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Date: Mon, 7 Mar 2016 08:10:21 -0500
From: Pedro Giffuni <>
To: Felix Fietkau <>,
Subject: Re: FreeBSD's Google Summer of Code 2016

On 03/07/16 03:32, Felix Fietkau wrote:
> On 2016-03-06 01:25, Rich Felker wrote:
>> On Sat, Mar 05, 2016 at 07:14:34PM -0500, Pedro Giffuni wrote:
... <snip>

>>> Of course I ahould have meant "based on regular FreeBSD kernel services".
>>>>> I am certain this will require more research but it would be useful
>>>>> for other ports as well.
>>> We could ask Petr Salinger for the details, but I am pretty sure
>>> FreeBSD has the required functionality natively.
>>>> Glibc/NPTL has a lot of what I'd call "gratuitous abstraction" (like
>>>> the lll stuff) in their pthread primitives which makes this
>>>> "possible". I call it gratuitous because it's really really hard to
>>>> achieve correct implementations of the pthread sync primitives that
>>>> don't have serious corner-case bugs, and it's unlikely that their
>>>> abstractions actually suffice to make correct alternate
>>>> implementations.
>>>> musl does not have any such abstraction. We require a compare-and-swap
>>>> operation or equivalent on which arbitrary atomic operations can be
>>>> constructed, a futex or equivalent operation that's roughly
>>>> while(*addr==expected) sleep(), and implement all the sync primitives
>>>> just once on top of these.
>>> I am not a threading expert (or even a CS guy), but it sounds like
>>> mutex(9) with condvar(9) would do [1]:
>> No, they don't satisfy the needs of musl; they have their own
>> additional storage requirements and are probably not AS-safe. It might
>> be possible to use them to implement a userspace-emulated futex queue
>> (only if they are AS-safe), but I don't see a way to extend that to
>> the process-shared case.
> What about umtx? It's supposed to be just like linux futex.

Yes, that's it.

It is not well documented beyond the <sys/umtx.h>  header but it
is indeed used to implement the linux abi futex and the native
libthr on FreeBSD.


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