[ANNOUNCE] libxshmfence 1.0

[Alan Coopersmith]

On 11/ 2/13 12:39 AM, Keith Packard wrote:
> Alan Coopersmith <alan.coopersmith at oracle.com> writes:
>
>> What about platforms which don't have <linux/futex.h> ?
>
> I looked into alternate synchronization mechanisms; I couldn't figure
> out how to use standard pthread APIs to provide the desired semantics.
>
> Here's the detailed write up I did that describes the SyncFence
> semantics and how those map directly to Linux Futexes:
>
>          http://keithp.com/blogs/Shared_Memory_Fences/
>
> If you can figure out how to build this with pthread mutexes and/or
> semaphores, then we could create a version of xshmfence that worked
> using regular pthread functions. As I understand it, pthread
> synchronization primitives are supposed to work across processes that
> share memory, even if they use different virtual addresses to reference
> the same object.

I've checked with the Solaris kernel & libc folks, and they confirm there's
no problem using the POSIX API's on Solaris - ours are correctly implemented
using uint{16,32,64}_t types so they are the same on 32-bit & 64-bit ABI's,
and they're documented & supported as being compatible across the two.

Specifically they said:

   "On Solaris, mutexes in shared memory, shared among 32-bit and 64-bit
    processes, work correctly.  All of the other locking thingies work
    correctly too:

    condvars
    rwlocks
    semaphores
    barriers
    message queues"

And one of our old timers pointed out:

   "Note that this is exactly the same problem that Solaris (SunOS first)
    solved with the winlock driver.

 
https://hg.java.net/hg/solaris~on-src/file/tip/usr/src/uts/common/io/winlockio.c#l27

    This was an real factor in the excellent performance of sundgadoom 8-)."

(Honestly, I vaguely knew we had winlockio, but not what it was for.  Our Xorg
  doesn't use it, only our old Xsun implementation with its SUN-DGA extension.)

So between this & the comments from the BSD guys, it seems like POSIX API's
should be the default implementation for all platforms except Linux multi-lib
platforms with incompatible implementations, where you have to use futexes
instead to work around platform bugs.

-- 
	-Alan Coopersmith-              alan.coopersmith at oracle.com
	 Oracle Solaris Engineering - http://blogs.oracle.com/alanc