Re: Futex queue_me/get_user ordering (was: 2.6.10-rc1-mm5 [u])

[Chuck Ebbert <76306.1226@compuserve.com>]

On Sun, 14 Nov 2004 at 09:00:23 +0000 Emergency Services Jamie Lokier wrote:

>+       * The basic logical guarantee of a futex is that it blocks ONLY
>+       * if cond(var) is known to be true at the time of blocking, for
>+       * any cond.  If we queued after testing *uaddr, that would open
>+       * a race condition where we could block indefinitely with
>+       * cond(var) false, which would violate the guarantee.
>+       *
>+       * A consequence is that futex_wait() can return zero and absorb
>+       * a wakeup when *uaddr != val on entry to the syscall.  This is
>+       * rare, but normal.


   Why can't it absorb a wakeup and still return -EAGAIN when this happens?

   IOW why not apply this patch to the original code?

================================================================================
        return -EINTR;
 
  out_unqueue:
-       /* If we were woken (and unqueued), we succeeded, whatever. */
-       if (!unqueue_me(&q))
-               ret = 0;
+       unqueue_me(&q); /* ignore result from unqueue */
  out_release_sem:
        up_read(&current->mm->mmap_sem);
        return ret;
================================================================================

   ...and what is "Emergency Services", BTW?

--Chuck Ebbert  14-Nov-04  21:28:56