Re: Futex queue_me/get_user ordering

From: Hidetoshi Seto
Date: Sun Nov 14 2004 - 19:57:55 EST

Jamie Lokier wrote:
Andrew Morton wrote:

The patch wasn't supposed to optimise anything. It fixed a bug which was
causing hangs. See

Or are you saying that userspace is buggy??

I haven't looked at the NPTL code, but that URL's pseudo-code is buggy.
The call to FUTEX_WAKE should be doing wake++ conditionally on the
return value, not unconditionally.

So I don't know if NPTL is buggy, but the pseudo-code given in the bug
report is (because of unconditional wake++), and so is the failure
example (because it doesn't use a mutex).

-- Jamie

from glibc-2.3.3(RHEL4b2):

31 int
32 __pthread_cond_signal (cond)
33 pthread_cond_t *cond;
34 {
35 /* Make sure we are alone. */
36 lll_mutex_lock (cond->__data.__lock);
38 /* Are there any waiters to be woken? */
39 if (cond->__data.__total_seq > cond->__data.__wakeup_seq)
40 {
41 /* Yes. Mark one of them as woken. */
42 ++cond->__data.__wakeup_seq;
43 ++cond->__data.__futex;
45 /* Wake one. */
46 lll_futex_wake (&cond->__data.__futex, 1);
47 }
49 /* We are done. */
50 lll_mutex_unlock (cond->__data.__lock);
52 return 0;
53 }

Ingo, is this buggy?

We should start again with a question:
Is this a kernel's bug or NPTL's bug?


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at