On Tue, May 24, 2011 at 08:37:50PM +0100, Jamie Lokier wrote:
Daniel P. Berrange wrote:
On Wed, May 11, 2011 at 03:45:39PM +0200, Paolo Bonzini wrote:
On 05/11/2011 03:05 PM, Anthony Liguori wrote:
A very slow way, too (on Windows at least if you use qemu_cond...).
That doesn't mean you can't do a fiber implementation for Windows... but
having a highly portable fallback is a good thing.
I agree but where would you place it, since QEMU is only portable to
POSIX and Windows?
osdep-$(CONFIG_POSIX) += coroutine-posix.c
osdep-$(CONFIG_WIN32) += coroutine-win32.c
osdep-??? += coroutine-fallback.c
NetBSD forbids the use of 'makecontext' in any application
which also links to libpthread.so[1]. We used makecontext in
GTK-VNC's coroutines and got random crashes in threaded
apps running on NetBSD. So for NetBSD we tell people to use
the thread based coroutines instead.
You have to use swapcontext(), no wait, you have to use setjmp(), no wait,
_setjmp(), no wait, threads.... Read on.
From Glibc's FAQ, setjmp/longjmp are not portable choices:
- UNIX provides no other (portable) way of effecting a synchronous
context switch (also known as co-routine switch). Some versions
support this via setjmp()/longjmp() but this does not work
universally.
So in principle you should use swapcontext() in portable code.
(By the way, Glibc goes on about how it won't support swapcontext()
from async signal handlers, i.e. preemption, on some architectures
(IA-64/S-390), and I know it has been very subtly broken from a signal
handler on ARM. Fair enough, somehow disappointing, but doesn't
matter for QEMU coroutines.)
But swapcontext() etc. have been withdrawn from POSIX 2008:
- Functions to be deleted
Legacy: Delete all legacy functions except utimes (which should not be
legacy).
OB: Default position is to delete all OB functions.
XSI Functions to change state
....
_setjmp and _longjmp. Should become obsolete.
....
getcontext, setcontext, makecontext and swapcontext are already
marked OB and should be withdrawn. And header file<ucontext.h>.
OB means obsolescent. They were marked obsolescent a few versions
prior, with the rationale that you can use threads instead...
Yep, aware of this but at the end of the day these functions are
commonly available.
It's not surprising that NetBSD forbids makecontext() with
libpthread.so. I suspect old versions of FreeBSD, OpenBSD, DragonFly
BSD, (and Mac OS X?), have the same restriction, because they have a
similar pthreads evolutionary history to LinuxThreads. LinuxThreads
also breaks when using coroutines that switch stacks, because it uses
the stack pointer to know the current thread.
(LinuxThreads is old now, but that particular quirk still affects me
because some uCLinux platforms, on which I wish to use coroutines, still
don't have working NPTL - but they aren't likely to be running QEMU :-)
That is nasty.
Finally, if you are using setjmp/longjmp, consider (from FreeBSD man page):
The setjmp()/longjmp() pairs save and restore the signal mask
while _setjmp()/_longjmp() pairs save and restore only the
register set and the stack. (See sigprocmask(2).)
As setjmp/longjmp were chosen for performance, you may wish to use
_setjmp/_longjmp instead (when available), as swizzling the signal
mask on each switch may involve a system call and be rather slow.
Thanks, I read about that but didn't try to implement special cases
because I don't have relevant OSes here to test against.
My current plan is to try using sigaltstack(2) instead of
makecontext()/swapcontext() as a hack since OpenBSD doesn't have
makecontext()/swapcontext().
TBH I'm almost at the stage where I think we should just use threads
and/or async callbacks, as appropriate. Hopefully I'll be able to cook
up a reasonably portable implementation of coroutines though, because
the prospect of having to go fully threaded or do async callbacks isn't
attractive in many cases.