Threads

A thread is a struct proc that shares three counted objects with others of
its group: the address space (struct mm: page tables, heap top, VMAs and a
lock for changing them), the descriptor table, and the signal dispositions.
clone() shares what its flags say and fork() copies.

- clone(): pthread_create's shape (VM, FILES, SIGHAND, THREAD, SETTLS,
  PARENT_SETTID, CHILD_CLEARTID) and posix_spawn's (VM, VFORK, a stack).
- futex: WAIT, WAIT_BITSET with timeouts, WAKE, REQUEUE, CMP_REQUEUE, keyed
  by the word's kernel address so shared pages meet. set_tid_address keeps
  the word that exit zeroes and wakes, for pthread_join.
- exit ends a thread; exit_group, a fatal signal, or the leader exiting ends
  them all. reapd frees exited threads; the leader is reaped by its parent
  once the rest are gone. getpid is the group, gettid the thread.
- Stops and resumes reach every thread; other process signals the leader.
- exec kills the other threads and unshares the dispositions.
- No TLB shootdown, so pages unmapped from a shared address space are freed
  only once every CPU has reloaded CR3 or is idle.
- procfs: /proc lists processes, task/ lists threads, comm, Threads:, Tgid.
- The console honours VMIN 0 / VTIME (btop's input drain hung on it).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/sys/sys_compat.c

@@ -32,10 +32,12 @@ sys_enable_linux_abi(void)
 addr_t
 sys_set_tid_address(void)
 {
+    // The word zeroed and woken when this thread exits -- what pthread_join
+    // waits on. It used to be *written*, with the tid, which is CHILD_SETTID's
+    // job and not this one's.
     addr_t tidp;
     argaddr(0, &tidp);
-    if (tidp)
-        put_user(myproc()->pid, tidp);
+    myproc()->clear_child_tid = tidp;
     return myproc()->pid;
 }
 
@@ -46,12 +48,15 @@ addr_t sys_set_robust_list(void) { return 0; }
 addr_t
 sys_futex(void)
 {
-    int op;
+    addr_t uaddr, utime, uaddr2;
+    int op, val, val3;
+    argaddr(0, &uaddr);
     argint(1, &op);
-    op &= ~(FUTEX_WAIT_PRIVATE - FUTEX_WAIT);
-    if (op == FUTEX_WAKE || op == FUTEX_WAKE_PRIVATE)
-        return 0;
-    return -EAGAIN; // no blocking, musl retries
+    argint(2, &val);
+    argaddr(3, &utime);
+    argaddr(4, &uaddr2);
+    argint(5, &val3);
+    return futex_op(uaddr, op, val, utime, uaddr2, val3);
 }
 
 // arch_prctl: save FS_BASE for TLS.