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Date: Mon, 7 Mar 2016 09:32:17 +0100
From: Felix Fietkau <>
To:, Pedro Giffuni <>
Subject: Re: FreeBSD's Google Summer of Code 2016

On 2016-03-06 01:25, Rich Felker wrote:
> On Sat, Mar 05, 2016 at 07:14:34PM -0500, Pedro Giffuni wrote:
>> On 03/05/16 18:32, Rich Felker wrote:
>> >On Sat, Mar 05, 2016 at 05:41:25PM -0500, Pedro Giffuni wrote:
>> >>First of all, great to hear there is interest on the musl side too.
>> >>
>> >>I think the biggest precedent of porting linux-oriented C libraries
>> >>came from Debian's kFreeBSD. We accomodated a little by for them
>> >>by defining __FreeBSD_kernel__ in sys/param.h.
>> >>
>> >>While using the optional linux-abi futex in FreeBSD could be an option,
>> >>it is not really the cleanest option. The Debian guys did a port of
>> >>NPTL using regular pthreads:
>> >>
>> 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.

- Felix

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