Date: Mon, 7 Mar 2016 08:10:21 -0500 From: Pedro Giffuni <pfg@...eBSD.org> To: Felix Fietkau <nbd@...nwrt.org>, musl@...ts.openwall.com 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". >>> >>>>> http://thread.gmane.org/gmane.linux.debian.ports.bsd/11702 >>>>> >>>>> 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 : >> >> 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. Pedro.
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