */
+                        a_store(&target_tid, tid);
+
+                        /* Thread-already-caught is a success condition. */
+                        for (cp = head; cp && cp->tid != tid; cp=cp->next);
+                        if (cp) continue;
+
+                        r = -__syscall(SYS_tgkill, pid, tid, SIGSYNCCALL);
+
+                        /* Target thread exit is a success condition. */
+                        if (r == ESRCH) continue;
+
+                        /* The FUTEX_LOCK_PI operation is used to loan priority
+                         * to the target thread, which otherwise may be unable
+                         * to run. Timeout is necessary because there is a race
+                         * condition where the tid may be reused by a different
+                         * process. */
+                        clock_gettime(CLOCK_REALTIME, &ts);
+                        ts.tv_nsec += 10000000;
+                        if (ts.tv_nsec >= 1000000000) {
+                                ts.tv_sec++;
+                                ts.tv_nsec -= 1000000000;
+                        }
+                        r = -__syscall(SYS_futex, &target_tid,
+                                FUTEX_LOCK_PI|FUTEX_PRIVATE, 0, &ts);
+
+                        /* Obtaining the lock means the thread responded. ESRCH
+                         * means the target thread exited, which is okay too. */
+                        if (!r || r == ESRCH) continue;
+
+                        miss_cnt++;
+                }
+                if (!miss_cnt) break;
+                rewinddir(&dir);
+        }
+        close(dir.fd);
+
+        /* Serialize execution of callback in caught threads. */
+        for (cp=head; cp; cp=cp->next) {
+                sem_post(&cp->target_sem);
+                sem_wait(&cp->caller_sem);
+        }
+
+        sa.sa_handler = SIG_IGN;
+        __libc_sigaction(SIGSYNCCALL, &sa, 0);
+
+single_threaded:
+        func(ctx);
+
+        /* Only release the caught threads once all threads, including the
+         * caller, have returned from the callback function. */
+        for (cp=head; cp; cp=next) {
+                next = cp->next;
+                sem_post(&cp->target_sem);
+        }
+
+out:
+        a_store(&__block_new_threads, 0);
+        __wake(&__block_new_threads, -1, 1);
+
+        pthread_setcancelstate(cs, 0);
+        UNLOCK(synccall_lock);
+        __restore_sigs(&oldmask);
+}