On Fri, 23 Jan 2015, Torvald Riegel wrote:
> Second, the current documentation for EINTR is that it can happen due to
> receiving a signal *or* due to a spurious wake-up. This is difficult to
I don't think so. I went through all callchains again with a fine comb.
futex_wait()
retry:
ret = futex_wait_setup();
if (ret) {
/*
* Possible return codes related to uaddr:
* -EINVAL: Not u32 aligned uaddr
* -EFAULT: No mapping, no RW
* -ENOMEM: Paging ran out of memory
* -EHWPOISON: Memory hardware error
*
* Others:
* -EWOULDBLOCK: value at uaddr has changed
*/
return ret;
}
futex_wait_queue_me();
if (woken by futex_wake/requeue)
return 0;
if (timeout)
return -ETIMEOUT;
/*
* Spurious wakeup, i.e. no signal pending
*/
if (!signal_pending())
goto retry;
/* Handled in the low level syscall exit code */
if (!timed_wait)
return -ERESTARTSYS;
else
return -ERESTARTBLOCK;
Now in the low level syscall exit we try to deliver the signal
if (!signal_delivered())
restart_syscall();
if (sigaction->flags & SA_RESTART)
restart_syscall();
ret_to_userspace -EINTR;
So we should never see -EINTR in the case of a spurious wakeup here.
But, here is the not so good news:
I did some archaeology. The restart handling of futex_wait() got
introduced in kernel 2.6.22, so anything older than that will have
the spurious -EINTR issues.
futex_wait_pi() always had the restart handling and glibc folks back
then (2006) requested that it should never return -EINTR, so it
unconditionally restarts the syscall whether a signal had been
delivered or not.
So kernels >= 2.6.22 should never return -EINTR spuriously. If that
happens it's a bug and needs to be fixed.
> Third, I think it would be useful to -- somewhere -- explain which
> behavior the futex operations would have conceptually when expressed by
> C11 code. We currently say that they wake up, sleep, etc, and which
> values they return. But we never say how to properly synchronize with
> them on the userspace side. The C11 memory model is probably the best
> model to use on the userspace side, so that's why I'm arguing for this.
> Basically, I think we need to (1) tell people that they should use
> memory_order_relaxed accesses to the futex variable (ie, the memory
> location associated with the whole futex construct on the kernel side --
> or do we have another name for this?), and (2) give some conceptual
> guarantees for the kernel-side synchronization so that one use this to
> derive how to use them correctly in userspace.
>
> The man pages might not be the right place for this, and maybe we just
> need a revision of "Futexes are tricky". If you have other suggestions
> for where to document this, or on the content, let me know. (I'm also
> willing to spend time on this :) ).
The current futex code in the kernel has gained documentation about
the required memory ordering recently. That should be a good starting
point.
Thanks,
tglx
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