On 1/17/15, 1:16 AM, "Michael Kerrisk (man-pages)"
[off-list ref] wrote:

Hello Darren,

On 01/17/2015 02:33 AM, Darren Hart wrote:

quoted

Corrected Davidlohr's email address.

Thanks!

quoted

On 1/15/15, 7:12 AM, "Michael Kerrisk (man-pages)"
[off-list ref] wrote:

quoted

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!

quoted

quoted

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.

quoted

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).

quoted

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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