Re: [Qemu-devel] coroutine-ucontext broken for x86-32

[Anthony Liguori]

On 05/09/2012 06:38 AM, Jan Kiszka wrote:
> On 2012-05-09 08:15, Peter Maydell wrote:
> > On 9 May 2012 11:11, Kevin Wolf<address@hidden>  wrote:
> > > Am 08.05.2012 21:35, schrieb Jan Kiszka:
> > > > I hunted down a fairly subtle corruption of the VCPU thread signal mask
> > > > in KVM mode when using the ucontext version of coroutines:
> > > >
> > > > coroutine_new calls getcontext, makecontext, swapcontext. Those
> > > > functions get/set also the signal mask of the caller. Unfortunately,
> > > > they only use the sigprocmask syscall on i386, not the rt_sigprocmask
> > > > version. So they do not properly save/restore the blocked RT signals,
> > > > namely our SIG_IPI - it becomes unblocke this way.
> > >
> > > If other coroutine backends work (sigaltstack?), we could try to detect
> > > the situation in configure and set the right default. Not sure what the
> > > condition is, glibc + i386?
> >
> > I don't think you can do a compile-time test for this short of
> > just disabling use of the ucontext code on all i386/Linux platforms.
> >
> > I think it's becoming increasingly obvious that the setcontext/getcontext
> > code path is not very well used and prone to nasty libc bugs. Trying
> > to implement coroutines in C is just a really bad idea and I think
> > we should be trying to reduce our use of them if we possibly can,
> > presumably by switching to actually using threads where we really
> > need the parallelism.
>
> I tend to agree.
>
> FWIW, sigaltstack works around the issue here, but I'm still looking s
> bit skeptical at its implementation.

Is there any downside to using SIGUSR1?

Regards,

Anthony Liguori

> Jan