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Re: futex(3) man page, final draft for pre-release review


On 12/18/2015 12:21 PM, Torvald Riegel wrote:
> On Tue, 2015-12-15 at 13:18 -0800, Darren Hart wrote:
>> On Tue, Dec 15, 2015 at 02:43:50PM +0100, Michael Kerrisk (man-pages) wrote:
>>>
>>>        When executing a futex operation that requests to block a thread,
>>>        the kernel will block only if the futex word has the  value  that
>>>        the  calling  thread  supplied  (as  one  of the arguments of the
>>>        futex() call) as the expected value of the futex word.  The loadâ
>>>        ing  of the futex word's value, the comparison of that value with
>>>        the expected value, and the actual blocking  will  happen  atomiâ
>>>
>>> FIXME: for next line, it would be good to have an explanation of
>>> "totally ordered" somewhere around here.
>>>
>>>        cally  and totally ordered with respect to concurrently executing
>>
>> Totally ordered with respect futex operations refers to semantics of the
>> ACQUIRE/RELEASE operations and how they impact ordering of memory reads and
>> writes. The kernel futex operations are protected by spinlocks, which ensure
>> that that all operations are serialized with respect to one another.
>>
>> This is a lot to attempt to define in this document. Perhaps a reference to
>> linux/Documentation/memory-barriers.txt as a footnote would be sufficient? Or
>> perhaps for this manual, "serialized" would be sufficient, with a footnote
>> regarding "totally ordered" and a pointer to the memory-barrier documentation?
> 
> I'd strongly prefer to document the semantics for users here.  

Yes, please.

> And I
> don't think users use the kernel's memory model -- instead, if we assume
> that most users will call futex ops from C or C++, then the best we have
> is the C11 / C++11 memory model.  

Agreed.

> Therefore, if we want to expand that,

I think we should. And by we, I mean you ;-)

> we should specify semantics in terms of as-if equivalence to C11 pseudo
> code.  I had proposed that in the past but, IIRC, Michael didn't want to
> add a C11 "dependency" in the semantics back then, at least for the
> initial release.

I'd like to avoid it if possible, since many of us don't understand
all the details of those C11 semantics--and by us, I mean
me :-/. But maybe I'll be forced to educate myself better.

> Here's what I wrote back then (atomic_*_relaxed() is like C11
> atomic_*(..., memory_order_relaxed), lock/unlock have normal C11 mutex
> semantics):
> 
> ========================
> 
> For example, we could say that futex_wait is, in terms of
> synchronization semantics, *as if* we'd execute a piece of C11 code.
> Here's a part of the docs for a glibc-internal futex wrapper that I'm
> working on; this is futex_wait ... :
> 
> /* Atomically wrt other futex operations, this blocks iff the value at
>    *FUTEX matches the expected value.  This is semantically equivalent to: 
>      l = <get lock associated with futex> (FUTEX);
>      wait_flag = <get wait_flag associated with futex> (FUTEX);
>      lock (l);
>      val = atomic_load_relaxed (FUTEX);
>      if (val != expected) { unlock (l); return EAGAIN; }
>      atomic_store_relaxed (wait_flag, 1);
>      unlock (l);
>      // Now block; can time out in futex_time_wait (see below)
>      while (atomic_load_relaxed(wait_flag));
> 
>    Note that no guarantee of a happens-before relation between a woken
>    futex_wait and a futex_wake is documented; however, this does not matter
>    in practice because we have to consider spurious wake-ups (see below),
>    and thus would not be able to reason which futex_wake woke us anyway.
> 
> 
> ... and this is futex_wake:
> 
> /* Atomically wrt other futex operations, this unblocks the specified
>    number of processes, or all processes blocked on this futex if there are
>    fewer than the specified number.  Semantically, this is equivalent to:
>      l = <get lock associated with futex> (futex);
>      lock (l);
>      for (res = 0; processes_to_wake > 0; processes_to_wake--, res++) {
>        if (<no process blocked on futex>) break;
>        wf = <get wait_flag of a process blocked on futex> (futex);
>        // No happens-before guarantee with woken futex_wait (see above)
>        atomic_store_relaxed (wf, 0);
>      }
>      return res;
> 
> This allows a programmer to really infer the guarantees he/she can get
> from a futex in terms of synchronization, without the docs having to use
> prose to describe that.  This should also not constrain the kernel in
> terms of how to implement it, because it is a conceptual as-if relation
> (e.g., the kernel won't spin-wait the whole time, and we might want to
> make this clear for the PI case).
> 
> Of course, there are several as-if representations we could use, and we
> might want to be a bit more pseudo-code-ish to make this also easy to
> understand for people not familiar with C11 (e.g., using mutex + condvar
> with some relaxation of condvar guaranteees).

Okay -- I'm open to all of the above.

Cheers,

Michael



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/


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