On Sat, Aug 08, 2015 at 08:57:35AM +0200, Michael Kerrisk (man-pages) wrote:

...

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.\" FIXME ===== End of adapted Hart/Guniguntala text =====



.\" FIXME We need some explanation in the following paragraph of *why*
.\"       it is important to note that "the kernel will update the
.\"       futex word's value prior
       It is important to note to returning to user space" . Can someone
       explain?   that  the  kernel  will  update the futex word's value
       prior to returning to user space.  Unlike the other futex  opera‐
       tions  described  above, the PI futex operations are designed for
       the implementation of very specific IPC mechanisms.

If the kernel didn't perform the update prior to returning to userspace,
we could end up in an invalid state. Such as having an owner, but the
value being 0. Or having waiters, but not having FUTEX_WAITERS set.

So I've now reworked this passage to read:

       It  is  important  to  note that the kernel will update the futex
       word's value prior to returning to user  space.   (This  prevents
       the possibility of the futex word's value ending up in an invalid
       state, such as having an owner but the value being 0,  or  having
       waiters but not having the FUTEX_WAITERS bit set.)

Okay?

Yes.

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.\"
.\" FIXME XXX In discussing errors for FUTEX_CMP_REQUEUE_PI, Darren Hart
.\"       made the observation that "EINVAL is returned if the non-pi 
.\"       to pi or op pairing semantics are violated."
.\"       Probably there needs to be a general statement about this
.\"       requirement, probably located at about this point in the page.
.\"       Darren (or someone else), care to take a shot at this?

We can probably borrow from either the futex.c comments or the
futex-requeue-pi.txt in Documentation. Also, it is important to note
that the PI requeue operations require two distinct uadders (although
that is implied by requiring "non-pi to pi" as a futex cannot be both.

Or... perhaps something like:

	Due to the kernel imposed futex word value policy, PI futex
	operations have additional usage requirements:
	
	FUTEX_WAIT_REQUEUE_PI must be paired with FUTEX_CMP_REQUEUE_PI
	and be performed from a non-pi futex to a distinct pi futex.
	Failing to do so will return EINVAL. 

For which operation does the EINVAL occur: FUTEX_WAIT_REQUEUE_PI or 
FUTEX_CMP_REQUEUE_PI?

FUTEX_WAIT_REQUEUE_PI can return -EINVAL if called with invalid parameters, such
as uaddr==uaddr2, or (in the case of SHARED futexes), the associated keys match
(meaning it's the same futex word - shared memory, inode, etc.). This can't
happen if the stated policy of requeueing from non-pi to pi is followed as the
same word cannot be both non-pi and pi at the same time, requiring them to be
unique futex words.

FUTEX_CMP_REQUEUE_PI will fail similarly if uaddr and uaddr2 are the same futex
word. Also, if nr_wake != 1.

But, to the point I was making above, FUTEX_CMP_REQUEUE_PI must reque uaddr to
same uaddr2 specified in the previous FUTEX_WAIT_REQUEUE_PI call.
FUTEX_WAIT_REQUEUE_PI sets up the operation, FUTEX_CMP_REQUEUE_PI completes it,
and they must agree on uaddr and uaddr2.

...

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And their PRIVATE counterparts of course (which is assumed as it is a
flag to the opcode).

Yes. But I don't think that needs to be called out explicitly here (?).

Agreed.

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.\" FIXME XXX ===== Start of adapted Hart/Guniguntala text =====
.\"       The following text is drawn from the Hart/Guniguntala paper
.\"       (listed in SEE ALSO), but I have reworded some pieces
.\"       significantly. Please check it.

       The PI futex operations described below  differ  from  the  other
       futex  operations  in  that  they impose policy on the use of the
       value of the futex word:

       *  If the lock is not acquired, the futex word's value  shall  be
          0.

       *  If  the  lock is acquired, the futex word's value shall be the
          thread ID (TID; see gettid(2)) of the owning thread.

       *  If the lock is owned and there are threads contending for  the
          lock,  then  the  FUTEX_WAITERS  bit shall be set in the futex
          word's value; in other words, this value is:

              FUTEX_WAITERS | TID


       Note that a PI futex word never just has the value FUTEX_WAITERS,
       which is a permissible state for non-PI futexes.

The second clause is inappropriate. I don't know if that was yours or
mine, but non-PI futexes do not have a kernel defined value policy, so
==FUTEX_WAITERS cannot be a "permissible state" as any value is
permissible for non-PI futexes, and none have a kernel defined state.

Perhaps include a Note under the third bullet as:

	  Note: It is invalid for a PI futex word to have no owner and
	        FUTEX_WAITERS set.

Done.

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       With this policy in place, a user-space application can acquire a
       not-acquired lock or release a lock that no other threads try  to

"that no other threads try to acquire" seems out of place. I think
"atomic instructions" is sufficient to express how contention is
handled.

Yup, changed.

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       acquire using atomic instructions executed in user space (e.g., a
       compare-and-swap operation such as cmpxchg on the  x86  architec‐
       ture).   Acquiring  a  lock simply consists of using compare-and-
       swap to atomically set the futex word's value to the caller's TID
       if  its  previous  value  was 0.  Releasing a lock requires using
       compare-and-swap to set the futex word's value to 0 if the previ‐
       ous value was the expected TID.

       If a futex is already acquired (i.e., has a nonzero value), wait‐
       ers must employ the FUTEX_LOCK_PI operation to acquire the  lock.
       If other threads are waiting for the lock, then the FUTEX_WAITERS
       bit is set in the futex value; in this case, the lock owner  must
       employ the FUTEX_UNLOCK_PI operation to release the lock.

       In  the  cases  where  callers  are forced into the kernel (i.e.,
       required to perform a futex() call), they then deal directly with
       a so-called RT-mutex, a kernel locking mechanism which implements
       the required priority-inheritance semantics.  After the  RT-mutex
       is  acquired,  the futex value is updated accordingly, before the
       calling thread returns to user space.

This last paragraph relies on kernel implementation rather than
behavior. If the RT-mutex is renamed or the mechanism is implemented
differently in futexes, this section will require updating. Is that
appropriate for a user-space man page?

In the end, I'm not sure. This is (so far) my best attempt at trying
to convey an explanation of the behavior provided by the API.

          results).

But see my question just above. I'll tweak the first bullet point a
little after I hear back from you.

Arg, lost context. Which question?

In my humble opinion, the paragraph about RT-mutex above should perhaps instead
read something like:


	    In  the  cases  where  callers  are forced into the kernel (i.e.,
	    required to perform a futex() call), they then deal directly with
	    Linux kernel internal mechanisms which implement the required
	    priority-inheritance semantics.  Once the internal locking structure
	    is  acquired,  the futex value is updated accordingly, before the
	    calling thread returns to user space.

I'm not terribly particular about this, but in my opinion, we should not refer
to internal-only kernel structures in the man page. Feel free to ignore, or to
defer to a differing opinion from Thomas or others.

Thanks for all your work on this!

-- 
Darren Hart
Intel Open Source Technology Center
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