Re: futex(2) man page update help request

[Darren Hart]

On 1/24/15, 3:35 AM, "Thomas Gleixner" <tglx@xxxxxxxxxxxxx> wrote:

>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.

Lots of paging in here... If I recall correctly there was something about
not being able to return to userspace in these events without owning the
lock (waiters but no owner, breaking pi chains and promotion, etc.), so
restarting was the preferable path.

-- 
Darren Hart
Intel Open Source Technology Center



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/