Re: futex(2) man page update help request

[Darren Hart]

On 1/17/15, 1:16 AM, "Michael Kerrisk (man-pages)"
<mtk.manpages@xxxxxxxxx> wrote:

>Hello Darren,
>
>On 01/17/2015 02:33 AM, Darren Hart wrote:
>> Corrected Davidlohr's email address.
>
>Thanks!
>
>> On 1/15/15, 7:12 AM, "Michael Kerrisk (man-pages)"
>> <mtk.manpages@xxxxxxxxx> wrote:
>> 
>>> Hello Darren,
>>>
>>> I give you the same apology as to Thomas for the
>>> long-delayed response to your mail.
>>>
>>> And I repeat my note to Thomas:
>>> In the next day or two, I hope to send out the new version
>>> of the futex(2) page for review. The new draft is a bit
>>> bigger (okay -- 4 x bigger) than the current page. And there
>>> are a quite number of FIXMEs that I've placed in the page
>>> for various points--some minor, but a few major--that need
>>> to be checked or fixed. Would you have some time to review
>>> that page?
>> 
>> I'll make the time for that. I've wanted to see this for a while, so
>>thank
>> you for working on it!
>
>Great!
>
>>> In the meantime, I have a couple of questions, which, if
>>> you could answer them, I would work some changes into the
>>> page before sending.
>>>
>>> 1. In various places, distinction is made between non-PI
>>>   futexs and PI futexes. But what determines that distinction?
>>>   From the kernel's perspective, hat make a futex one type
>>>   or another? I presume it is to do with the types of blocking
>>>   waiters on the futex, but it would be good to have a formal
>>>   definition.
>> 
>> You're right in that a uaddr is a uaddr is a uaddr. Also "there is no
>>such
>> thing as a futex", it doesn't exist as any kind of identifiable object,
>>so
>> these discussions can get rather confusing :-)
>
>So, I want to make sure that I am clear on what you mean you say this.
>You say "there is no such thing as a futex" because from the kernel's
>perspective there is no visible entity in the uncontended case
>(where everything can be dealt with in user space). And from user-space,
>in the uncontended case all we're doing is memory operations. Right?
>
>On the other hand, from a kernel perspective, we could say that a
>futex "exists" in the contended phases, since the kernel has allocated
>state associated with the uaddr. Right?


Sorry, this was more anecdotal, and probably more of a distraction than
constructive. I just meant that unlike other things which you can point to
a specific struct for (task, rt_mutex, etc.), a "futex" has it's state
distributed across the backing store (uaddr), the queue (futex_q), the
pi_state, the rt_mutex, etc, and these span kernel space and userspace.
Your description above is correct.

>
>> A "futex" becomes a PI futex when it is "created" via a PI futex op
>>code.
>
>Precisely which PI op codes? Is it: FUTEX_LOCK_PI, FUTEX_TRYLOCK_PI, and
>FUTEX_CMP_REQUEUE_PI, and not FUTEX_WAIT_REQUEUE_PI or FUTEX_UNLOCK_PI?

Based on your wording below about taking a user POV on this, I'm going to
say "yes" here. These opcodes paired with the PI futex value policy
(described below) defines a "futex" as PI aware. These were created very
specifically in support of PI pthread_mutexes, so it makes a lot more
sense to talk about a PI aware pthread_mutex, than a PI aware futex, since
there is a lot of policy and scaffolding that has to be built up around it
to use it properly (this is what a PI pthread_mutex is).

>> At that point, the syscall will ensure a pi_state is populated for the
>> futex_q entry. See futex_lock_pi() for example. Before the locks are
>> taken, there is a call to refill_pi_state_cache() which preps a pi_state
>> for assignment later in futex_lock_pi_atomic(). This pi_state provides
>>the
>> necessary linkage to perform the priority boosting in the event of a
>> priority inversion. This is handled externally from the futexes via the
>> rt_mutex construct.
>> 
>> Clear as mud?
>
>Not quite that bad, but... The thing is, still, the man page has text
>such as the following (based on your wording):
>
>       FUTEX_CMP_REQUEUE_PI (since Linux 2.6.31)
>              This operation is a PI-aware variant of FUTEX_CMP_REQUEUE.
>              It    requeues    waiters    that    are    blocked    via
>              FUTEX_WAIT_REQUEUE_PI  on uaddr from a non-PI source futex
>              (uaddr) to a PI target futex (uaddr2).
>
>And elsewhere you said
>
>    EINVAL is returned if the non-pi to pi or
>    op pairing semantics are violated.
>
>When someone in user-land (e.g., me) reads pieces like that, they then
>want to find somewhere in the man page a description of what makes a
>futex a *PI futex* and probably some statements of the distinction
>between PI and non-PI futexes. And those statements should be from a
>perspective that is somewhat comprehensible to user-space. I'm not
>yet confident that I can do that. Do you care to take a shot at it?

Hrm, tricky indeed. From userspace, what makes a "futex" PI is the policy
agreement between kernel and userspace (which is the value of the futex:
0, TID, TID|WAITERS, and never just WAITERS, and the use of PI aware futex
op codes when making the futex syscalls.

For a longer discussion of this policy, see Documentation/pi-futex.txt.
Also note that this policy can be combined with that for robust futexes,
adding the OWNERDIED component.

--
Darren Hart
Intel Open Source Technology Center






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